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Ground Water Acid Mine Drainage: Sources and Treatment in Artificial Recharge of Unconfined Aquifer 11 Groundwater and Arsenic: Chemical Behavior Treatment of Arsenic, Chromium, and Biof

GROUND WATER

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Library of Congress Cataloging-in-Publication Data is available.

Ground Water

Acid Mine Drainage: Sources and Treatment in

Artificial Recharge of Unconfined Aquifer 11

Groundwater and Arsenic: Chemical Behavior

Treatment of Arsenic, Chromium, and Biofouling

Modeling Contaminant Transport and

In Situ Bioremediation of Contaminated

Sensitivity of Groundwater to Contamination 56

Water Contamination by Low Level Organic

Waste Compounds in the Hydrologic System 60

Recharge in Desert Regions Around The World 72

Hydrologic Feasibility Assessment and Design in

Physical Properties of DNAPLs and

Earthquakes—Rattling the Earth’s Plumbing

In Situ Electrokinetic Treatment of MtBE,

Fluoride Contamination in Ground Water 130

Geochemical Modeling—Computer Code

Hydraulic Properties Characterization 184Mobility of Humic Substances in

Assessment of Groundwater Quality in District

Irrigation Water Quality in District Hardwar,

Installation And In Situ Remediation of

Metal Organic Interactions in Subtitle D LandfillLeachates and Associated Ground Waters 258

Soil and Water Contamination by Heavy

Modeling Non-Point Source Pollutants in the

Modeling Techniques for Solute Transport in

Ambient Groundwater Monitoring Network

v

Enhanced Bioremediation 319

Treatment for Nitrates in Groundwater 323

Groundwater Vulnerability to Pesticides: An

Overview of Approaches and Methods of

High pH Groundwater—The Effect of The

Dissolution of Hardened Cement Pastes 362

Phytoextraction and Phytostabilization:

Technical, Economic and Regulatory

Considerations of the Soil-Lead Issue 365

Phytoextraction of Zinc and Cadmium from Soils

Phytoremediation Enhancement of Natural

Bacteria Role in the Phytoremediation of Heavy

Phytoremediation of Lead-Contaminated Soils 381

Phytoremediation of Methyl Tertiary-Butyl

Phytoremediation of Selenium-Laden Soils 397

Low Flow Groundwater Purging and

Sub-Surface Redox Chemistry: A Comparison of

Equilibrium and Reaction-Based Approaches 413

Groundwater Remediation by Injection and

Groundwater Remediation Project Life Cycle 436

Innovative Contaminated Groundwater

Characterizing Soil Spatial Variability 465

Reactive Transport in The Saturated Zone: CaseHistories for Permeable Reactive Barriers 518Transport of Reactive Solute in Soil and

Groundwater and Vadose Zone Hydrology 533

Vapor Transport in the Unsaturated Zone 543Applications of Soil Vapor Data to Groundwater

Microbial Processes Affecting Monitored NaturalAttenuation of Contaminants in the

Groundwater Vulnerability to Pesticides:

Pharmaceuticals, Hormones, and Other OrganicWastewater Contaminants in U.S Streams 605The Environmental Impact of Iron in

Groundwater and Perchlorate: Chemical

Groundwater and Vinyl Chloride: Chemical

Groundwater and Uranium: Chemical Behavior

Throughout history, groundwater has played a major role

in providing the resource needs of the world It accounts

for 97% of the world’s freshwater and serves as the base

flow for all streams, springs, and rivers In the United

States, one half of the population relies on groundwater

for its drinking water and is the sole source of supply for

20 of the 100 largest cities Well over 90% of rural America

is totally dependent on groundwater An inventory of the

total groundwater resources in the United States can be

visualized as being equal to the flow of the Mississippi

River at Vicksburg for a period of 250 years

One of the first groundwater scientists was a French

engineer who was in charge of public drinking water

in Dijon In 1856, Henri Darcy conducted experiments

and published mathematical expressions describing the

flow of water through sand filters His work remains one

of the cornerstones of today’s groundwater hydrologists

At about the same time, a Connecticut court ruled that

the influences of groundwater movement are so secret,

changeable, and uncontrollable that they could not be

subject to regulations of law, nor to a system of rules, as

had been done with surface streams

In this volume of the Water Encyclopedia, we have

attempted to erase the ignorance that existed in the

early years of groundwater science by presenting the most

current knowledge on the subject as provided by authors

from around the globe In addition to excellent articles

from many American scholars, equally superb writingsfrom such diverse countries as England, Nigeria, India,Iran, Thailand, and Greece are provided

As the origins of the selected articles are diverse, soare the subjects of discussion Along with straightforwarddescriptions of basic groundwater concepts (drawdownaround pumping wells, hydraulic head, field capacity, andflow), the reader is introduced to more complex subjects

(isotope technologies, aquifer tests, in situ remediation,

tritium dating, modeling, and geophysical properties).There are also articles for more practical applications (wellmaintenance, subsurface drainage, nitrate contamination,tracer tests, well yields, and drilling technologies) Ofcourse, for the more fanciful reader, we have selectedarticles that remind us of the way windmills sounded

at night, the ancient use of qanats in Persia to providesustainable groundwater resources, and the development

of Darcy’s Law

In the end, we feel that the information providedwill afford an educational home for readers approaching

the Water Encyclopedia from a variety of needs as well

as different levels of scientific acumen We are alsoconfident that many readers will simply be expandingtheir knowledge base by these sets of enjoyable reading

Jay LehrJack Keeley

ix

Segun Adelana, University of Ilorin, Ilorin, Nigeria, Summary of Isotopes

in Contaminant Hydrogeology, Environmental Isotopes in Hydrogeology

Mohammad N Almasri, An-Najah National University, Nablus,

Palestine, Groundwater Flow and Transport Process

Tom A Al, University of New Brunswick, Fredericton, New Brunswick,

Canada, River-Connected Aquifers: Geophysics, Stratigraphy,

Hydroge-ology, and Geochemistry

Larry Amskold, University of New Brunswick, Fredericton, New

Brunswick, Canada, River-Connected Aquifers: Geophysics,

Stratigra-phy, Hydrogeology, and Geochemistry

Ann Azadpour-Keeley, National Risk Management Research Laboratory,

ORD, U.S EPA, Ada, Oklahoma, Microbial Processes Affecting

Monitored Natural Attenuation of Contaminants in the Subsurface,

Nitrate Contamination of Groundwater

Mukand Singh Babel, Asian Institute of Technology, Pathumthani,

Thailand, Groundwater Velocities, Groundwater Flow Properties, Water

in The Unsaturated Zone

Philip B Bedient, Rice University, Houston, Texas, Transport of Reactive

Solute in Soil and Groundwater

David M Bednar, Jr., Michael Baker, Jr Inc., Shreveport, Louisiana,

Karst Hydrology, Karst Topography, Groundwater Dye Tracing in Karst

Milovan Beljin, Cincinnati, Ohio, Horizontal Wells

Craig H Benson, University of Wisconsin-Madison, Madison, Wisconsin,

Reactive Transport in The Saturated Zone: Case Histories for Permeable

Reactive Barriers

Robert A Bisson, Alexandria, Virginia, Megawatersheds

William J Blanford, Louisiana State University, Baton Rouge, Louisiana,

Vadose Zone Monitoring Techniques

Thomas B Boving, University of Rhode Island, Kingston, Rhode

Island, Organic Compounds in Ground Water, Innovative Contaminated

Groundwater Remediation Technologies

Richard C Brody, UC Berkeley, Berkeley, California, Connate Water

Kristofor R Brye, University of Arkansas, Fayetteville, Arkansas,

Lysimeters, Soil and Water Contamination by Heavy Metals

Mobility of Humic Substances in Groundwater

Bureau of Indian Affairs and Arizona Department of Water

Program

Karl E Butler, University of New Brunswick, Fredericton, New Brunswick,

Canada, River-Connected Aquifers: Geophysics, Stratigraphy,

Hydroge-ology, and Geochemistry

Herbert T Buxton, United States Geological Survey, Pharmaceuticals,

Hormones, and Other Organic Wastewater Contaminants in U.S.

Streams

Natalie L Capiro, Rice University, Houston, Texas, Transport of Reactive

Solute in Soil and Groundwater

Harendra S Chauhan, G.B Pant University of Agriculture and

Technology, Uttar Pradesh, India, Steady-State Flow Aquifer Tests,

Subsurface Drainage

Bernard L Cohen, University of Pittsburgh, Pittsburgh, Pennsylvania,

Risk Analysis of Buried Wastes From Electricity Generation

David P Commander, Water and Rivers Commission, East Perth,

Australia, Water Dowsing (Witching), Artesian Water

Dennis L Corwin, USDA-ARS George E Brown, Jr., Salinity Laboratory,

Riverside, California, Characterizing Soil Spatial Variability, Modeling

Non-Point Source Pollutants in the Vadose Zone Using GIS, Groundwater

Vulnerability to Pesticides: An Overview of Approaches and Methods of

Evaluation

Colin C Cunningham, The University of Edinburgh, Edinburgh,

Scotland, United Kingdom, In Situ Bioremediation of Contaminated

Groundwater

William L Cunningham, U.S Geological Survey, Denver, Colorado,

Earthquakes—Rattling the Earth’s Plumbing System

Uwe Dannwolf, URS Australia Pty Ltd., Turner, Australia, Groundwater

and Vadose Zone Hydrology

Diganta Bhusan Das, Oxford University, Oxford, United Kingdom,

Viscous Flow, Finite Element Modeling of Coupled Free and Porous

Flow, Combined Free and Porous Flow in the Subsurface, Modeling Techniques for Solute Transport in Groundwater

Rupali Datta, University of Texas at San Antonio, San Antonio, Texas,

Remediation of Contaminated Soils, Genetics of Metal Tolerance and Accumulation in Higher Plants, Phytoextraction of Zinc and Cadmium from Soils Using Hyperaccumulator Plants, Phytoremediation

of Selenium-Laden Soils, Phytoextraction and Phytostabilization: Technical, Economic and Regulatory Considerations of the Soil-Lead Issue

Ali H Davani, University of Texas at San Antonio, San Antonio, Texas,

Remediation of Contaminated Soils

L.C Davis, (from Phytoremediation: Transformation and Control of

Contaminants, Wiley 2003), Phytoremediation of Methyl Tertiary-Butyl

Ether

Melissa R Dawe, University of New Brunswick, Fredericton, New

Brunswick, Canada, River-Connected Aquifers: Geophysics,

Stratigra-phy, Hydrogeology, and Geochemistry

Steven A Dielman, ENVIRON International Corporation, Arlington,

Virginia, Hydraulic Conductivity/Transmissibility

Craig E Divine, Colorado School of Mines, Golden, Colorado,

Ground-water Sampling with Passive Diffusion Samplers, Detecting Modern Groundwaters with 85 Kr, Groundwater Dating with H–He

Shonel Dwyer, Environmental Bio-Systems, Inc., Mill Valley, California,

Groundwater and Perchlorate: Chemical Behavior and Treatment

Aly I El-Kadi, University of Hawaii at Manoa, Honolulu, Hawaii,

Unconfined Groundwater

Environment Canada, Groundwater—Nature’s Hidden Treasure

L.E Erickson, (from Phytoremediation: Transformation and Control of

Contaminants, Wiley 2003), Phytoremediation of Methyl Tertiary-Butyl

Ether

Thomas R Fisher, Horn Point Laboratory—UMCES, Solomons,

Maryland, What is a Hydrochemical Model?

Craig Foreman, Environmental Bio-Systems, Inc., Mill Valley, California,

Groundwater and Cadmium: Chemical Behavior and Treatment

Devin L Galloway, U.S Geological Survey, Denver, Colorado,

Earth-quakes—Rattling the Earth’s Plumbing System

Lorraine Geddes-McDonald, Environmental Bio-Systems, Inc., Mill

Valley, California, Groundwater and Nitrate: Chemical Behavior and

Treatment

M ´ario Abel Gon ¸calves, Faculdade de Ciˆencias da Universidade de Lisoba,

Lisoba, Portugal, Metal Organic Interactions in Subtitle D Landfill

Leachates and Associated Ground Waters, Geochemical Computer Codes, Geochemical Models

Modeling-Jason J Gurdak, U.S Geological Survey, Lakewood, Colorado and

Colorado School of Mines, Golden, Colorado, Groundwater Vulnerability

to Pesticides: Statistical Approaches

Navraj S Hanspal, Loughborough University, Loughborough, United

Kingdom, Modeling Techniques for Solute Transport in Groundwater,

Viscous Flow, Laminar Flow, Finite Element Modeling of Coupled Free and Porous Flow

Thomas Harter, University of California, Davis, California, Specific Yield

Storage Equation, Vulnerability Mapping of Groundwater Resources, Aquifers

Blayne Hartman, H&P Mobile Geochemistry, Solana Beach, California,

Applications of Soil Vapor Data to Groundwater Investigations

Joseph Holden, University of Leeds, Leeds, United Kingdom,

Infiltrom-eters, Soil Pipes and Pipe Flow, Infiltration and Soil Water Processes, Darcy’s Law, Infiltration/Capacity/Rates

Ekkehard Holzbecher, Humboldt Universit ¨at Berlin, Berlin, Germany,

Groundwater Modeling, Ghijben–Herzberg Equilibrium

Paul F Hudak, University of North Texas, Denton, Texas, Mass Transport

in Saturated Media

John D Humphrey, Colorado School of Mines, Golden, Colorado,

Groundwater Dating with H–He

S.L Hutchinson, (from Phytoremediation: Transformation and Control

of Contaminants, Wiley 2003), Hydrologic Feasibility Assessment and

Design in Phytoremediation

Th.A Ioannidis, Aristotle University of Thessaloniki, Thessaloniki,

Greece, Phytoremediation of Lead-Contaminated Soils

xi

Based Approaches

Irena B Ivshina, Institute of Ecology and Genetics of Microorganisms

of the RAS, Perm, Russia, In Situ Bioremediation of Contaminated

Groundwater

James A Jacobs, Environmental Bio-Systems, Inc., Mill Valley,

Cali-fornia, Groundwater and Cobalt: Chemical Behavior and Treatment,

Limiting Geochemical Factors in Remediation Using Monitored

Nat-ural Attenuation and Enhanced Bioremediation, The Role of Heat in

Groundwater Systems, Horizontal Wells in Groundwater Remediation,

Groundwater Flow in Heterogenetic Sediments and Fractured Rock

Systems, Groundwater and Cadmium: Chemical Behavior and

Treat-ment, Groundwater and Benzene: Chemical Behavior and TreatTreat-ment,

Groundwater and Lead: Chemical Behavior and Treatment,

Groundwa-ter and Nitrate: Chemical Behavior and Treatment, GroundwaGroundwa-ter and

Uranium: Chemical Behavior and Treatment, Groundwater and

Mer-cury: Chemical Behavior and Treatment, The Environmental Impact of

Iron in Groundwater, Water Well Drilling Techniques, Water-Jetting

Drilling Technologies for Well Installation And In Situ Remediation of

Hydrocarbons, Solvents, and Metals, Source, Mobility, and Remediation

of Metals, Particulate Transport in Groundwater—Bacteria and

Col-loids, Groundwater and Arsenic: Chemical Behavior and Treatment,

In Situ Groundwater Remediation for Heavy Metal Contamination,

Groundwater Remediation by In Situ Aeration and Volatilization,

MTBE, Phytoremediation Enhancement of Natural Attenuation

Pro-cesses, Groundwater Remediation by Injection and Problem Prevention,

Chemical Oxidation Technologies for Groundwater Remediation,

Phys-ical Properties of DNAPLs and Groundwater Contamination, Process

Limitations of In Situ Bioremediation of Groundwater, Water

Contam-ination by Low Level Organic Waste Compounds in the Hydrologic

System, Applications of Soil Vapor Data to Groundwater

Investi-gations, Groundwater Remediation Project Life Cycle, Groundwater

Remediation: In Situ Passive Methods, Groundwater and Vinyl

Chlo-ride: Chemical Behavior and Treatment, Groundwater and Perchlorate:

Chemical Behavior and Treatment, Groundwater Sampling Techniques

for Environmental Projects, Low Flow Groundwater Purging and

Surging

Hamid R Jahani, Water Research Institute, Hakimieh, Tehran, Iran,

Groundwater Tracing, Resistivity Methods

Chakresh K Jain, National Institute of Hydrology, Roorkee, India,

Assessment of Groundwater Quality in District Hardwar, Uttaranchal,

India, Nonpoint Sources, Fluoride Contamination in Ground Water,

Irrigation Water Quality in District Hardwar, Uttaranchal, India

John R Jansen, Aquifer Science & Technology, Waukesha, Wisconsin,

Geophysics and Remote Sensing

Anthea Johnson, University of Auckland, Auckland, New Zealand,

Bacteria Role in the Phytoremediation of Heavy Metals

Silvia Johnson, Environmental Bio-Systems, Inc., Mill Valley, California,

Groundwater and Mercury: Chemical Behavior and Treatment

Tracey Johnston, University of Texas at San Antonio, San Antonio,

Texas, Phytoextraction and Phytostabilization: Technical, Economic and

Regulatory Considerations of the Soil-Lead Issue

Jagath J Kaluarachchi, Utah State University, Logan, Utah,

Ground-water Flow and Transport Process

A Katsoyiannis, Aristotle University of Thessaloniki, Thessaloniki,

Greece, The Use of Semipermeable Membrane Devices (SPMDs) for

Monitoring, Exposure, and Toxicity Assessment

Jack Keeley, Environmental Engineer, Ada, Oklahoma, Nitrate

Contam-ination of Groundwater

David W Kelley, University of St Thomas, St Paul, Minnesota, Leaching

Lisa Kirkland, Environmental Bio-Systems, Inc., Mill Valley, California,

Groundwater and Lead: Chemical Behavior and Treatment

Dana W Kolpin, United States Geological Survey, Pharmaceuticals,

Hormones, and Other Organic Wastewater Contaminants in U.S.

