Natural and Enhanced Remediation Systems - Chapter 3 pot

Protocols 3.3.1 Protocols for Natural Attenuation3.3.2 Patterns of Natural Attenuation3.3.2.1 Various Patterns of Natural Attenuation3.4 Processes Affecting Natural Attenuation of Compou

Suthersan, Suthan S “Monitored Natural Attenuation”

Natural and Enhanced Remediation Systems

Edited by Suthan S SuthersanBoca Raton: CRC Press LLC, 2001

©2001 CRC Press LLC

CHAPTER 3 Monitored Natural AttenuationCONTENTS

3.1 Introduction3.1.1 Definitions of Natural Attenuation 3.2 Approaches for Evaluating Natural Attenuation3.3 Patterns vs Protocols

3.3.1 Protocols for Natural Attenuation3.3.2 Patterns of Natural Attenuation3.3.2.1 Various Patterns of Natural Attenuation3.4 Processes Affecting Natural Attenuation of Compounds3.4.1 Movement of Contaminants in the Subsurface3.4.1.1 Dilution (Recharge)

3.4.1.2 Advection3.4.1.3 Dispersion 3.4.2 Phase Transfers 3.4.2.1 Sorption3.4.2.2 Stabilization 3.4.2.3 Volatilization3.4.3 Transformation Mechanisms3.4.3.1 Biodegradation3.5 Monitoring and Sampling of Natural Attenuation3.5.1 Dissolved Oxygen (DO)

3.5.2 Oxidation–Reduction (REDOX) Potential (ORP)3.5.3 pH

3.5.4 Filtered vs Unfiltered Samples for Metals 3.5.4.1 Field Filtration and the Nature of

Groundwater Particulates3.5.4.2 Reasons for Field Filtration3.5.5 Low-Flow Sampling as a Paradigm for Filtration 3.5.6 A Comparison Study

References

…natural attenuation (NA) is not a “no action (NA)” alternative Monitored natural Attenuation (MNA) defines the required monitoring parameters to dem- onstrate that the ongoing natural processes will continue to meet the remediation objectives…

3.1 INTRODUCTION

The term monitored natural attenuation (MNA) refers to an approach to clean upsubsurface contamination, specifically in groundwater, by relying on natural processesand monitoring MNA is also referred to as natural degradation and intrinsic or passive remediation