Streams

C.P Kumar, National Institute of Hydrology, Roorkee, India, Groundwater

Balance

Maria S Kuyukina, Institute of Ecology and Genetics of Microorganisms

of the RAS, Perm, Russia, In Situ Bioremediation of Contaminated

Groundwater

Kung-Yao Lee, Horn Point Laboratory—UMCES, Solomons, Maryland,

Scott M Lesch, USDA-ARS George E Brown, Jr., Salinity Laboratory,

Riverside, California, Characterizing Soil Spatial Variability

Len Li, University of Wisconsin-Madison, Madison, Wisconsin, Reactive

Transport in The Saturated Zone: Case Histories for Permeable Reactive Barriers

Keith Loague, Stanford University, Stanford, California, Groundwater

Vulnerability to Pesticides: An Overview of Approaches and Methods of Evaluation, Modeling Non-Point Source Pollutants in the Vadose Zone Using GIS

Walter W Loo, Environmental & Technology Services, Oakland,

California, Treatment for Nitrates in Groundwater, Treatment of

Arsenic, Chromium, and Biofouling in Water Supply Wells, In Situ

Electrokinetic Treatment of MtBE, Benzene, and Chlorinated Solvents, Hydraulic Properties Characterization

Kerry T Macquarrie, University of New Brunswick, Fredericton, New

Brunswick, Canada, River-Connected Aquifers: Geophysics,

Stratigra-phy, Hydrogeology, and Geochemistry

Mini Mathew, Colorado School of Mines, Golden, Colorado, Modeling of

DNAPL Migration in Saturated Porous Media

S.C Mccutcheon, (from Phytoremediation: Transformation and Control

of Contaminants, Wiley 2003), Hydrologic Feasibility Assessment and

Design in Phytoremediation

John E McCray, Colorado School of Mines, Golden, Colorado,

Groundwater Vulnerability to Pesticides: Statistical Approaches

M.S Mohan Kumar, Indian Institute of Science, Bangalore, India,

Modeling of DNAPL Migration in Saturated Porous Media

John E Moore, USGS (Retired), Denver, Colorado, Well Hydraulics and

Aquifer Tests, Drawdown, Groundwater Quality, Hot Springs, Overdraft, Saline Seep, Geological Occurrence of Groundwater

Angela Munroe, Environmental Bio-Systems, Inc., Mill Valley, California,

Groundwater and Vinyl Chloride: Chemical Behavior and Treatment

Jean-Christophe Nadeau, University of New Brunswick, Fredericton,

New Brunswick, Canada, River-Connected Aquifers: Geophysics,

Stratig-raphy, Hydrogeology, and Geochemistry

NASA Earth Science Enterprise Data and Services, Squeezing Water

from Rock

Vahid Nassehi, Loughborough University, Loughborough, United

King-dom, Combined Free and Porous Flow in the Subsurface, Viscous Flow

Sascha E Oswald, UFZ Centre for Environmental Research,

Leipzig-Halle, Germany, Modeling Contaminant Transport and Biodegradation

in Groundwater

Timothy K Parker, Groundwater Resources of California, Sacramento,

California, Water Contamination by Low Level Organic Waste

Compounds in the Hydrologic System

Jim C Philp, Napier University, Edinburgh, Scotland, United Kingdom,

In Situ Bioremediation of Contaminated Groundwater

Laurel Phoenix, Green Bay, Wisconsin, Fossil Aquifers Nitish Priyadarshi, Ranchi University, Ranchi, Jharkhand, India,

Geothermal Water, Rock Fracture, Consolidated Water Bearing Rocks, Groundwater Contamination from Runoff, Groundwater Dating with Radiocarbon, Methane in Groundwater, Permeability

S.N Rai, National Geophysical Research Institute, Hyderabad, India,

Artificial Recharge of Unconfined Aquifer

Todd Rasmussen, The University of Georgia, Athens, Georgia, Head, Deep

Soil-Water Movement, Soil Water, Specific Gravity, Tidal Efficiency

Microbial Processes Affecting Monitored Natural Attenuation of Contaminants in the Subsurface

Philip R Rykwalder, University of Texas at San Antonio, San Antonio,

Texas, Vadose Zone Monitoring Techniques

Bahram Saghafian, Soil Conservation and Watershed Management

Research Institute, Tehran, Iran, Qanats: An Ingenious Sustainable

Groundwater Resource System

C Samara, Aristotle University of Thessaloniki, Thessaloniki, Greece,

The Use of Semipermeable Membrane Devices (SPMDs) for Monitoring, Exposure, and Toxicity Assessment

Dibyendu Sarkar, University of Texas at San Antonio, San Antonio,

Texas, Remediation of Contaminated Soils, Genetics of Metal Tolerance

and Accumulation in Higher Plants, Phytoextraction of Zinc and

of Selenium-Laden Soils, Phytoextraction and Phytostabilization:

Technical, Economic and Regulatory Considerations of the Soil-Lead

Issue

J.L Schnoor, (from Phytoremediation: Transformation and Control of

Contaminants, Wiley 2003), Phytoremediation of Methyl Tertiary-Butyl

Ether

Guy W Sewell, National Risk Management Research Laboratory, ORD,

U.S EPA, Ada, Oklahoma (formerly with Dynamac Corporation),

Microbial Processes Affecting Monitored Natural Attenuation of

Contaminants in the Subsurface

Raj Sharma, University of KwaZulu-Natal, Durban, South Africa,

Laminar Flow

Caijun Shi, CJS Technology, Inc., Burlington, Ontario, Canada, High

pH Groundwater—The Effect of The Dissolution of Hardened Cement

Pastes

Naresh Singhal, University of Auckland, Auckland, New Zealand,

Bacteria Role in the Phytoremediation of Heavy Metals, Sub-Surface

Redox Chemistry: A Comparison of Equilibrium and Reaction-Based

Approaches

V.P Singh, Louisiana State University, Baton Rouge, Louisiana, Artificial

Recharge of Unconfined Aquifer

Joseph Skopp, University of Nebraska, Lincoln, Nebraska, Field Capacity

Jeffrey G Skousen, West Virginia University, Morgantown, West

Virginia, Acid Mine Drainage: Sources and Treatment in the United

States

Ricardo Smalling, Environmental Bio-Systems, Inc., Mill Valley,

California, Groundwater and Uranium: Chemical Behavior and

Treatment

James A Smith, University of Virginia, Charlottesville, Virginia, Vapor

Transport in the Unsaturated Zone

Stuart A Smith, Smith-Comeskey GroundWater Science LLC, Upper

Sandusky, Ohio, Well Maintenance, Biofouling in Water Wells, Soil and

Groundwater Geochemistry and Microbiology

Michelle Sneed, U.S Geological Survey, Denver, Colorado,

Earth-quakes—Rattling the Earth’s Plumbing System

Roger Spence, Oak Ridge National Laboratory, Oak Ridge, Tennessee,

High pH Groundwater—The Effect of The Dissolution of Hardened

Cement Pastes

Kenneth F Steele, University of Arkansas, Fayetteville, Arkansas, Soil

and Water Contamination by Heavy Metals

Mark D Steele, MDC Systems, Inc., Berwyn, Pennsylvania, Water Level

Drawdown

P Takis Elefsiniotis, University of Auckland, Auckland, New Zealand,

Bacteria Role in the Phytoremediation of Heavy Metals

Henry Teng, The George Washington University, Washington, DC,

Water/Rocks Interaction

Stephen M Testa, Mokelumne Hill, California, Dating Groundwaters

with Tritium, Brine Deposits

Geoffrey Thyne, Colorado School of Mines, Golden, Colorado, Detecting

Modern Groundwaters with 85 Kr, Geochemical Modeling—Computer

Code Concepts

Fred D Tillman, U.S Environmental Protection Agency, Athens, Georgia,

Vapor Transport in the Unsaturated Zone

David J Tonjes, Cashin Associates PC, Hauppauge, New York,

Ground-water Contamination From Municipal Landfills in the USA

Douglas C Towne, Phoenix, Arizona, Ambient Groundwater Monitoring

Network Strategies and Design

Michael D Trojan, Minnesota Pollution Control Agency, St Paul,

Minnesota, Land Use Impacts on Groundwater Quality, Sensitivity of

Groundwater to Contamination

Kristine Uhlman, University of Arizona, Tucson, Arizona, Recharge in

Desert Regions Around The World

Matthew M Uliana, Texas State University—San Marcos, San Marcos,

Texas, Regional Flow Systems, Hydraulic Head, Storage Coefficient

David B Vance, ARCADIS G&M, Inc., Midland, Texas, Groundwater

Remediation by In Situ Aeration and Volatilization, Source, Mobility, and

Remediation of Metals, Particulate Transport in Groundwater—Bacteria and Colloids, The Environmental Impact of Iron in Groundwater, Groundwater Remediation by Injection and Problem Prevention, Chemi- cal Oxidation Technologies for Groundwater Remediation, Physical Prop- erties of DNAPLs and Groundwater Contamination, Process Limitations

of In Situ Bioremediation of Groundwater, Phytoremediation

Enhance-ment of Natural Attenuation Processes, Groundwater and Arsenic: Chemical Behavior and Treatment, Low Flow Groundwater Purging and Surging, The Role of Heat in Groundwater Systems, Horizontal Wells

in Groundwater Remediation, Groundwater Flow in Heterogenetic iments and Fractured Rock Systems, Limiting Geochemical Factors

Sed-in Remediation UsSed-ing Monitored Natural Attenuation and Enhanced Bioremediation

Keith Villiers, Environmental Bio-Systems, Inc., Mill Valley, California,

Groundwater and Benzene: Chemical Behavior and Treatment

Nikolay Voutchkov, Poseidon Resources Corporation, Stamford,

Con-necticut, Well Design and Construction

Atul N Waghode, Loughborough University, Leicestershire, United

Kingdom, Finite Element Modeling of Coupled Free and Porous Flow

Roger M Waller, U.S Geological Survey,, Ground Water: Wells Lise Walter, Environmental Bio-Systems, Mill Valley, California,

Groundwater and Cobalt: Chemical Behavior and Treatment

J.W Weaver, (from Phytoremediation: Transformation and Control of

Contaminants, Wiley 2003), Hydrologic Feasibility Assessment and

Design in Phytoremediation

Jason J Wen, City of Downey, Downey, California, Treatment for Nitrates

in Groundwater, Treatment of Arsenic, Chromium, and Biofouling in Water Supply Wells

Dennis E Williams, Geoscience Support Services, Claremont, California,

Well TEST, Radial Wells, Well Screens

Eric S Wilson, E L Montgomery & Associates, Inc., Tucson, Arizona,

Safe Yield of an Aquifer, Specific Capacity

S.K Winnike-McMillan, (from Phytoremediation: Transformation and

Control of Contaminants, Wiley 2003), Phytoremediation of Methyl Tertiary-Butyl Ether

Q Zhang, (from Phytoremediation: Transformation and Control of

Contaminants, Wiley 2003), Phytoremediation of Methyl Tertiary-Butyl

Ether

A.I Zouboulis, Aristotle University of Thessaloniki, Thessaloniki, Greece,

Phytoremediation of Lead-Contaminated Soils

ACID MINE DRAINAGE: SOURCES AND

TREATMENT IN THE UNITED STATES

JEFFREYG SKOUSEN

West Virginia University Morgantown, West Virginia

Acid mine drainage (AMD) occurs when metal sulfides

are exposed to oxidizing conditions Leaching of reaction

products into surface waters pollute over 20,000 km of

streams in the United States alone Mining companies

must predict the potential of creating AMD by using

overburden analyses Where a potential exists, special

handling of overburden materials and quick coverage

of acid-producing materials in the backfill should be

practiced The addition of acid-neutralizing materials

can reduce or eliminate AMD problems Placing

acid-producing materials under dry barriers can isolate

these materials from air and water Other AMD

control technologies being researched include injection of

alkaline materials (ashes and limestone) into abandoned

underground mines and into buried acid material in mine

backfills, remining of abandoned areas, and installation of

alkaline recharge trenches Chemicals used for treating

AMD are Ca(OH)2, CaO, NaOH, Na2CO3, and NH3,

with each having advantages under certain conditions

Under low-flow situations, all chemicals except Ca(OH)2

are cost effective, whereas at high flow, Ca(OH)2 and

CaO are clearly the most cost effective Floc, the metal

hydroxide material collected after treatment, is disposed of

in abandoned deep mines, refuse piles, or left in collection

ponds Wetlands remove metals from AMD through

formation of oxyhydroxides and sulfides, exchange and

organic complexation reactions, and direct plant uptake

Aerobic wetlands are used when water contains enough

alkalinity to promote metal precipitation, and anaerobic

wetlands are used when alkalinity must be generated

by microbial sulfate reduction and limestone dissolution

Anoxic limestone drains are buried trenches of limestone

that intercept AMD underground to generate alkalinity

Under anoxia, limestone should not be coated with Fe+3

hydroxides in the drain, which decreases the likelihood of

clogging Vertical flow wetlands pretreat oxygenated AMD

with organic matter to remove oxygen and Fe+3, and then

the water is introduced into limestone underneath the

organic matter Open limestone channels use limestone in

aerobic environments to treat AMD Coating of limestone

occurs, and the reduced limestone dissolution is designed

into the treatment system Alkaline leach beds, containing

either limestone or slag, add alkalinity to acid water At

present, most passive systems offer short-term treatment

and are more practical for installation on abandoned sites

or watershed restoration projects where effluent limits do

not apply and where some removal of acid and metals will

benefit a stream

Acid mine drainage (AMD) forms when sulfide minerals

deep in the earth are exposed during coal and metal

mining, highway construction, and other large-scale

excavations Upon exposure to water and oxygen, sulfide

minerals oxidize to form acidic products, which then can bedissolved in water The water containing these dissolvedproducts often has a low pH, high amounts of dissolvedmetals such as iron (Fe) and aluminum (Al), and sulfate.The metal concentrations in AMD depend on the typeand quantity of sulfide minerals present, and the overallwater quality from disturbed areas depends on theacid-producing (sulfide) and acid-neutralizing (carbonate)minerals contained in the disturbed rock The carbonatecontent of overburden determines whether there is enoughneutralization potential or base to counteract the acidproduced from pyrite oxidation Of the many types of acid-neutralizing compounds present in rocks, only carbonates(and some clays) occur in sufficient quantity to effectivelyneutralize acid-producing rocks A balance between theacid-producing potential and neutralizing capacity of thedisturbed overburden will indicate the ultimate acidity oralkalinity that might be expected in the material uponcomplete weathering

Approximately 20,000 km of streams and rivers in theUnited States are degraded by AMD, but sulfide mineralsoccur throughout the world causing similar problems.About 90% of the AMD reaching streams originates

in abandoned surface and deep mines No company orindividual claims responsibility for reclaiming abandonedmine lands and contaminated water flowing from thesesites is not treated

Control of AMD before land disturbance requires anunderstanding of three important factors: (1) overburdengeochemistry, (2) method and precision of overburdenhandling and placement in the backfill during reclamation,and (3) the postmining hydrology of the site

OVERBURDEN ANALYSES, HANDLING, AND PLACEMENT

Premining analysis of soils and overburden are required bylaw (1) Identifying the acid-producing or acid-neutralizingstatus of rock layers before disturbance aids in developingoverburden handling and placement plans Acid-baseaccounting provides a simple, relatively inexpensive, andconsistent procedure to evaluate overburden chemistry Itbalances potential acidity (based on total or pyritic sulfurcontent) against total neutralizers Samples containingmore acid-producing than acid-neutralizing materialsare ‘‘deficient’’ and can cause AMD, whereas thoserock samples with the reverse situation have ‘‘excess’’neutralizing materials and will not cause AMD Rocklayers with equal proportions of each type of materialshould be subjected to leaching or weathering analyses (2).Kinetic tests such as humidity cells and leach columnsare important because they examine the rate of acid-producing and neutralization reactions This informationfrom kinetic tests can supplement information given byacid-base accounting and help regulators in permittingdecisions (3)

The prevailing approach to control AMD is to keepwater away from pyritic material Once overburdenmaterials have been classified, an overburden handlingand placement plan for the site can be designed

1

Segregating and placing acid-producing materials above

the water table is generally recommended (2,4) Where

alkaline materials overwhelm acid-producing materials,

no special handling is necessary Where acid-producing

materials cannot be neutralized by onsite alkaline

materials, it is necessary to import a sufficient amount to

neutralize the potential acidity or the disturbance activity

may not be allowed

Postmining Hydrology

The hydrology of a backfill and its effect on AMD

are complex Generally, the porosity and hydraulic

conductivity of the materials in a backfill are greater than

those of the consolidated rock overburden that existed

before mining, and changes in flow patterns and rates

should be expected after mining (5) Water does not move

uniformly through the backfill by a consistent wetting

front As water moves into coarse materials in the backfill,

it follows the path of least resistance and continues

downward through voids or conduits until it encounters a

barrier or other compacted layer Therefore, the chemistry

of the water from a backfill will reflect only the rock types

encountered in the water flow path, and not the entire

geochemistry of the total overburden (6)

Diverting surface water above the site to decrease

the amount of water entering the mined area is highly

recommended If it cannot be diverted, incoming water can

be treated with limestone to improve water quality Under

certain conditions, pyritic material can be placed where

it will be rapidly and permanently inundated, thereby

preventing oxidation Inundation is only suggested where

a water table may be reestablished, such as below drainage

deep mines (seeWET COVERS)

CONTROL OF AMD

Acid mine drainage control can be undertaken where AMD

exists or is anticipated Control methods treat the

acid-producing rock directly and stop or retard the production

of acidity Treatment methods add chemicals directly to

acidic water exiting the rock mass Companies disturbing

land in acid-producing areas must often treat AMD, and

they face the prospect of long-term water treatment and

its liabilities and expense Cost-effective methods, which

prevent the formation of AMD at its source, are preferable

Some control methods are most suited for abandoned

mines, and others are only practical on active operations

Other methods can be used in either setting

Land Reclamation

Backfilling (regrading the land back to contour) and

revegetation together are effective methods of reducing

acid loads from disturbed lands (7) Water flow from seeps

can be reduced by diversion and reclamation, and on

some sites where flow may not be reduced, water quality

can change from acid to alkaline by proper handling of

overburden Diverting surface water or channeling surface

waters to control volume, direction, and contact time can

minimize the effects of AMD on receiving streams Surface

diversion involves construction of drainage ditches to move

surface water quickly off the site before infiltration or byproviding impervious channels to convey water across thedisturbed area

Alkaline Amendment to Active Disturbances

Certain alkaline amendments can control AMD fromacid-producing materials (8–11) All alkaline amendmentschemes rely on acid-base accounting or kinetic tests

to identify the required alkalinity for neutralization ofacidic materials Special handling of overburden seeks

to blend acid-producing and acid-neutralizing rocks inthe disturbance/reclamation process to develop a neutralrock mass The pit floor or material under coal isoften rich in pyrite, so isolating it from groundwatermay be necessary by building highwall drains (whichmove incoming groundwater away from the pit floor) orplacing impermeable barriers on the pit floor Acid-formingmaterial can be compacted or capped within the spoil (12)

If insufficient alkalinity is available in the spoil, thenexternal sources of alkalinity must be imported (13,14).Limestone is often the least expensive and most readilyavailable source of alkalinity It has a neutralizationpotential of between 75% and 100%, and it is safeand easy to handle On the other hand, it has

no cementing properties and cannot be used as abarrier Fluidized bed combustion ashes generally haveneutralizing amounts of between 20% and 40%, and theytend to harden into cement after wetting (15) Otherpower-generation ashes, like flue gas desulfurizationproducts and scrubber sludges, may also have significantneutralization potential, which make them suitablealkaline amendment materials (16) Other materials, likekiln dust, produced by lime and cement kilns, or limemuds, grit, and dregs from pulp and paper industriescontain neutralization products (10) Steel slags, whenfresh, have neutralizing amounts from 45% to 90% Slagsare produced by several processes, so care is needed toensure that candidate slags are not prone to leachingmetal ions like Cr, Mn, and Ni Phosphate rock hasbeen used in some studies to control AMD It may reactwith Fe released during pyrite oxidation to form insolublecoatings (17), but phosphate usually costs much more thanother calcium-based amendments and is needed in aboutthe same amounts (18)

Alkaline Recharge Trenches

Alkaline recharge trenches (19) are surface ditches orcells filled with alkaline material, which can minimize oreliminate acid seeps through an alkaline-loading processwith infiltrating water Alkaline recharge trenches wereconstructed on top of an 8-ha, acid-producing coal refusedisposal site, and after 3 years, the drainage watershowed 25% to 90% acidity reductions with 70% to 95%reductions in Fe and sulfate (20) Pumping water intoalkaline trenches greatly accelerates the movement ofalkalinity into the backfill and can cause acid seeps toturn alkaline (21)

Dry Barriers

Dry barriers retard the movement of water and oxygeninto areas containing acid-producing rock These ‘‘water

Surface barriers can achieve substantial reductions in

water flow through piles, but generally they do not control

AMD completely Grouts can separate acid-producing rock

and groundwater Injection of grout barriers or curtains

may significantly reduce the volume of groundwater

moving through backfills Gabr et al (22) found that a

1.5-m-thick grout wall (installed by pumping a mixture of

Class F fly ash and Portland cement grout into vertical

boreholes near the highwall) reduced groundwater inflow

from the highwall to the backfill by 80%, which results

in some seeps drying up and others being substantially

reduced in flow At the Heath Steele Metal Mine in New

Brunswick, a soil cover was designed to exclude oxygen

and water from a tailings pile (23) It consisted of a

10-cm gravel layer for erosion control, 30-10-cm gravel/sand

layer as an evaporation barrier, 60-cm compacted till

(conductivity of 10−6 cm/sec), 30-cm sand, and pyritic

waste rock This barrier excluded 98% of precipitation,

and oxygen concentrations in the waste rock dropped from

20% initially to around 1% At the Upshur Mining Complex

in West Virginia, Meek (12) reported covering a 20-ha spoil

pile with a 39-mil PVC liner, and this treatment reduced

acid loads by 70%

Wet Covers

Disposal of sulfide tailings under a water cover, such

as in a lake or fjord, is another way to prevent acid

generation by excluding oxygen from sulfides Wet covers

also include flooding of aboveground tailings in ponds

Fraser and Robertson (24) studied four freshwater lakes

used for subaqueous tailings disposal and found that

the reactivity of tailings under water was low and that

there were low concentrations of dissolved metals, thereby

allowing biological communities to exist

Alkaline Amendment to Abandoned Mines

Abandoned surface mines comprise huge volumes of

spoil of unknown composition and hydrology Abandoned

underground mines are problematic because they are often

partially caved and flooded, cannot be accessed, and have

unreliable or nonexistent mine maps Re-handling and

mixing alkalinity into an already reclaimed backfill is

generally prohibitively expensive

Filling abandoned underground mine voids with

nonpermeable materials is one of the best methods to

prevent AMD Underground mine voids are extensive (a

60-ha mine with a coal bed height of 1.5 m and a recovery

rate of 65% would contain about 600,000 m3 of voids), so

fill material and the placement method must be cheap

Mixtures of Class F fly ash and 3–5% Portland cement

control subsidence in mined-under residential areas and

these slurries are generally injected through vertical

boreholes at between 8- and 16-m centers Pneumatic

(air pressure) and slurry injection for placing fly ash in

abandoned underground mines can extend the borehole

spacing to about 30 m (25) On reclaimed surface mines

still producing AMD, researchers in Pennsylvania saw

small improvements in water quality after injecting coal

combustion residues into buried pods of pyritic materials

‘‘Remining’’ means returning to abandoned surface orunderground mines for further coal removal Where AMDoccurs, remining reduces acid loads by (1) decreasinginfiltration rates, (2) covering acid-producing materials,and (3) removing the remaining coal, which is the source ofmost of the pyrite Hawkins (26) found contaminant loads

of 57 discharges from remined sites in Pennsylvania to bereduced after remining and reclamation Short-term loadswere sometimes increased during the first six monthsafter remining and reclamation, but reduction in loadsafter six months resulted from decreased flow rather thanlarge changes in concentrations Ten remining sites inPennsylvania and West Virginia were reclaimed to currentstandards (which included eliminating highwalls, coveringrefuse, and revegetating the entire area), and all sites hadimproved water quality (15)

CHEMICAL TREATMENT OF AMD

If AMD problems develop during mining or afterreclamation, a plan to treat the discharge must bedeveloped A water treatment system consists of an inflowpipe or ditch, a storage tank or bin holding the treatmentchemical, a valve to control its application rate, a settlingpond to capture precipitated metal oxyhydroxides, and adischarge point At the discharge point, water samplesare analyzed to monitor whether specified parametersare being attained Water discharge permits (NPDES)

on surface mines usually require monitoring of pH, totalsuspended solids, and Fe and Mn concentrations Thetype and size of a chemical treatment system is based

on flow rate, pH, oxidation status, and concentrations ofmetals in the AMD The receiving stream’s designateduse and seasonal fluctuations in flow rate are alsoimportant After evaluating these variables over a period

of time, the operator can consider the economics ofdifferent chemicals

Six chemicals treat AMD (Table 1) Each is more

or less appropriate for a specific condition The bestchoice depends on both technical (acidity levels, flow, andthe types and concentrations of metals) and economicfactors (chemical prices, labor, machinery and equipment,treatment duration, and interest rates) Enough alkalinitymust be added to raise pH to between 6 and 9 soinsoluble metal hydroxides will form and settle out.Treatment of AMD with high Fe (ferric) concentrationsoften affords coprecipitation of other metals with the Fehydroxide, thereby removing them from AMD at a lower

pH Limestone has been used for decades to raise pH andprecipitate metals in AMD It has the lowest materialcost and is the safest and easiest to handle of the AMDchemicals Unfortunately, it is limited because of its lowsolubility and tendency to develop an external coating,

or armor, of Fe(OH)3 when added to AMD Fine-groundlimestone may be dumped in streams directly or thelimestone may be pulverized by water-powered rotatingdrums and metered into the stream Limestone has alsotreated AMD in anaerobic (anoxic limestone drains) andaerobic environments (open limestone channels)

Table 1 Chemical Compounds Used in AMD Treatment

2000 Costc

$ per Mg or L

Conversion Factora

Neutralization

aThe conversion factor may be multiplied by the estimated milligrams acid/yr to get milligrams of chemical needed for neutralization per year For liquid caustic, the conversion factor gives liters needed for neutralization.

bNeutralization efficiency estimates the relative effectiveness of the chemical in neutralizing AMD acidity For example, if 100 Mg of acid/yr was the amount

of acid to be neutralized, then it can be estimated that 82 Mg of hydrated lime would be needed to neutralize the acidity in the water (100(0.74)/0.90).

c Price of chemical depends on the quantity being delivered Bulk means delivery of chemical in a large truck, whereas < Bulk means purchased in small

quantities Liquid caustic prices are for liters Others in milligrams.

Lime

Hydrated lime is common for treating AMD As a powder,

it tends to be hydrophobic, and extensive mechanical

mixing is required for dissolution Hydrated lime is

particularly useful and cost effective in large-flow,

high-acidity situations where a lime treatment plant with a

mixer/aerator is constructed to help dispense and mix the

chemical with the water (27) Hydrated lime has limited

effectiveness if a very high pH (>9) is required to remove

ions such as Mn Unfortunately, increasing the lime rate

increases the volume of unreacted lime that enters the

floc-settling pond

Pebble quicklime (CaO) is used with the Aquafix Water

Treatment System using a water wheel concept (28) A

water wheel is turned based on water flow, which causes

a screw feeder to dispense the chemical This system was

initially used for small and/or periodic flows of high acidity

because CaO is very reactive, but water wheels have been

attached to large silos for high-flow/high-acidity situations

Tests show an average of 75% cost savings over NaOH

systems and about 20% to 40% savings over NH3systems

Soda Ash

Soda ash (Na2CO3) generally treats AMD in remote areas

with low flow and low amounts of acidity and metals

This choice is usually based on convenience rather than

on chemical cost Soda ash comes as solid briquettes and

is gravity fed into water through bins The number of

briquettes used per day is determined by the rate of

flow and quality of the water One problem is that the

briquettes absorb moisture, expand, and stick to the

corners of the bin and will not drop into the stream

For short-term treatment, some operators use a much

simpler system that employs a wooden box or barrel with

holes that allows water inflow and outflow The operator

simply fills the barrel with briquettes on a regular basis

and places the barrel in the flowing water This system

offers less control of the amount of chemical used

Caustic Soda

Caustic soda (i.e., lye, NaOH) is often used in remote

low-flow, high-acidity situations, or if Mn concentrations

in the AMD are high The system can be gravity fed bydripping liquid NaOH directly into the AMD Caustic isvery soluble, disperses rapidly, and raises the pH quickly.Caustic should be applied at the surface of ponds becausethe chemical is denser than water The major drawbacks

of using liquid NaOH for AMD treatment are high costand dangers in handling

Ammonia

Ammonia compounds (NH3 or NH4OH) are extremelyhazardous NH3is compressed and stored as a liquid butreturns to the gaseous state when released Ammonia isextremely soluble, reacts rapidly, and can raise the pH ofreceiving water to 9.2 At pH 9.2, it buffers the solution tofurther pH increases, and therefore very high amounts of

NH3must be added to go beyond 9.2 Injection of NH3intoAMD is one of the quickest ways to raise water pH, and

it should be injected near the bottom of the pond or waterinlet because NH3is less dense than water NH3is cheap,and a cost reduction of 50% to 70% is usually realized when

NH3 is substituted for NaOH (29) Major disadvantages

of using NH3 include (1) the hazards; (2) uncertaintyconcerning nitrification, denitrification, and acidificationdownstream; and (3) consequences of excessive applicationrates, which cause toxic conditions to aquatic life

Costs of Treating AMD

Costs were estimated for five treatment chemicals underfour sets of flow and acid concentration conditions [Table 1from Skousen et al (30)] Na2CO3 had the highest laborrequirements (10 hours per week) because the dispensersmust be filled by hand and inspected frequently Caustichad the highest reagent cost per mole of acid-neutralizingcapacity, and Na2CO3 had the second highest Hydratedlime treatment systems had the highest installation costs

of the five chemicals because of the need to construct alime treatment plant and install a pond aerator However,the cost of Ca(OH)2was very low, and the combination ofhigh installation costs and low reagent cost made Ca(OH)2systems particularly appropriate for long-term treatment

of high-flow/high-acidity conditions

had about the same cost as the NH3 system, but slightly

higher installation costs Caustic was third because of

its high labor and reagent costs, and Na2CO3was fourth

because of high labor costs Hydrated lime was the most

expensive because of its high installation costs At

high-flow/high-acidity, the Ca(OH)2 and CaO systems were

clearly the cheapest treatment systems (annual costs of

about $250,000 less than NH3, the next best alternative)

After chemical treatment, the treated water flows

into sedimentation ponds so metals in the water can

precipitate All AMD treatment chemicals cause the

formation of metal hydroxide sludge or floc Sufficient

residence time of the water (dictated by pond size and

depth) is important for adequate metal precipitation The

amount of metal floc generated depends on water quality

and quantity, which in turn determines how often the

ponds must be cleaned Knowing the chemical and AMD

being treated will provide an estimate of the stability

of metal compounds in the floc Floc disposal options

include (1) leaving it submerged indefinitely, (2) pumping

or hauling it to abandoned deep mines or to pits dug

on surface mines, and (3) dumping it into refuse piles

Pumping flocs onto land and letting them age and dry is a

good strategy for disposal, because they become crystalline

and behave like soil material

Each AMD is unique, requiring site-specific treatment

Each AMD source should be tested with various chemicals

by titration tests to evaluate the most effective chemical

for precipitation of the metals The costs of each AMD

treatment system based on neutralization (in terms of the

reagent cost, capital investment, and maintenance of the

dispensing system) and floc disposal should be evaluated

to determine the most cost-effective system

PASSIVE TREATMENT OF AMD

Active chemical treatment of AMD is often an expensive,

long-term proposition Passive treatment systems have

been developed that do not require continuous chemical

inputs and that take advantage of natural chemical and

biological processes to cleanse contaminated mine waters

Passive technologies include constructed wetlands, anoxic

limestone drains, vertical flow wetlands (also known

as SAPS), open limestone channels, and alkaline leach

beds (Fig 1) In low-flow and low-acidity situations,

passive systems can be reliably implemented as a single

permanent solution for many AMD problems

Constructed Wetlands

Wetlands are of two basic types: aerobic and anaerobic

Metals are retained within wetlands by (1) formation of

metal oxides and oxyhydroxides, (2) formation of metal

sulfides, (3) organic complexation reactions, (4) exchange

with other cations on negatively charged sites, and

(5) direct uptake by living plants Other beneficial

reactions in wetlands include generation of alkalinity

caused by microbial mineralization of dead organic matter,

microbial dissimilatory reduction of Fe oxyhydroxides and

SO , and dissolution of carbonates

promote metal oxidation and hydrolysis, thereby causingprecipitation and physical retention of Fe, Al, and Mnoxyhydroxides Successful metal removal depends ondissolved metal concentrations, dissolved oxygen content,

pH and net acidity of the mine water, the presence ofactive microbial biomass, and detention time of the water

in the wetland The pH and net acidity/alkalinity of thewater are particularly important because pH influencesboth the solubility of metal hydroxide precipitatesand the kinetics of metal oxidation and hydrolysis.Therefore, aerobic wetlands are best used in conjunctionwith water that contains net alkalinity to neutralizemetal acidity

Anaerobic wetlands consisting of deep ponds (>30 cm)

with substrates of soil, peat moss, spent mushroomcompost, sawdust, straw/manure, hay bales, or otherorganic mixtures, often underlain or admixed withlimestone Anaerobic wetlands are most successful whenused to treat small flows of acidic water Anaerobicwetlands use chemical and microbial reduction reactions

to precipitate metals and neutralize acidity The waterinfiltrates through a thick permeable organic subsurfacethat becomes anaerobic because of high biological oxygen

demand Other chemical mechanisms that occur in situ

include metal exchanges, formation and precipitation

of metal sulfides, microbial-generated alkalinity, andformation of carbonate alkalinity (because of limestonedissolution) As anaerobic wetlands produce alkalinity,they can be used in net acidic and high dissolved oxygen

(>2 mg/L) AMD Microbial mechanisms of alkalinity

production are critical to long-term AMD treatment

Under high acid loads (>300 mg/L), pH-sensitive microbial

activities are eventually overwhelmed At present, thesizing value for Fe removal in these wetlands is 10 gs perday per meter squared (31)

Sorption onto organic materials (such as peat andsawdust) can initially remove 50% to 80% of the metals

in AMD (32), but the exchange capacity declines withtime Over the long term, metal hydroxide precipitation

is the predominant form of metal retention in a wetland.Wieder (33) reported up to 70% of the Fe in a wetland to becomposed of Fe+3oxyhydroxides, whereas the other 30%

is reduced and combined with sulfides (34)

Sulfate reducing bacteria (SRB) reactors have beenused to generate alkalinity by optimizing anaerobicconditions Good success has been noted for severalsystems receiving high and low flows (35,36)

Anoxic Limestone Drains

Anoxic limestone drains are buried cells or trenches oflimestone into which anoxic water is introduced Thelimestone raises pH and adds alkalinity Under anoxicconditions, the limestone does not coat or armor with Fehydroxides because Fe+2does not precipitate as Fe(OH)2at

pH 6.0 Faulkner and Skousen (37) reported both successesand failures among 11 anoxic drains in WV Failuresresulted when ferric iron and Al precipitate as hydroxides

in the limestone causing plugging and coating

Figure 1 Diagram of possible passive

treatment systems to treat mine water

based on water flow and chemistry.

Determine flow rateanalyze water chemistrycalculate loading

Net acidic waterNet alkaline water

Determine do,ferric iron, al

Do < 1 mg/L andferric < 1 mg/L and

al < 1 mg/L

Settlingpond

Netalkalinewater

Anoxiclimestonedrain

Netacidwater

Settlingpond

Sulfatereducingbioreactor

Anaerobicwetland

limestonechannel

Slag or lsleach bed

Settlingpond

Settlingpond

Settlingpond

Settlingpond

Settlingpond

Aerobicwetland

Meet effluentstandards?

Meet effluentstandards?

Re-evaluatedesign

al > 1 mg/L

Longevity of treatment is a major concern for anoxic

drains, especially in terms of water flow through the

lime-stone Selection of the appropriate water and

environmen-tal conditions is critical for long-term alkalinity generation

in an anoxic drain Eventual clogging of the limestone pore

spaces with precipitated Al and Fe hydroxides, and

gyp-sum is predicted (38) For optimum performance, no Fe+3,

dissolved oxygen, or Al should be present in the AMD

Like wetlands, anoxic limestone drains may be a solution

for AMD treatment for specific water conditions or for a

finite period after which the system must be replenished

or replaced

Vertical Flow Wetlands

In these modified wetlands [called SAPS by Kepler and

McCleary (39)], 1 to 3 m of acid water is ponded over

an organic compost of 0.2 to 0.3 m, underlain by 0.5 to

1 m of limestone Below the limestone are drainage pipes

that convey the water into an aerobic pond where metalsare precipitated The hydraulic head drives ponded waterthrough the anaerobic organic compost, where oxygenstripping as well as Fe and sulfate reduction can occurbefore water entry into the limestone Water with highmetal loads can be successively cycled through additionalwetlands Iron and Al clogging of limestone and pipes can

be removed by flushing the system (40) Much work isbeing done on these wetlands presently, and refinementsare being made for better water treatment

Open Limestone Channels

Open limestone channels are another means of introducingalkalinity to acid water (41) We usually assume thatarmored limestone ceases to dissolve, but Ziemkiewicz

et al (42) found armored limestone to be 50% to 90%effective in neutralizing acid compared with unarmoredlimestone Seven open channels in the field reduced acidity

promise for neutralizing AMD in watershed restoration

projects and AML reclamation projects where there can be

only a one-time installation cost, little to no maintenance

is required, and water exiting the system does not have to

meet water quality standards Long channels of limestone

can convey acid water to a stream or other discharge point

Cross sections of channels can be designed with calculated

amounts of limestone (which will become armored) to treat

the water Open limestone channels work best on steep

slopes (>20%), where flow velocities keep metal hydroxides

in suspension, thereby limiting plugging If constructed

correctly, open limestone channels should be maintenance

free and provide AMD treatment for decades

Alkaline Leach Beds

Limestone, when placed in an open pond or leach

bed, will dissolve slowly over time and continually add

alkalinity to water unless the limestone gets coated

with metal hydroxides, thereby reducing its dissolution

rate (41) Therefore, limestone treatment in aerobic

systems works best in low-pH, metal-free water, and

can add alkalinity to streams before encountering acid

water downstream (42) As limestone generally reacts

relatively slowly under field conditions, steel slag, a

byproduct of steel making and composed of hydrated

amorphous silica and calcium compounds, can be used as

an alkaline material to add alkalinity to water Steel slags

have high neutralization potentials (from about 50–70%),

can generate exceptionally high levels of alkalinity in

water, and do not armor (43) Steel slag fines can be

used in leach beds Effluents from slag leach beds attain

high pH (>10) and have alkalinity concentrations in the

thousands of milligrams/liter Slag leach beds may receive

AMD directly, or effluent from ‘‘fresh water’’ beds may

be combined with an AMD source downstream to treat

acid indirectly

SUMMARY

Acid mine drainage occurs when metal sulfides are

oxidized Leaching of reaction products into surface waters

pollute over 20,000 km of streams in the United States

alone Companies must predict AMD before mining by

using overburden analyses On sites where a potential

exists, special handling of overburden materials and quick

coverage of acid-producing materials in the backfill should

be practiced Alkaline addition with materials such as kiln

dust and FBC ash can reduce or completely eliminate

AMD problems Other control techniques include dry

barriers, wet barriers, injection of alkaline materials

into underground mines, remining of abandoned areas,

and alkaline recharge trenches Five chemicals typically

treat AMD, and each has characteristics that make it

suitable for specific applications Companies must select

a chemical that treats the water adequately and

cost-effectively Passive systems are low maintenance systems

that are implemented on abandoned mine land and stream

restoration projects Certain systems are more suited to

BIBLIOGRAPHY

1 Sobek, A., Skousen, J., and Fisher, S (2000) Chemical and physical properties of overburdens and minesoils In:

Reclamation of Drastically Disturbed Lands, 2nd Edn.

American Society of Agronomy, Madison, WI.

2 Skousen, J.G., Sencindiver, J.C., and Smith, R.M (1987) A Review of Procedures for Surface Mining and Reclamation

in Areas with Acid-Producing Materials EWRC 871, West

Virginia University, Morgantown, WV.

3 Geidel, G., Caruccio, F.T., Hornberger, R., and Brady, K (2000) Guidelines and recommendations for use of kinetic tests for coal mining (AMD) prediction in the eastern U.S In:

Prediction of Water Quality at Surface Coal Mines National

Mine Land Reclamation Center, West Virginia University, Morgantown, WV.

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Hellier, W (1998) Handbook of Technologies for Avoidance and Remediation of Acid Mine Drainage National Mine Land

Reclamation Center, West Virginia University, Morgantown, WV.

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really beneficial? In: 1989 Multinational Conference on Mine Planning and Design 16–17 Sept 1989, University of

Kentucky, Lexington, KY.

6 Ziemkiewicz, P.F and Skousen, J.G (1992) Prevention of

acid mine drainage by alkaline addition Green Lands 22(2):

42–51.

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12 Meek, F.A (1994) Evaluation of acid prevention techniques

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13 Skousen, J and Larew, G (1994) Alkaline overburden addition to acid-producing materials to prevent acid mine

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14 Wiram, V.P and Naumann, H.E (1995) Alkaline additions

to the backfill: A key mining/reclamation component to acid

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15 Skousen, J., Bhumbla, D., Gorman, J., and Sencindiver, J.

(1997) Hydraulic conductivity of ash mixtures and metal

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16 Stehouwer, R., Sutton, P., Fowler, R., and Dick, W (1995).

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17 Evangelou, V.P (1995) Pyrite Oxidation and its Control CRC

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18 Ziemkiewicz, P.F and Meek, F.A (1994) Long term behavior

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19 Caruccio, F.T., Geidel, G., and Williams, R (1984) Induced

alkaline recharge zones to mitigate acid seeps In:

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Sedimentology and Reclamation 7–10 Dec 1984, Univ of

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20 Nawrot, J.R., Conley, P.S., and Sandusky, J.E (1994)

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23 Bell, A.V., Riley, M.D., and Yanful, E.G (1994) Evaluation

of a composite soil cover to control acid waste rock pile

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24 Fraser, W.W and Robertson, J.D (1994) Subaqueous

dis-posal of reactive mine waste: an overview and update of case

studies-MEND/Canada In: International Land Reclamation

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25 Burnett, J.M., Burnett, M., Ziemkiewicz, P., and Black, D.C.

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26 Hawkins, J.W (1994) Assessment of contaminant load

changes caused by remining abandoned coal mines In:

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Pitts-27 Skousen, J and Ziemkiewicz, P (1996) Acid Mine Drainage Control and Treatment, 2nd Edn National Research Center

for Coal and Energy, National Mine Land Reclamation Center, West Virginia University, Morgantown, WV.

28 Jenkins, M and Skousen, J (2001) Acid mine drainage treatment costs with calcium oxide and the Aquafix Machine.

Green Lands 31: 46–51.

29 Skousen, J., Politan, K., Hilton, T., and Meek, A (1990) Acid

mine drainage treatment systems: chemicals and costs Green

Lands 20(4): 31–37.

30 Skousen, J.G., Sexstone, A., and Ziemkiewicz, P (2000) Acid

mine drainage control and treatment In: Reclamation of Drastically Disturbed Lands, 2nd Edn American Society of

Agronomy, Madison, WI.

31 Hedin, R.S and Nairn, R.W (1992) Passive treatment of coal

mine drainage Course Notes for Workshop U.S Bureau of

Mines, Pittsburgh, PA.

32 Brodie, G.A., Hammer, D.A., and Tomljanovich, D.A (1988).

An evaluation of substrate types in constructed wetlands acid

drainage treatment systems In: Mine Drainage and Surface Mine Reclamation 19–21 April 1988, Vol 1, Info Circular

9183, U.S Bureau of Mines, Pittsburgh, PA.

33 Wieder, R.K (1993) Ion input/output budgets for wetlands

constructed for acid coal mine drainage treatment Water,

Air, and Soil Pollution 71: 231–270.

34 Wieder, R.K (1992) The Kentucky wetlands project: A field study to evaluate man-made wetlands for acid coal mine drainage treatment Final Report to the U.S Office of Surface Mining, Villanova Univ., Villanova, PA.

35 Canty, M (2000) Innovative in situ treatment of acid mine drainage using sulfate-reducing bacteria In: Proceedings, Fifth International Conference on Acid Rock Drainage Society

for Mining, Metallurgy, and Exploration, Inc., Denver, CO.

36 Gusek, J., Mann, C., Wildeman, T., and Murphy, D (2000) Operational results of a 1200 gpm passive bioreactor for metal

mine drainage, Missouri In: Proceedings, Fifth International Conference on Acid Rock Drainage Society for Mining,

Metallurgy, and Exploration, Inc., Denver, CO.

37 Faulkner, B.B and Skousen, J.G (1994) Treatment of Acid Mine Drainage by Passive Treatment Systems In: Interna- tional Land Reclamation and Mine Drainage Conference.

24–29 April 1994, USDI, Bureau of Mines SP 06A-94, burgh, PA.

Pitts-38 Nairn, R.W., Hedin, R.S., and Watzlaf, G.R (1991) A liminary review of the use of anoxic limestone drains in the

pre-passive treatment of acid mine drainage In: Proceedings, Twelfth Annual West Virginia Surface Mine Drainage Task Force Symposium 3–4 April 1991, West Virginia University,

Morgantown, WV.

39 Kepler, D.A and McCleary, E (1997) Passive aluminum

treatment successes In: Proceedings, Eighteenth Annual West Virginia Surface Mine Drainage Task Force Symposium.

15–16 April 1997, West Virginia University, Morgantown, WV.

40 Kepler, D.A and McCleary, E (1994) Successive producing systems (SAPS) for the treatment of acidic mine

alkalinity-drainage In: International Land Reclamation and Mine Drainage Conference 24–29 April 1994, USDI, Bureau of

Mines SP 06A-94, Pittsburgh, PA.

41 Ziemkiewicz, P.F., Skousen, J., and Lovett, R (1994) Open limestone channels for treating acid mine drainage: a new

look at an old idea Green Lands 24(4): 36–41.

Environ Qual 26: 718–726.

43 Ziemkiewicz, P.F., Skousen, J., and Simmons, J (2001).

Cost benefit analysis of passive treatment systems In:

Proceedings, 22nd West Virginia Surface Mine Drainage Task

Force Symposium 3–4 April, Morgantown, WV.

44 Ziemkiewicz, P.F and Skousen, J (1998) The use of steel

slag in acid mine drainage treatment and control In:

Proceedings, 19th West Virginia Surface Mine Drainage Task

Force Symposium 7–8 April, Morgantown, WV.

READING LIST

Rich, D.H and Hutchison, K.R (1990) Neutralization and

stabilization of combined refuse using lime kiln dust at

High Power Mountain In: Proceedings, 1990 Mining and

Reclamation Conference 23–26 April 1990, West Virginia

University, Morgantown, WV.

Skousen, J.G., Hedin, R., and Faulkner, B.B (1997) Water

quality changes and costs of remining in Pennsylvania and

West Virginia In: 1997 National Meeting of the American

Society for Surface Mining and Reclamation 10–15 May 1997,

Austin, TX.

AQUIFERS

THOMASHARTER

University of California Davis, California

GENERAL DEFINITION

An aquifer is a geologic formation or geologic unit

from which significant amounts of groundwater can

be pumped for domestic, municipal, or agricultural

uses The four major types of rock formations that

serve as aquifers are unconsolidated sand and gravel,

sandstone, carbonate rocks, and fractured volcanic rocks

Aquifers may also occur in other geologic formations,

particularly in fractured zones of igneous, metamorphic,

or sedimentary rocks

ORIGIN OF THE WORD

The word aquifer was probably adopted around the early

twentieth century from the French word aquif`ere, which

originates from the two Latin words aqua (water) and ferre

(to carry, to bear) Hence, literally translated from Latin,

aquifer means ‘that which carries water.’

FURTHER DEFINITIONS

There is no strict definition of the hydrogeologic attributes

or volumetric extent necessary to make a geologic

forma-tion or geologic unit an aquifer Rather, the term aquifer

is used for local formations that have relatively higher

permeability than surrounding formations Geologic units

that form an aquifer in one setting may therefore not be

sists of very shallow, sandy loam deposits a few feet thickmay supply enough water to maintain a pumping rate

of 0.5–2 gallons per minute, enough for domestic watersupply wells and some stock supply wells This may besignificantly more water than would be provided by thehardrock formations underlying and bounding such a shal-low aquifer In contrast, the same sandy loam deposits ofthe same thickness would not be considered an aquifer ifthey were part of an unconsolidated sedimentary sequence

in a larger alluvial basin, where gravel and sand aquifersyield from 50 to more than 1,000 gallons per minute

ROLE OF AN AQUIFER IN THE HYDROLOGIC CYCLE

Aquifers are part of the hydrologic cycle They receivewater through

• recharge from precipitation,

• recharge from irrigation return water,

• seepage from rivers and streams,

• lateral transfer of water from neighboring aquiferbasins, and

• leakage from aquifer formations situated either above

or below the aquifer

Water that collects in aquifers from those sources overperiods of years, decades, centuries, and even millennia isdischarged back to the surface through (Fig 1)

• springs,

• subsurface discharge into rivers and streams,

• lateral outflow to downgradient aquifers,

• vertical leakage to overlying or underlying aquifers,and

AQUIFER SIZE

Aquifers can be vastly different in size: a small localaquifer in a mountainous setting may be only a few feetthick and extend over an area of a few acres to tens ofacres Other aquifers span entire regions For example,the Ogallala aquifer in the western-central United Statesunderlies most of the High Plains region, which extends

Artesianwell

Piezometric surface

Stream

Water table

Unconfined aquiferAquitard

gw flow

Confined aquifer

UnconfinedaquiferImpermeablehardrock(aquitard)

Figure 1 Schematic representation of uncontinued aquifers, confined aquifers, aquitards, and

aquicludes Blade vertical arrows indicate recharge Black horizontal arrows indicate pumping.

Light colored arrows indicate the direction of groundwater movement.

eastward from the Rocky Mountains through parts of

Texas, Oklahoma, Colorado, Kansas, and Nebraska The

aquifer consists of alluvial sediments, predominantly

sands and gravel It is an important production aquifer

An aquifer is characterized by its geologic extent

(regional extent and thickness), the type of geologic

formations that makes up the aquifer, the hydraulic

conductivity, the transmissivity (which is defined as the

product of hydraulic conductivity and aquifer thickness),

the specific yield (the drainable porosity), the specific

storage (the amount of water and rock compressed by

hydrostatic pressure in a confined aquifer, see below),

and the specific capacity (specific capacity is the amount of

water pumped from a well per foot of water level drawdown

created by pumping) The hydraulic conductivity of

aquifers typically ranges from 1 m/day to more than

100 m/day The specific capacity of wells located in aquifers

may range from less than 0.1 gpm/ft (small, low-yielding

aquifers suitable for domestic water supplies) to more

than 100 gpm/ft (large production aquifers suitable for

municipal and irrigation pumping)

AQUIFER CHARACTERIZATION

The amount of water that can be pumped from an aquifer

depends primarily on four parameters: the hydraulic

conductivity (also called the permeability) of the aquifer,

the thickness of the aquifer, the specific yield or specific

storage of the aquifer (related mostly to its porosity),

and the amount of competition for water between wells

All four of these may change from location to location

The amount of pumping wells and the rate at which

wells are pumping may be different from area to area;

the thickness of the aquifer naturally changes with

the thickness of the geologic formation With respect

to hydraulic conductivity, porosity, and specific yield orspecific storage, hydrogeologists have found that smallvariations that occur in the geologic composition of aquiferformations often result in large localized changes inhydraulic conductivity This latter phenomenon is referred

to as ‘‘natural aquifer heterogeneity.’’ As a result of allthis variability, each well within the same aquifer willhave a different specific capacity Sometimes, the specificcapacity of wells can vary quite significantly from well

to well, especially in fractured rock aquifers, but also inunconsolidated aquifers with sand and gravel

Hydraulic conductivity, thickness, and specific yield orspecific storage of an aquifer are determined indirectly

by using literature values available for specific geologicformations, by using computer models in conjunction withlocal observations of groundwater fluxes or groundwatertable fluctuations, or directly by performing an aquifertest (pumping test)

AQUIFERS, AQUITARDS, AND AQUICLUDES

Aquifers are the major hydrogeologic units withinthe hydrogeologic framework of a region from whichgroundwater is or can be extracted The description

of local or regional hydrogeology centers around thedescription of aquifers, that is, of those geologic formationswith the highest significance—locally or regionally—withrespect to (potential) groundwater production Geologicformations that bound aquifers are referred to asaquicludes or aquitards Aquicludes are, for all practicalpurposes, impermeable Important aquicludes are thick,continuous clay formations and unfractured igneous rocks.Aquitards are geologic formations that have a lower

flow between overlying aquifers Aquitards can consist

of material similar to aquifers, but either the amount

of fine sediments is much larger (in unconsolidated

formations) relative to the aquifer formation, or the degree

of fracturing and size of fractures is smaller than that in

the aquifer formation (in hardrock formations)

CONFINED AND UNCONFINED AQUIFERS

Aquifers can be either unconfined or confined, depending

on the existence of an overlying aquitard or aquiclude In

an unconfined aquifer, there is no overlying aquitard or

aquiclude Recharge to the aquifer from the land surface

or from and to streams is not restricted The water table

moves freely up and down, depending on the water stored,

added to, or removed from the unconfined aquifer The

water level in a borehole drilled into an unconfined aquifer

will be the same as the water level in the aquifer (if we

ignore the effects of the capillary fringe)

In a confined aquifer, on the other hand, water

is ‘‘sandwiched’’ between two aquitards or between an

aquitard and an aquiclude above and below the aquifer

Water in a confined aquifer is under hydrostatic pressure

created by the weight of the overlying geologic formations

and the water pressure created by the higher water levels

in the usually remote recharge area of a confined aquifer

Due to the pressure in a confined aquifer, the water level

in a borehole drilled into a confined aquifer will rise

significantly above the top of the aquifer An artesian

well occurs where the pressure is so large that the water

level in a well drilled into the confined aquifer rises above

the land surface A confined aquifer does not have a

water table—it is completely filled with groundwater

The water level in wells drilled into a confined aquifer,

instead, corresponds to the hydrostatic pressure head or

potentiometric surface of the aquifer, which is located

higher than the upper boundary of the aquifer itself If

the hydrostatic pressure head falls below the top of the

confined aquifer, it becomes unconfined

An aquifer that is confined by an aquitard rather

than an aquiclude is referred to as a ‘‘leaky aquifer’’

or a ‘‘semiconfined aquifer.’’ The aquitard is not always

a contiguous layer of less permeable material Local

accumulations of multiple, smaller clay lenses and other

clay-rich or otherwise impermeable layers dispersed

within a more permeable formation may render the

entire formation an aquitard The actual low permeable

lenses are not contiguous, but the overall effect of their

presence within such a heterogeneous formation on the

regional aquifer below is identical to that of a continuous

aquitard formation

PERCHED WATER TABLE

Occasionally, water collects above an impermeable or

low permeability layer within the unsaturated (aerated)

zone and forms a ‘‘perched’’ water table By definition, a

‘‘perched’’ water table is a saturated groundwater zone

an unconfined shallow aquifer that is separated from adeeper confined aquifer through thick but saturated layers

Louisiana State University Baton Rouge, Louisiana

INTRODUCTION

Groundwater plays a major role in augmenting watersupply to meet the ever-increasing domestic, agricultural,and industrial demands Increasing dependence of watersupply on groundwater resources is resulting in increasinguse of aquifers as a source of fresh water supply andsubsurface reservoir for storing excess surface water.Aquifers are the geological formations that can store water

as well as allow the flow of significant amount of waterthrough their pores under ordinary field conditions If theaquifer is bounded by two impermeable formations fromtop and bottom, it is called a confined aquifer If the upperboundary of the aquifer is the water table, it is called anunconfined aquifer The advantage of unconfined aquifersover confined aquifers to serve as a subsurface reservoir

is that the storage of groundwater in large quantity ispossible only in unconfined aquifer, which is becausethe storativity of the unconfined aquifer is linked to theporosity and not to the elastic properties of the water andsolid matrix, as in case of the confined aquifer (1) Also, thevast surface area of the unconfined aquifer above the watertable is available to receive the surface applied recharge,whereas in case of the confined aquifer, only a small openarea exposed near to the ground surface or leaky portion ofthe aquifer boundary is available to receive the recharge(Fig 1) This article deals with the artificial recharging ofunconfined aquifer and related problems

Natural replenishment of aquifers occurs very slowly.Therefore, withdrawal of groundwater at a rate greaterthan the natural replenishment rate causes declining ofgroundwater level, which may lead to decreased watersupply, contamination of fresh water by intrusion ofpollutant water from nearby sources, seawater intrusioninto the aquifer of coastal areas, etc To increase thenatural replenishment, artificial recharging of the aquifer

is becoming increasingly important in groundwatermanagement The artificial recharge may be defined as

an augmentation of surface water into aquifers by someartificially planned operations The source of water forrecharge may be direct precipitation, imported water, orreclaimed wastewater The purpose of artificial recharging

Water table Unconfined aquifer

Leakage Confined aquifer

Figure 1 Aquifer types.

of groundwater systems has been to reduce, stop, or

even reverse the declining trend of groundwater level;

to protect fresh groundwater in coastal aquifers against

saline water intrusion from the ocean; and store surface

water, including flood or other surplus water, imported

water, and reclaimed wastewater for future use

RECHARGE METHODS

A variety of direct surface, direct subsurface, and indirect

recharge techniques have been developed to recharge

groundwater systems The choice of a technique depends

on the source of water, quality of the water, the

type of aquifer, topographical condition, etc The most

widely practiced methods are direct surface techniques,

which include surface flooding in basins, ponds, lakes,

ditches, trenches, and furrow systems; stream and channel

modification; and bunds (2–5) Trenches are constructed

mostly in foothill regions to arrest the runoff and put it

into the aquifers for storage Stream channel modification

involves alteration in the course of stream flow to detain

stream flow and increasing the stream bed area for

recharging purposes Construction of check dams across

the stream flow is one technique of stream channel

modification It enhances artificial recharge in two ways

Above the dam, impoundments enhance recharge by

increasing the recharge area and detaining water for a

longer period by reducing the rate of water flow Below the

dam, recharge is enhanced through exposure of a larger

area than the original area of stream channel flow Bunds,

which are small earthen barriers, are constructed in

agricultural lands with slopes to facilitate impounding of

runoff for a longer duration, thereby increasing recharge

In indirect subsurface recharge techniques, water is

injected directly into an aquifer through (a) natural

openings in the aquifers, (b) pits or shafts, and (c) wells

In contrast to the direct surface techniques, groundwater

recharge by indirect subsurface techniques is practiced

mostly for recharging the confined aquifer and where the

topography or existing land use, such as in urban areas,

makes recharge by surface flooding impractical Indirect

recharge techniques involve special cases in which potable

water supply is provided by river bank or sand dune

filtration of generally polluted river water (6,7)

In many cases, excess recharging leads to the growth

of water table near the ground surface and causes several

types of environmental problems, such as water logging,soil salinity, etc In these situations, proper management

of groundwater resources is needed to overcome theshortage of water supply on one hand and to preventthe environmental problems on the other hand In order

to address the management problem, one must be able topredict the response of the aquifer system to any proposedoperational policy of groundwater resources developmentsuch as artificial recharging Such problems are referred

to as forecasting problems Their solution will provide thenew state of the groundwater system Once the new state

is known, one can check whether the related rechargescheme is feasible Then one can compare responses ofdifferent proposed recharge schemes in order to selectthe best scheme that can meet the preset objectives ofgroundwater resources development without disturbingthe regional water balance and without creating any kind

of environmental problems The forecasting problems areeffectively tackled by application of modeling techniques

A model is the simplified representation of a complex realphysical system and the processes taking place in it It can

be physical (for example, a laboratory sand pack model),electrical analog, or mathematical Development andapplications of mathematical models are much easier thanthe other two types of models Therefore, mathematicalmodels are mostly in use today for solving groundwatermanagement problems

MATHEMATICAL MODELING

Modeling begins with a conceptual understanding of thephysical problem (in this case, groundwater flow in theunconfined aquifer) The next step is translating thephysical problem into a mathematical framework resulting

in equation forms that describe the groundwater flow.Mathematical models may be deterministic, statistical,

or some combination of the two Deterministic modelsretain a good measure of physical insight while permittingany number of problems of the same class to be tackledwith the same model Here, discussion will be confined todeterministic models

Formulations of groundwater flow equations are based

on the conservation principles dealing with mass andmomentum These principles require that the net quantity

of mass (or momentum) entering or leaving a specifiedvolume of aquifer during a given time interval be equal tothe change in the amount of mass (or moment) stored inthe volume Groundwater flow equations are formulated

by combining the equation of motion in the form of Darcy’slaw, which follows principle of conservation of momentumwith the mass balance equations, also known as continuityequations, which follows the principle of conservation

of mass Some mathematical models commonly used forsolving the forecasting problem in the presence of rechargeare discussed below:

2-D groundwater flow in an inhomogeneous anisotropicunconfined aquifer with a horizontal base is described bythe following equation (1,8):

hydraulic conductivities in x and y directions, respectively,

S y is the specific yield, t is time of observation, and N(x, y, t)

is the sum of all recharge rates from distributed sources

(recharge basins, ponds, streams, etc.) and withdrawal

rates from distributed sinks (wells, leakage boundaries,

etc.) and is represented by

where n is the total number of basins, N i (t) is the

time-varying recharge (or pumping) rate for the ith basin

(or well, respectively), and x i1 , x i2 , y i1 , and y i2 are the

coordinates of ith basin (or well) N i (t) is positive for

recharge to the aquifer and negative for pumping

For an inhomogeneous isotropic aquifer, Eq (1)

Equations (1 and 3) are nonlinear second-order partial

differential equation The nonlinearity is because of

the presence of h as a coefficient of partial derivatives

on the left-hand side Solving these equations because

of nonlinearity is possible only by numerical methods,

such as finite difference, finite element, and boundary

elements (1,9,10) These equations need to be linearized

for their analytical solution

For homogenous isotropic aquifers (K= constant),

Eq (3) can be written in the following two forms:

facilitate analytical solutions of Eqs (4 and 5) According

to the first procedure, i.e., the Baumann procedure of

lineraization, if the variation in h is much less than the

initial height of the water table h0, then the coefficient h

appearing on the left-hand side of Eq (4) can be replaced

by h0(11) Then Eq (4) can be rewritten as

where T = Kh0(known as transmissivity) Now, Eq (6) is

linear in h Sometimes the mean depth of saturation is

also used in place of h0

In the second procedure, i.e., the Hantush procedure of

linearization, h appearing in the denominator on the

right-hand side of Eq (5) is replaced by the weighted mean of

the depth of saturation h, a constant of linearization that

is approximated by 0.5[h + h(t )], and t is the period at

Now, Eq (7) becomes linear in h2 By substituting a new

variable H, defined as H = h2− h0 , into Eq (7) gives

where s = h2, a = θ/2D, θ = slope of the base, D =

the mean depth of saturation,  = KD/S y , x, y= space

coordinates, t = time of observation, and N(t) = time

varying rate of recharge

One-dimensional groundwater flow equations can beobtained by substituting zero for the derivative of y in theabove equations These equations are used to predict thewater table fluctuations in response to artificial rechargefrom strip basins, canals, channels, etc

GROUNDWATER FLOW EQUATIONS IN CYLINDRICAL COORDINATES

These types of equations are used to describe groundwaterflow induced by recharging through circular-shapedrecharge basins/wells and is given by (15,16)

Initial Conditions

Initial conditions describe distribution of h at all points of

the flow domain at the beginning of the investigation, that

is, at t= 0, which is expressed as

where ψ is a known value of h for all points of the

flow domain

Boundary Conditions

These conditions describe the nature of interaction of

the flow system with its surroundings Three types

of boundary conditions are generally encountered in

groundwater flow problems

• Dirichlet boundary condition—In this case, h is

prescribed for all points of the boundary for the entire

period of investigation, which is expressed as

where ψ (x, y, t) are known values of h at all points

on the boundary

• Neumann boundary condition—This type of

bound-ary condition prescribes the flux across the boundbound-ary

of the flow system and can be expressed as

where ψ1(x, y, t) are known values at the boundary.

A special case of this boundary condition is the no

flow boundary condition in which flux is zero This

condition occurs at impermeable surface or at the

groundwater divide, a surface across which no flow

takes place

• Cauchy boundary condition—This boundary

condi-tion is encountered at the semipervious boundary

layer between the aquifer and an open water body

such as a river As a result of the resistance to the

flow offered by the semipervious boundary that lies

between the aquifer and the river, the water level in

the river differs from that in the aquifer on the other

side of the semipervious boundary In this case, the

flux is defined by

q = Kh − h0

where h is the head at x = 0, h0 is the water level

in the river, and b and K are the thickness and

hydraulic conductivity of the semipervious boundary,

respectively

The purpose of solving a groundwater flow equation is

to obtain the values of h(x, y, t) Generally, two types of

methods, namely analytical methods and numerical

meth-ods, are used for this purpose Numerical methods are

used to solve the nonlinear groundwater flow equation

to tackle the real field problems, and analytical methods

are used to solve the linearized form of groundwater flow

equations Analytical methods commonly used for the

solu-tion of groundwater problems include Fourier transforms,

Laplace transforms, integral balance methods, method of

separation of variables, approximate analytic methods,

etc Details about these methods can be found in many

books (19–25) Most of the analytical solutions developed

earlier for this purpose were based on the assumption of

constant recharge Warner et al (26) have reviewed the

performance of some such analytical solutions (27–31).However, the rate of recharge largely depends on the infil-tration rate, which initially decreases because of swellingand dispersion of soil particles After some time, theinfiltration rate increases because of the release of airentrapped into soil pores and reaches to a maximumvalue Then, it starts decreasing because of clogging ofsoil pores beneath the bottom of the basin Recharge ratealso follows a more or less similar pattern of variationwith some time lag and less intensity When it falls below

a prescribed low value, the recharge operation is continued for some time After drying and, if necessary,scrapping of the silted base of the basin, a high rechargerate closer to its initial value is rejuvenated in the nextphase of recharge operation (1,32–34) Zomorodi (35) hasalso shown that the analytical solution of Dagan (36),which is based on the assumption of constant rechargerate, fails to predict the recession of the water tablecaused by decrease in the recharge rate Therefore, itwould be more appropriate to consider recharge rate astime-dependent to simulate the actual field conditions.Some solutions have been developed for the time-varyingrecharge cases in which the decreasing rate of rechargehas been represented by two linear elements (37–39) or

dis-by exponential function (14,16,40–44) However, mation of time-varying recharge by two linear elements orexponential function is possible only for one recharge cycle.However, recharge is applied intermittently for more thanone cycle separated by dry periods Manglik et al (45), Rai

approxi-et al (46), Manglik and Rai (48), and Rai and Manglik (49)used a general scheme of recharge approximation for anynumber of recharge cycles In this scheme, time-varyingrecharge is approximated by a number of linear elements

of different lengths and slopes depending on the nature

of variation of recharge rate Later on, this scheme wasmodified to represent rates of recharge from any num-ber of basins In mathematical form, this scheme can berepresented by

of different lengths and slopes depending on the nature

of variation of recharge rate By using this rechargescheme, several analytical solutions to describe watertable fluctuation in different flow systems representingdifferent physical conditions have been developed (50–53).The following analytical solution given by Manglik andRai (50) is considered as an example to demonstrate theapplication of these solutions in prediction of water tablefluctuation in the presence of time-varying recharge and

sin nπ y B

in which A and B are the length and width of the aquifer,

a = Kh/S y , and m and n are integers representing number

This solution is obtained by solving Eq (8) with

recharge/pumping rates defined by Eq (16) and subjected

to the horizontal water table as an initial condition and

Dirichlet boundary condition In order to demonstrate the

application of Eq (17) in the prediction of water table

fluc-tuation, we consider an example in which an unconfined

aquifer of 10× 10 km2 dimension is having two recharge

basins of dimension 60× 40 m2 and 50× 50 m2 centered

at (4470 m, 4500 m) and (5875 m, 5530 m), respectively,

and two wells each of 10× 10 cm2 dimension centered at

(5000 m, 4500 m) and (5000 m, 5500 m), respectively The

pattern of time-varying recharge rate and pumping rate

are shown in Fig 2 The recharge operation for both the

basins consists of two wet periods and one dry period, each

of 20 days duration During the first wet period, the rate

of recharge decreases from its initial value of 0.8 m d−1to

a lower value of 0.7 m d−1after 2 days It again increases

and attains maximum value of 0.9 m d−1on the fourth day

After that, it starts decreasing and reduces to zero on

twen-tieth day The second cycle of recharge operation begins on

the fortieth day and continues until the sixtieth day The

nature of variation of recharge rate for the second cycle is

considered similar to the first cycle Pumping of

ground-water at a rate of 105m d−1from each well is considered

for two periods The first period is from the tenth to the

twentieth day, and the second period is from the fortieth

to the fiftieth day after a gap of 20 days Numerical values

of other controlling parameters are h0= 20 m, K = 8 m

d−1, and S y = 0.20 Two water table profiles computed for

t = 45 days along a line parallel to the x-axis at y = 4500 m

are shown in Fig 3 These profiles pass through the center

of one recharge basin and one well centered at (4470 m,

4500m) and (5000 m, 4500 m) The profile represented by

the dotted curve is in response to recharge only Hence,

it shows only growth of groundwater mound The profile

represented by continuous curve shows growth as well

as depression of the water table at the respective site

of recharging and pumping This example demonstratesthe capabilities of prediction of water table variations inresponse to time-varying recharge and withdrawal.Accurate estimation of the varying recharge rate is amajor problem in groundwater resources management

If the time history of water table variation at asite of an observation well is known, then analytical

Only recharge Both recharge and pumping

1 0

Figure 3 Water table profiles in the presence of only recharge

(dashed curve), and both recharge and pumping (solid curve).

solutions can be used for the estimation of varying

recharge rate by making a judicious selection of recharge

rate using trial and error method, such that the

computed water table variation matches well with the

observed one Although the application of analytical

solutions is restricted to the relatively homogeneous

isotropic aquifer system having boundaries of simple

geometrical shapes, their application is fast and simple

in comparison with that of the numerical methods

Analytical solutions are also useful for other purposes,

such as analysis of the effects of various controlling

parameters, such as aquifers properties, initial and

boundary conditions, intensity and duration of recharge

rate, shape, size, and location of a recharge basin,

etc., on the response of the aquifer system Such

information is very essential for the judicious selection

of a suitable recharge scheme out of many proposed

schemes to achieve the preset objectives of groundwater

resource management

Acknowledgment

We wish to thank Dr S Thiagarajan for his help in preparation

of this work SNR wish to thank Dr V.P Dimri, Director, NGRI

for according permission to publish the work.

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Recharge of Groundwater T Asano (Ed.) Butterworth Publ.,

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of Earth’s Fluid Systems S.N Rai, D.V Ramana, and

A Manglik (Eds.) A A Balkema Publ., Holland, pp 49–62.

9 Pinder, G.F and Gray, W.G (1977) Finite Element

Simula-tion in Surface and Subsurface Hydrology Academic Press,

New York, p 295.

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Bound-ary element analysis of unconfined flow in porous media

using B-splines In: Proceedings of the 8th Intl Conference on

‘Computational methods in water resources, Part B Venice,

pp 405–411.

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mound in response to recharge Groundwater 18: 587–595.

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and decay of groundwater ridges J Geophys Res 72:

1195–1205.

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recharge In: Advances in Hydroscience Vol 2 V.T Chow

(Ed.) Academic Press, New York, pp 209–279.

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with inclined base Water Resour Manag 9: 127–138.

15 Mercer, J.W and Faust, C.R (1980) Groundwater modeling;

Mathematical models Groundwater 16: 212–227.

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through spreading Amer Soc Civ Eng Trans 117:

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GROUNDWATER AND ARSENIC: CHEMICAL BEHAVIOR AND TREATMENT

Mill Valley, California

20 mg/Kg (3) to 1 to 50 mg/Kg (2), with concentrations ashigh as 70 mg/Kg being unremarkable (3) Human activitygenerates anthropogenic arsenic, which makes it the thirdmost common regulated inorganic contaminant found atU.S Superfund sites

Arsenical copper was in use by 4000BC, and the toxiceffects of arsenic were documented by early Greek writers.More recently, arsenic has been linked to skin, bladder,and other cancers (4) The U.S Environmental ProtectionAgency (USEPA) lowered the arsenic standard in drinkingwater from 50µg/L to 10 µg/L, effective January 23, 2006.Modern usage of arsenic includes formulation ofpesticides and herbicides, decolorization of glass, paintmanufacturing, the production of semiconductors, andthe treatment/preservation of wood Pressure treatedlumber was commonly treated for decades using copper-chromium-arsenate (CCA) This product, also called ‘‘greenwood,’’ has been used for foundation lumber and morerecently as wood for outdoor children’s play structuresand picnic tables The CCA wood is being phased outfor toxicity concerns and environmental reasons Many ofthe pressure treatment lumber facilities have significantsoil and groundwater contaminated with arsenic as well

as chromium

CHEMICAL CHARACTER

Although arsenic occurs in more than 20 minerals, only afew are commonly found in ore deposits (5) Arsenic mayoccur as a semimetallic element (As0), arsenate (As5+),arsenite (As3+), or arsine (As3−) The biogeochemistry ofarsenic involves adsorption, biotransformation, REDOXreactions, and precipitation-dissolution processes (6,7)

The chemical character of arsenic is labile and

read-ily changes oxidation state or chemical form through

chemical or biological reactions that are common in the

environment Therefore, rather than solubility

equilib-rium controlling the mobility of arsenic, it is usually

controlled by REDOX conditions, pH, biological activity,

and adsorption/desorption reactions Arsenic in

ground-water most often occurs from geogenic sources, although

anthropogenic arsenic pollution does occur Geogenic

arsenic is almost exclusively an arsenite or arsenate The

most oxidized pentavalent form, arsenic, forms

oxyan-ions (H3ASO4 −, H2ASO4 −, H2ASO4-, HASO4 −, ASO3 −)

These arsenic oxyanions are isomorphous with

oxyan-ions of phosphorous, substituting for phosphate in both

marine organisms and phosphate deposits (4) Arsenite is

the trivalent form that also forms a series of oxyanions

that change specific configuration and charge with pH

Of critical importance with regard to the controls of the

mobility of arsenite is the fact that at a pH of 9.5 or lower,

the arsenite oxyanion is not charged This result obviates

all ionic interactions of the species

Common arsenic minerals are arsenopyrite (FeAsS),

enargite (Cu,AsS), proustite (Ag,AsS), and lollingite

(FeAs2) Late-stage magmatic crystallization

(pyrometaso-matic and hydrothermal stages) contributes to arsenic-rich

sulfides In sedimentary rocks, arsenic is commonly found

adsorbed onto fine-grained sedimentary rocks, such as iron

and manganese oxides (4) According to the U.S Geological

Survey, arsenic concentrations in sedimentary iron-ores

range from 65 to 650 mg/Kg (8) Arsenic is also

associ-ated with sedimentary pyrite at concentrations of 100 to

77,000 mg/Kg (6)

Anthropogenic arsenic may have any form including

organic arsine species Groundwater in acidic to

interme-diate volcanic rocks, or in sediments derived from those

rocks, will often have arsenic concentrations exceeding

50µg/L

Figure 1 illustrates the difference in molecular

struc-ture between arsenate and arsenite The double bond

oxygen in the arsenate molecule influences its ability to

become ionized through the of hydrogen ions The

pro-cess is termed dissociation A negative charge develops

on the arsenate molecule when dissociation occurs The

double bond oxygen increases the capacity to delocalize

that charge, which cases the loss of hydrogen ions The

propensity for ionization is expressed by the constant

of dissociation, pKa The pKa value, which is a

nega-tive log, shows a greater degree of dissociation with a

The pH at which these ionization steps occur is

sig-nificantly different between arsenate and arsenite, as

*These pKa values are extrapolated from the strength of oxygen

H

HAs

ArseniteArsenate

Common species of Arsenic

Figure 2 Control of arsenic speciation by Eh and pH conditions.

illustrated in Fig 2 (10,11) Figure 2 also shows the trol of REDOX potential (Eh) on the arsenate/arsenitetransition This Eh/pH relationship is key in understand-ing arsenic mobility in groundwater and the effectiveness

con-of arsenic water treatment systems

Arsenic Immobilization

The previous section described the conditions under whicharsenic can become an ionized species The most commonlyrecognized adsorption reactions are based on ion exchangebetween charged adsorption sites and charged solubleions However, London Van der Waals bonding is anothermechanism that is also responsible for adsorption Thistype of bonding is the result of complex interactionsamong the electron clouds of molecules, molecular polarity,and attractive forces of an atomic nucleus for electronsbeyond its own electron cloud Consequently, some degree

of immobilization can occur with soluble species thatare not ionized Arsenic immobilization through ionicadsorption can be controlled within normal oxidizingEh/pH conditions London Van der Waals bonding iscomplex to the point of unpredictability except for arsenic

1 Zone of arsenicimmobility

Ferrous iron Ferric iron

Arsenate Arsenite

Eh/pH vs arsenic immobilization in groundwater

Figure 4 Arsenic mobility in groundwater as controlled by the

effect of Eh/pH conditions on the speciation of arsenic and iron.

mobility at extreme Eh/pH conditions that can be obtained

in industrial settings, but not in groundwater

Components of soil that participates in both types of

adsorptive reactions include clays, carbonaceous material,

and oxides of iron, aluminum, and manganese In the most

shallow soils, the organic fractions typically dominate,

whereas at greater depths, iron oxyhydroxides play the

principal adsorptive role

The typical iron content of soil ranges from 0.5% and

5% Not only is iron common, but as with arsenic, it is

also labile and readily reflects changes in surrounding

Eh/pH conditions This relationship for iron is illustrated

in Fig 3 (12)

Ferric hydroxide acts as an amphoteric ion exchanger

Depending on pH conditions, the ferric hydroxide has

the capacity for cation or anion exchange Given

the average iron concentration in soil and soluble

arsenic concentrations in groundwater at 50µg/L, ferric

hydroxides in sediment can potentially adsorb 0.5 to 5

pounds of arsenic per cubic yard of aquifer matrix, which

may then act as a significant potential reservoir for arsenic

release under changing Eh/pH conditions

Arsenic concentrations up to 12,000µg/L have beenreported for the St Peter aquifer in eastern Wisconsin (4)

In this case, the oxidation of arsenic sulfides in a sulfidecement horizon (SCH) within the aquifer is a source of thehigh arsenic concentrations

Figure 4 superimposes the Eh/pH relationship for thearsenic and iron systems; it illustrates the conditionsunder which arsenic will be immobilized in a groundwatersystem Of equal importance, it illustrates how arsenicadsorbed to ferric hydroxides in sediment can be released

at exposure to groundwater that is chemically reducing.Two effects would be at work: Arsenate is reduced toarsenite that will not remain ionically bound to thegeologic substrate, and ferric iron is reduced to ferrous,which is soluble under normal pH conditions Outside theimmobilized zone, arsenic mobility is variable LondonVan der Waals bonding of arsenite is in effect, but it is notsufficient to assure complete immobilization

WATER TREATMENT SYSTEMS Introduction

Following is a brief review of various technologies used forthe removal of arsenic from drinking water and industrialwastewater Table 1 summarizes the effectiveness of eachand gives the source for the information

or inorganic species, reagent costs, and operationalissues are all factors Oxygen would be ideal, as it isthermodynamically capable of this oxidizing step Thekinetics for oxidizing arsenic compounds in groundwaterare exceedingly slow (22) It is possible to use gasdiffusion technologies that slowly release dissolved oxygeninto aquifers and that have demonstrated the capacity

to convert anaerobic groundwater systems into aerobicsystems within 3 to 6 months (23) This process willconvert soluble ferrous iron to insoluble ferric iron oxidescapable of attracting arsenate to their surfaces Theoxidation of arsenite is complex and may take additionaltime or the presence of other abiotic or biological (24)

stimulants In Bangladesh, in situ concentrations of

arsenic less than 0.1 mg/L were readily removed by theoxygenation of groundwater; concentrations greater thanthat had only 50% removal (25)

Other chemicals can affect arsenite oxidation includingfree chlorine, hypochlorite, ozone, permanganate, andhydrogen peroxide with ferrous iron

Table 1 Effectiveness of Arsenic Water Treatment Methods

Treatmenttechnology

Initial Arsenic Concentration

∗The current limit for drinking water is 50µg/L.

Eh/pH conditions & water treatment for arsenic

Figure 5 Eh/pH range required for effective treatment of soluble

arsenic.

Iron Coprecipitation

Coprecipitation of arsenate with ferric iron is recognized as

overall the most effective and practical existing method of

arsenic removal Ferric iron coprecipitation is particularly

useful in the mining industry, where large amounts of

ferric iron and arsenic can be byproducts of production

or refining Adding ferric iron salts for the treatment of

drinking water is usually necessary Figure 4 shows that

arsenate is readily removed by iron coprecipitation and

the Eh/pH conditions that must be maintained to effect

that removal Because of London Van der Waals bonding,

ferric iron coprecipitation of arsenite is also moderatelyeffective, with 50% removal at a pH of 7.0 (26)

The use of iron hydroxides for the coprecipitation ofarsenic in industrial wastewater (in which arsenic is inthe mg/L range) requires iron dosage four to eight timeshigher than that of the soluble arsenic; a greater irondosage yields no further benefit (27)

Alum Coprecipitation

Suspended aluminum salts (alum) can remove arsenatevia mechanisms similar to those for ferric hydroxides.However, it is less effective over a narrower pH range forarsenate removal and is ineffective for removal of arsenite(16)

Lime Precipitation

In testing by Nishimura and Tozawa (17), removal ofarsenic with lime precipitation was feasible, but notnecessarily practical High concentrations of arsenicwere removed to concentrations of 2 and 4 mg/L forarsenate and arsenite, respectively Removal to lowerlevels required a second treatment step in which initialarsenic concentrations of 2 mg/L were lowered to 20µg/Lfor arsenate and 160µg/L for arsenite However, to achievethese removal efficiencies, the lime dosage was between 5and 15 g/L

Activated Alumina

Activated alumina can be effective for the removal

of arsenate under moderately acidic pH conditions

alumina (18) The use of activated alumina for complete

arsenite removal is ineffective because of the nonionic

character of arsenite in that pH range (see Fig 1) Some

initial arsenite removal is observed because of London

Van der Waals bonding, but compared with arsenate

adsorption, this capacity is rapidly exhausted

Ion Exchange

Ion exchange has the potential for soluble arsenic removal

Anion exchange resins are available in two basic forms,

weak base and strong base Many weak-base anion

exchangers are capable of significant adsorption because

of London Van der Waals bonding in addition to ion

exchange, which gives them a higher level of adsorptive

capacity for nonionic arsenite The author has evaluated

anion exchange resins for arsenate removal The most

effective activation was in the hydroxyl form Chloride

and acetate were also tested Weak base resins had higher

loading capacities than did strong base (6% vs 4.8%),

but they did not have adequate removal efficiencies (75%

vs 99+% for the strong base resin) Anion exchange

resins are also prone to chromatographimg because of

the presence of competing anions in the treated water

However, ion exchange is an area of intense research

where the development of anionic chelating exchange

resins or ion exchange polymers may dramatically improve

the technology for arsenic treatment

Reverse Osmosis

Reverse osmosis (RO) has been shown to have a removal

efficiency greater than 97% Electrodialysis was only 73%

effective When used to treat 100% arsenite, removal was

only 28% (19)

Other

Rosehart (21) evaluated a series of removal technologies

including activated carbon and sulfide precipitation

Neither performed at a level adequate for use in the

treatment of drinking water

CONCLUSIONS

The behavior of arsenic in groundwater and industrial

wastewater is dominated by REDOX and pH conditions

Under a limited range of specific Eh/pH conditions,

the ability to predict total immobility of arsenic in

groundwater and in water treatment systems exists (see

Figs 4 and 5) Except for those conditions, arsenic will

be partially mobile, the magnitude of which is difficult

to predict

Implications of this behavior include:

• Arsenic treatment without control of Eh/pH is likely

to be ineffective

• The injection of water in a reduced oxidation

state into sediments with adsorbed arsenic may

• If ferric iron sludges used for the coprecipitation

of arsenic are disposed under improper Eh/pH

con-ditions, arsenic will remobilize In situ remediation

via recovery or stabilization of arsenic contaminatedgroundwater should be focused on Eh control throughchemical or biological methods

Site-specific arsenic chemistry including source,

mobil-ity, migration, fate, and transport is complex Any in situ

treatment of groundwater containing arsenic should beevaluated carefully with laboratory bench tests, computergroundwater modeling, and field pilot tests before full-scale remediation is attempted

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16 Sorg, T.J and Logsdon, G.S (1978) Treatment technology

to meet the interim primary drinking water regulations

for inorganics, Part 2 J Am.Water Works Assoc 70: 379–

393.

17 Nishimura, T and Tozawa, K (1985) Removal of arsenic

from wastewater by addition of calcium hydroxide and

stabilization of arsenic-bearing precipitates by calcination.

Proceedings of CIM Metallurgical Society, 15th Annual

Hydrometallurgical Meeting pp 3–1–3-18.

18 Frank, P and Clifford, D (1986) Arsenic (III) Oxidation and

Removal from Drinking Water U.S EPA, EPA-600-52-86/021.

19 Clifford, D and Lin, C-C (1991) Arsenic(III) and Arsenic(V)

Removal from Drinking Water in San Ysidro, New Mexico.

U.S EPA Proj Sum., EPA/600/S2-91/011.

20 Calmon, C (1973) Comment J Am Water Works Assoc 651:

568–569.

21 Rosehart, R and Lee, J (1972) Effective methods of arsenic

removal from gold mine wastes Can Min J June: 53–57.

22 Clifford, D., Ceber, L., and Chow, S (1983) Arsenic(III)/

Arsenic(V) separation by chloride- form ion-exchange resins.

XI Am Water Works Assoc Water Qual Tech Conf., Norfolk,

VA.

23 Kolhatkar, R (2002) Stable isotope analyses to

demon-strate MTBE biodegradation in ground water Groundwater

Resources Association of California MTBE Symposium, San

Jose, CA, Oct 17, Abstracts/presentation.

24 Phillips, S.E and Taylor, M.L (1976) Oxidation of arsenite

to arsenate by Alcaligenes faecalis App Env Microbiol 32(3):

392–399.

25 Sarkar, A.R and Rahman, O.T (2001) In-situ removal of

arsenci—experiences of DPHE-Danida Pilot Project In:

Tech-nologic for Arsenci Removal from Drinking Water M Amhed,

A Feroze, M Ashraf, and Z Adeel (Eds.) pp 201–206.

26 Pierce, M.L and Moore, C.B (1982) Adsorption of arsenite

and arsenate on amorphous iron hydroxide Water Res 16:

1247–1253.

27 Krause, E and Ettel, V.A (1985) Ferric arsenate compounds:

Are they environmentally safe? Solubilities of basic ferric

arsenates Proceedings of CIM Metallurgical Society, 15th

Annual Hydrometallurgical Meeting pp 5–1–5-20.

TREATMENT OF ARSENIC, CHROMIUM,

AND BIOFOULING IN WATER SUPPLY WELLS

INTRODUCTION

Arsenic

Arsenic has been used as a component of pesticides and

thus may enter streams or groundwater through waste

disposal or agricultural drainage Arsenic is also present in

volcanic gases and is a common constituent of geothermal

or spring water

As small amounts of arsenic can be toxic to humans,

it is considered a highly undesirable impurity inwater supplies; an upper concentration limit of 50µg/L,established in 1976 by the U.S EPA, has now been lowered

to 10µg/L

Arsenic may form metal arsenides in which its oxidationstate is negative (1) Arsenic may also form sulfidesand can be present as an accessory element in sulfideore deposits In solution in water, the stable formsare arsenate (As5+) or arsenite (As3+) oxyanions A

pH–Eh diagram showing fields of dominance of aqueousarsenates (1) indicates that the monovalent arsenateanion H2AsO−4 is expected to predominate between pH

3 and 7 and the divalent species HAsO2−4 takes over from

pH 7 to 11 Mildly reducing conditions favor the unchargedarsenite ion HAsO2(aq)

Chromium

Chromium is an inorganic element that is used inelectroplating, leather tanning, wood treatment, pigmentmanufacture, and cooling tower treatment for corrosioncontrol Chromium can contaminate drinking watersources through discharges from industries, leaching fromhazardous waste sites, or it may occur naturally fromthe erosion of natural deposits Two forms of chromiumcan occur in water sources: chromium (III) and chromium(VI) Chromium (III) is an essential nutrient at traceconcentrations Chromium (VI) is toxic and is the basisfor setting the chromium drinking water standard Theratio of the two forms can vary quite a bit in naturalwaters Evidence also exists to suggest that chromium(VI) may be converted to chromium (III) in the humanbody, particularly in the acidic digestive system

The U.S Environmental Protection Agency (EPA)classified chromium (VI) as a human carcinogen byinhalation In 1991, it reviewed the existing standard fortotal chromium and the most recent scientific researchavailable As a result, the EPA actually raised themaximum contaminant level from 50 to 100 parts perbillion as total chromium, based on its conclusion thatchromium (VI) is not carcinogenic by ingestion

In California, the drinking water standard is 50 partsper billion for total chromium [the sum of chromium(III) and chromium (VI)] A public health goal is a

‘‘risk assessment’’ of the concentration of a contaminant

in drinking water that poses no significant health risk

to the consumer The recommended public health goaldetermined by the Cal/EPA Office of EnvironmentalHealth Hazard Assessment (state health experts) for totalchromium is 2.5 parts per billion

Well Biofouling

Iron and manganese biofouling (usually associated with

‘‘iron bacteria’’) are common in water supply wells andattached appurtenances Although biofouling sometimeshas no overt symptoms, it may cause clogging, corrosion,and water quality degradation These problems pose aconsiderable challenge to water utilities and well owners

in North America and around the world Precise estimates

of increased operational and mitigation costs resultingfrom damage and loss of efficiency from well biofouling are

actual practice, resulting in frequent operating problems.

Although preventive maintenance would be more

cost-effective, the most common approach to iron biofouling

problems in wells is crisis management Preventive

maintenance is seldom employed, however, because

suitable maintenance monitoring methods and practical

protocols for detecting iron and manganese biofouling

problems before they severely affect well production and

water quality have not been available (2)

Understanding the cause of water well deterioration

and developing ways to sustain water well environments is

important in maintaining and improving the quality of life

in rural areas The deterioration of well yield and water

quality is a concern to individuals, small communities,

and industries that rely on water wells as their principal

source of water Currently, when the quality or quantity

of water produced declines dramatically, wells are

often abandoned, or treatments are applied with little

understanding of the cause of these problems The cost

of replacing these wells can have a significant economic

impact on well owners Correctly identifying the cause

of water well problems offers the possibility of effective

treatment and maintenance instead of well abandonment

Losses in water well production and water quality have

traditionally been attributed to the chemical and physical

properties of the water well environment Many of these

problems can be solved by well-established diagnostic and

rehabilitative techniques However, less recognized is that

groundwater contains microorganisms such as bacteria,

and the activities of these microorganisms also cause

significant water well problems Water well deterioration

caused by microbiological activity is termed biofouling

Installing and pumping a well increases the level of oxygen

and nutrients in the well and in the surrounding aquifer,

encouraging bacterial cells, which are naturally present

in groundwater, to anchor themselves to surfaces in the

well and around the well intake Once attached, these

bacteria quickly multiply and colonize these surfaces The

bacterial colonies form a gel-like slime or biofilm that

captures chemicals, minerals, and other deposits, such as

clays and silts, moves to the well during pumping, and

forms biomasses

Some of the byproducts of bacterial growth, such as

oxidized iron and manganese, also accumulate in these

secretions, which leads to the production of the red or

black slimes often found in toilet tanks or observed on

pumps and discharge lines when they are pulled from

a well Biofouling of a water well occurs when biofilms

accumulate a sufficient amount of debris to interfere

with water flow and affect water quality If uncontrolled,

well biofouling can affect well performance in various

ways Biofilms and the debris they collect can quickly

coat, harden, and plug the well screen, the sand pack,

the surrounding aquifer material, and may even plug

water lines and affect the performance of household

treatment systems In addition, the bacteria living within

the biofilm can increase the rate of iron oxidation and

iron buildup in the well and distribution pipes, which

leads to occasional discoloration of well water Biofouling

steel and iron casings and pipes Once developed, abiomass can protect the bacterial cells from environmentalchanges such as changes in pH, temperature, and fluidvelocities, making treatment chemicals less effectiveand removal of plugging material more difficult, whichemphasizes the importance of regular well maintenance

A number of field and laboratory tests exist that can

be used to monitor water quality and biological activity

in groundwater If performed regularly 1 month afterthe well is installed and then once every 6 months,these tests indicate when water quality is changing orwhen biological activity is increasing Changes in waterquality and increased levels of biological activity indicatethat well maintenance is required Ideally, appropriatewell maintenance chemicals should be applied beforewell performance is significantly affected Establishing

a monitoring schedule, where pumping water levels andwell pumping rates are recorded, is also an effective way

to identify when preventive maintenance measures are nolonger effective and well rehabilitation is required.Extracellular slimes are composed largely of polysac-charides and, in general, are the major component of thebiofouling mass Biomass associated with viable activecells is a relatively minor component Figure 1 illustrateshow the maximum level of biological activity commonlyoccurs under redox conditions that are at the periphery

of oxidation in the Ehrange of−50 to +150 mV ing can be complex and caused by a variety of bacteria.However, the appearance and odor of bacterial slimes arediagnostic

Biofoul-ARSENIC TREATMENT

Of the 14 treatment technologies that the EPA reviewed,five are relevant technologies for small systems—ionexchange, activated alumina, and membrane technolo-gies (reverse osmosis, nanofiltration, and electrodialysisreversal) Seven alternative technologies are categorized

as still emerging (iron oxide-coated sand, granular ric hydroxide, iron filings, sulfur-modified iron, greensandfiltration, iron addition with microfiltration, and conven-tional iron/manganese removal) The last two technolo-gies—coagulation/filtration and lime softening—are usedprimarily in larger systems and are not expected to beinstalled solely for arsenic removal

fer-Ion Exchange

Ion exchange, in particular, will probably be a verycommon technology used to comply with the arsenicregulation It is recommended for systems with low sulfate

(<120 mg/L) and total dissolved solids (TDS) The effect of

competing ions drives the regeneration frequency and, inturn, the cost Ions that compete with arsenic are sulfate(the most significant competitor), fluoride, selenium,and nitrate Systems that have high levels of thesecontaminants may need a pretreatment phase as well.The EPA has data on co-occurrence but would like to hearhow much competition is occurring in the field and how

Power supply polaritycan be reversed periodically

to shock and prevent bacteriaand scale mineral builduparound well screen

Not to scale

DC powersupplyLand surface

Sand & gravel

Sand & gravelAlternate

electrode

Increase

in cationexchangecapacity

in clay layers

Lower pH

DC current flowLess than 5 amperes

Clayey layers

Steel blindcasing

Orginal fluid level

EK depressed

water table

Soluble high valence

metals in well can be

pumped out and

neutralized above ground

Non-conductivecasing & screen

Insulatedwire &

primaryanode

EK inducedwater moundwill increasewell yield

Large diameterwater supplywells(cathodes)

Small diameterelectrode wellanode

Reduction zone Perforated screen

production zone Magnetised screen

to reduce mineral scaling

Lower valence cations oxides/OHs adsorbed on clay

CR +3 and As +3

Higher pH

+ + + + ++ + + + ++ + + + + + + + + + + + + ++ + + + +

ETSEnvironment & technology services

Generalized in situ ek treatment of arsenic, chromium,

and well biofouling protection

Figure 1 Generalized in situ EK treatment of arsenic, chromium, and well biofouling protection.

viable people think this technology would be The waste

stream, or brine, can be reused, which reduces the volume

of waste and increases its concentration The following are

frequently asked questions about ion exchange:

1 How often is regeneration feasible? Will it vary by

size category?

2 Can the EPA cap sulfate at 120 mg/L? Where is the

TDS cap?

3 How often will an ion exchange system need

regeneration? How long does it take to regenerate?

Can you afford to have two systems to switch back

and forth during regeneration? How much

back-up water sback-upply is necessary to provide water

during regeneration? Do you have that much

storage capacity available? How much regeneration

is feasible for small systems? Daily, once a week,

once a month? Can we expect variation by size?

4 Should we calculate costs for different removal

percentages assuming full stream treatment or

blending for ion exchange? Can all sources be treated

at one location? Do different wells/streams serve

different parts of the system?

Coagulation/filtration and lime softening are

intended for larger systems If small systems install these

technologies, the EPA expects it would be in package

plant form to reduce costs A package plant is one bought

‘‘off-the-shelf’’ versus one custom designed for a site Awell-trained operator is needed to run these technologies;

an off-site operator could utilize remote telemetry TheEPA does not expect these technologies to be installed onlyfor arsenic removal, but if there is another contaminant inthe water, it may be practical Sludge disposal needs to beconsidered and may be an issue for small systems Again,the EPA would like information from those experiencedwith these types of technologies

All the technologies looked at and discussed so farwork best when the arsenic is in the form of arsenic

(V) Pretreatment converts arsenic (III) to arsenic (V).

Surface water tends to favor arsenic (V), but groundwatertends to contain arsenic (III) Data shows that chlorineand potassium permanganate are effective in oxidizingarsenic (III) to arsenic (V) Possible problems includethe existence of chlorine, which increases the potential

to create disinfection byproducts, and membrane fouling

of subsequent treatments such as RO The EPA’s Office

of Research and Development is researching otherpreoxidants (including ozone and hydrogen peroxide,which are expected to be effective) to provide more data inthe next few months

Point of use (POU)/Point of entry (POE) devices

may be appropriate for small systems serving 10,000 orfewer and are new elements of the SDWA POE is whole-house treatment, whereas POU treats water at the tap.The EPA is looking at these devices as possible compliance

large flows of water might use POE and POU devices

to treat the minor part of the flow provided for potable

use The POE/POU technologies that are available for

arsenic removal are smaller versions of reverse osmosis,

activated alumina, and ion exchange Note that POU/POE

technologies must be maintained by the public water

system Therefore, the need exists for substantial

record-keeping It also increases the responsibility on the part of

customers, as it requires them to facilitate entry into their

homes by the utility for maintenance The water utility is

ultimately responsible for ensuring that these devices are

maintained properly The EPA is trying to determine the

system size cutoff where centralized treatment would be

more affordable than POU and POE devices

Waste Disposal will be an important issue for both

large and small plants If a plant is located inland and uses

membrane technologies, operators may have to pretreat

prior to discharge If the plant is discharging to a sanitary

sewer because of the membranes, there may be very high

salinity in the discharge as well as high levels of arsenic

that might be above local sewer regulations Ion exchange,

reverse osmosis, and activated alumina treatment brines

will be even more concentrated (on the order of 30,000

TDS) and more than likely will require pretreatment prior

to discharge to either a receiving body of water or the

sanitary sewer

CHROMIUM TREATMENT

Reverse osmosis (RO) systems can often improve the

quality of water Reverse osmosis water treatment has

been used extensively to convert brackish water or

seawater to drinking water, to clean up wastewater,

and to recover dissolved salts from industrial processes

It is becoming more popular in the home market

as homeowners become increasingly concerned about

contaminants that affect their health, as well as about

nonhazardous chemicals that affect the taste, odor, or color

of their drinking water People considering the installation

of a water treatment system to reduce toxic chemicals

should first have their water tested to determine how

much, if any, hazardous compounds are in the water

Public water supplies are routinely monitored and treated

as required under the federal Safe Drinking Water Act

and state regulations Private water systems should be

tested at the owner’s initiative based on knowledge of land

use and contamination incidents in the area (3)

Reducing Contaminants Through RO

Reverse osmosis reduces the concentration of dissolved

solids, including a variety of ions and metals and very

fine suspended particles such as asbestos, that may be

found in water An RO device may be installed following

a water softener to reduce the concentration of sodium

ions exchanged for hardness ions RO also removes

arsenic (As), chromium (Cr), nitrate (NO−3), certain organic

contaminants, some detergents, and specific pesticides

In reverse osmosis, a cellophane-like membrane separatespurified water from contaminated water An understand-ing of osmosis is needed before further describing RO.Osmosis occurs when two solutions containing differ-ent quantities of dissolved chemicals are separated by

a semipermeable membrane that allows only some pounds to pass through The osmotic pressure of thedissolved chemical causes pure water to pass throughthe membrane from the dilute to the more concentratedsolution In reverse osmosis, water pressure applied tothe concentrated side forces the process of osmosis intoreverse Under enough pressure, pure water is ‘‘squeezed’’through the membrane from the concentrated to the diluteside Salts dissolved in water as charged ions are repelled

com-by the RO membrane Treated water is collected in a age container The rejected impurities on the concentratedside of the membrane are washed away in a stream ofwastewater, not accumulated as on a traditional filter.The RO membrane also functions as an ultrafiltrationdevice, screening out particles, including microorganisms,that are physically too large to pass through the mem-brane’s pores RO membranes can remove compounds inthe 0.0001 to 0.1 micron size range (thousands of timessmaller than a human hair)

stor-Design of an RO System

Although reverse osmosis is simple, a complete watertreatment system is often complex, depending on thequality of the incoming water before treatment and theconsumer’s needs Most home RO systems are point-of-use(POU) units placed beneath the kitchen sink to treat waterfor cooking and drinking Point-of-entry (POE) systemsthat treat all water entering the household are moreexpensive to purchase and operate than POU systems

A typical home reverse osmosis system consists ofpretreatment and posttreatment filters as well as an

RO membrane, flow regulator, storage container for thetreated water, and dispensing faucet The pressure for RO

is usually supplied by the feed line pressure of the watersystem in the home, but a booster pump may be needed toproduce an adequate volume of treated water A sedimentprefilter is essential for removing relatively large sandgrains and silt that may tear or clog the RO membrane orclog a pump or flow regulator Water softeners are used

in advance of the RO system when household water isexcessively hard If the water is chlorinated or containsother oxidizing chemicals such as bromine, an activatedcarbon prefilter is needed to protect membranes sensitive

an activated carbon filter with RO expands the range

of chemicals the system can remove Furthermore, AC

treatment is improved because RO removes compounds

that adversely affect AC adsorption

The storage tank, tubing, and dispensing faucet

should be made of plastic, stainless steel, or other

nontoxic materials The low pH and mineral content

of RO-treated water may corrode copper pipes and

allow lead to leach into the drinking water from

brass components

RO Membrane Materials

The most common RO membrane materials are polyamide

thin film composites (TFC) or cellulosic types [cellulose

acetate (CA), cellulose triacetate (CTA), or blends]

Very thin membranes are made from these synthetic

fibers Membrane material can be spiral-wound around

a tube, or hollow fibers can be bundled together,

providing a tremendous surface area for water treatment

inside a compact cylindrical element Hollow fiber

membranes have greater surface area (and therefore

greater capacity) but are more easily clogged than

the spiral-wound membranes commonly used in home

RO systems

The flux, or capacity, of the RO membrane indicates

how much treated water it can produce per day Typically,

RO membranes for home systems are rated in the range of

10 to 35 gallons per day Thus, under standard operating

conditions, it could take from 2 to 6 hours to fill a

two and-a-half-gallon storage tank CA/CTA membranes

have adequate capacity for most households, but TFC

membranes should be used if large volumes of treated

water are needed

RO membranes are rated for their ability to reject

compounds from contaminated water A rejection rate (%

rejection) is calculated for each specific ion or contaminant

as well as for reduction of total dissolved solids (TDS)

It is important that consumers know their specific

requirements for water quality when buying a system

For example, high rejection rates are essential when

high nitrates or lead concentrations in the water must

be brought below the EPA maximum contaminant or

action levels

Efficiency of RO Systems

The performance of an RO system depends on membrane

type, flow control, feed water quality (e.g., turbidity, TDS,

and pH), temperature, and pressure The standard at

which manufacturers rate RO system performance is

77◦F, 60 pounds per square inch (psi), and TDS at 500

parts per million (ppm) Only part of the water that flows

into an RO system comes out as treated water Part of the

water fed into the system is used to wash away the rejected

compounds and goes down the drain as waste The recovery

rate, or efficiency, of the system is calculated by dividing

the volume of treated water produced by the volume of

water fed into the system If not properly designed, RO

systems can use large quantities of water to produce

relatively little treated water Most home RO systems

are designed for 20% to 30% recovery (i.e., 2–3 gallons

of treated water are produced for every 10 gallons putinto the system) Home RO systems can operate at higherrecovery rates but doing so may shorten membrane life.The flow regulator on the reject stream must be properlyadjusted If the flow is slow, the recovery rate is high, but

RO membranes are easily fouled if concentrated impuritiesare not washed away quickly enough If the flow is too fast,the recovery rate is low and too much water goes down thedrain Overall water quality affects the efficiency of an ROsystem and its ability to remove specific contaminants.The higher the TDS, the lower the recovery rate oftreated water The amount of treated water produceddecreases by 1% to 2% for every degree below the standardtemperature of 77◦F An RO system supplied with wellwater at a temperature of 60◦F produces only three-quarters of the volume it would produce at 77◦F For

an RO system to function properly, there must be enoughwater pressure Although most home RO systems arerated at 60 pounds per square inch, the incoming feed linepressure of many private water systems is less than 40 psi.The RO system must work against back pressure created

in the storage tank as it fills with water and compressesthe air in the tank The RO device must also overcomeosmotic pressure, bonding between water molecules, anddissolved impurities; the higher the TDS level, the greaterthe osmotic pressure The net water pressure at the ROmembrane can be calculated by subtracting back pressureand osmotic pressure from feed line pressure If the netwater pressure at the membrane is lower than 15 psi,treated water production is less efficient and contaminantrejection rates are lower Auxiliary pumps can be added

to the treatment system to boost pressure and improvethe quality and quantity of water produced High-quality

RO systems have valves that shut off the flow wheneverstorage tank pressure reaches two-thirds of the feedpressure; at that point, low net water pressure can result

in low rejection rates In some systems, once the storagetank is filled, surplus treated water is discarded; waterloss from such units is frequently excessive A system thatautomatically shuts off when the pressure on the tankreaches a given level saves water

Maintenance of an RO System

An RO system must be well maintained to ensure reliableperformance Clogged RO membranes, filters, or flowcontrols decrease water flow and systems performance

If fouling is detected in early stages, the membrane canoften be cleaned and regenerated The cleaning procedurevaries depending on the type of membrane and fouling.Completely clogged or torn RO membranes must bereplaced In addition, pre- or postfilters must be replacedonce a year or more often, depending on the volume ofwater fed through the system and the quality of thefeed water Damage to RO membranes cannot be easilyseen The treated water must be analyzed periodically

to determine whether the membrane is intact and doingits job Many systems now have a built-in continuousmonitor that indicates a high TDS level, a sign that thesystem is not operating properly It may also be necessary

to test regularly for specific health-related contaminantssuch as nitrates or lead Microorganisms, dead or alive,

other biocides provided by the manufacturer Continuous

chlorination can be used with cellulosic membranes to

protect the system from biofouling and eliminate the

particle-trapping slime that worsens other forms of fouling

such as scaling Chlorine and other oxidizing disinfectants

are harmful to thin film composite membranes If the

feed water is chlorinated, an activated carbon unit

must be used to remove the oxidizing chemicals before

they reach the TFC membrane Activated carbon (AC)

prefilters should not be used on nonchlorinated water

supplies because they provide a place for microorganisms

to multiply and lead to increased biofouling of the RO

membrane surface It is important to replace AC filters

periodically following the manufacturer’s instructions,

especially after an extended shutdown period during which

microorganisms can flourish

Choosing an RO System

Homeowners who are thinking about buying reverse

osmosis systems should determine their initial water

quality and their goals in adding water treatment systems

RO removes many inorganic impurities from drinking

water, especially nitrate Its effectiveness depends not

only on the type of membrane but on feed water quality,

temperature, pressure, and flow control, as well as the type

and concentration of specific contaminants to be removed

A typical RO system consists of a sediment filter, pump,

reverse osmosis membrane, flow regulator, storage tank,

final activated carbon filter (for taste and odors), and

dispensing faucet An AC prefilter is sometimes needed

for dechlorination RO is commonly used to treat only

the water used for drinking and cooking at the point of

use rather than at the point of entry for all household

use RO membrane types vary in their ability to reject

contaminants and differ in capacity (the volume of treated

water produced per day) Water pressure is an important

factor in determining the RO system’s rejection rate,

capacity, and recovery rate (amount of treated water

produced per amount of feed water used) Maintenance

of an RO system is essential for reliable performance

High levels of TDS and microorganisms in the system are

commonly the cause of fouled membranes Treated water

should be monitored for TDS and the level of any specific

contaminants that may affect health

A list of home water treatment devices certified by

various Department of Health Services can be found on

their websites

BIOFOULING TREATMENT

Historically, there have been three approaches to the

declining operation of a water well of any common type

One is to simply abandon the well and install a new well of

similar or greater capacity to replace the abandoned well

A second involves attempting to change the operating

techniques (e.g., pump times, volumes, sequences of

up-and downtimes, up-and control flow by drawdown limitations)

or change some components in the well (e.g., pump, screen)

the problem by determining

—first, the cause;

—second, confirm that the effects witnessed can berelated to the cause identified;

—and third, determine and apply a treatment strategythat counteracts the cause and allows the well tofunction as designed

Increasing economic and environmental costs andconcerns are now restricting the ability of a well usersimply to replace a failing well Economic concerns relate

to the increasing costs involved in well replacementand the growing sensitivity for maximizing the use ofeach well installation by extending its useful life (i.e.,environmental sustainability) Environmental concernsare being brought to the fore because groundwaters are

no longer seen as an infinite resource In some areas,aquifers are now being heavily depleted by the demand,and there is little flexibility to provide additional capacity.Another major environmental concern is the impact ofvarious forms of pollution on well fields In the pastdecades, general attitudes may be summarized by an ‘‘out

of sight, out of mind’’ approach in which groundwaterwas given a lower status than surface waters Variouschemical leakages from industry, agriculture, and variousservice industries were not considered as important asthose in surface waters When a pollutant impactedsurface water, the effects could often be relatively quicklyappreciated through radical eutrophication, deterioratingwater quality, and water unacceptable to users One majordifference between surface and groundwaters is the factthat the former flows as large unconfined masses whereasthe latter moves as a confined mass within porous media.This difference is very critical to the current understanding

of groundwater flow and quality

It is not easy to appreciate the complex interactions thatoccur between flowing groundwater and the media throughwhich it is passing as it moves to a well, a spring, orinterfaces with another aquifer For the last century, it hasbeen popularly believed that groundwater is essentiallysterile (devoid of biological activity) and that all activitieswithin an aquifer may be explained almost exclusively by

a combination of physical and chemical processes Today,the hydrology of groundwater systems still leans heavily

on this assumption Through the science of subsurfacemicrobiology (the study of microorganisms in the crust

of the planet), it is now becoming increasingly evidentthat groundwater movement and quality are affected bymicrobiological interactions In the past decades, thesehave been ignored, and one of the major consequenceshas been that the effects of these microorganisms asbiological filters (interface) have been ignored Pollutantswithin a groundwater system may become entrapped (andpossibly degraded) within these biological filters and sonot appear in the groundwater resurfacing through a well.Environmental monitoring of the product (postdiluvial,after the ‘‘event’’) water from a well may not necessarilygive an ‘‘accurate’’ picture of the chemical loading in

the transient (causal) water itself There has been a

tendency for groundwater users to rely on product

(‘‘biofiltered’’) water for environmental assessment, and

yet this water may not accurately allow a risk assessment

for that well (due to bioentrapment of some chemicals

of concern)

In the next two decades, the realization of the nature

of the biological interfaces within and around water wells

may cause much tighter environmental constraints to be

placed on new well installations, which would mean that

greater attention would be paid to extending the service

life of existing wells through preventive maintenance

and effective rehabilitative programs The mindset that

a water well is a physical object set within a chemical

and physical world has to change This mindset has

generated a ‘‘traditional’’ attitude that a dysfunctional

well is simply a result of chemically driven corrosion,

encrustation, clogging processes, or the physical collapse

of the system (through such events as ‘‘silting up’’ and

‘‘collapsed’’ aquifer and well structures)

Acidization has commonly been applied as a

remedi-ation technique to dissolve and disperse the clogs and

encrustations, and various disinfectants (such as different

formulations of chlorine) were used to control any coliform

and other bacteria that may be growing down the borehole

(and presents a potential health risk) Slime formations

were considered by many to be simply physical-chemical

accumulates that may result in clogging, encrustation,

and corrosion Even today, camera logging a water well

is considered sufficient to view all biological and much of

the chemical deposits (e.g., silts and salts) that can be

causing problems around a well Please see the section on

the ‘‘Preliminary Diagnosis of Biological Fouling of Water

Wells Using TV Camera Logging Methods.’’

Combinations of disinfectants, selected acids and even,

in more recent times, dispersants (a.k.a wetting agents)

have become part of the arsenal of weapons used to

rehabilitate problems in a well One of the findings

from these actions has been that ‘‘no one size fits all’’

and that each well should be treated as unique and

requires customization of the treatment parameters to

optimize maintenance practices This approach stems

from observations that each well can be characterized

as different from other wells in the same field Many

instances exist where two wells of the same construction

and characterization placed within feet (meters) of each

other in supposedly the same aquifer formation bear very

different characteristics An unfortunate result of this is

that a treatment may be successfully applied to one well in

a field but that same treatment may fail on a neighboring

well of exactly the same characteristics in construction,

operation, and mode of failure (4)

INNOVATIVE IN SITU TREATMENT

In situ electrokinetic treatment of chromium, arsenic,

and biofouling may provide a cost-effective solution (5)

The electrokinetic treatment process involves applying

direct current (dc) in a medium (soil and water)

The flow of electrons from anode to cathode creates

a migration of cations in the medium toward the

cathode The electrolysis of water creates a higher

pH and oxidizes the metal or reduces its valence,which renders the metal into a nontoxic form nearthe cathode Clayey material may show a dramaticincrease in cation exchange capacity under a high

pH near the cathode.(6–8) The proposed electrokinetic

process can be applied both in situ and ex situ Figure 1

presents a more realistic distribution of the various

valence states of metals under the proposed in situ

electrokinetic influence

Besides the treatment of metals, the beneficial side

effects of the in situ EK treatment are as follows:

The well itself is set up as a cathode, so iron bacteriawill not live on the surface of the well casing andperforation because of the high pH

The electrokinetically induced water migration towardthe cathode (well) may induce an increase inhydraulic head, thus increasing well yield

No such treatment has been tried on chromium, arsenic,and manganese in a saturated medium to date Thereare many successes of the proposed EK process in thelaboratory and in soil However, we have successfullydemonstrated the electrokinetic control of selenium andboron in clayey saturated media at two sites in the PanocheIrrigation District, Central Valley, California We believethat the same EK control can be applied to chromium,arsenic, and biofouling because of the similarity of themultiple valence forms to those of selenium and boron.Due to the simplicity of the cathode and anode setup, webelieve that the proposed EK processes will prove cost-

effective The proposed in situ EK treatment (once set up)

is permanent It has a one-time capital cost and minimumlong-term maintenance costs The continuous operatingelectricity demand will not exceed 50 amperes at 30 to

100 Vdc or 1500 to 5,000 watts per site

BIBLIOGRAPHY

1 Hem, J.D (1979) Reaction of metal ions at surfaces of hydrous

iron oxide Geochim Cosmochim Acta 41: 527–538.

2 Smith, S.A (1992) Methods for Monitoring Iron and ganese Biofouling in Water Wells AWWA Research Foundation

6 Driscoll, F.G., (1986) Groundwater and Wells—Well Failure

& Iron Bacteria Johnson Filtration Systems, Inc., St Paul,

Iron-Water-Resources Investigations Report 97-4032.

from rectangular areas Groundwater 19: 271–274.

32 Singh, V.P (1989) Hydrologic Systems—Watershed elling... transient (causal) water itself There has been a

tendency for groundwater users to rely on product

(‘‘biofiltered’’) water for environmental assessment, and

yet this water may not...

17 Todd, D.K (1980) Groundwater Hydrology, 2nd Edn John

Wiley & Sons, New York, p 535.

18 Rushtun, K.R (2003) Groundwater Hydrology: Conceptual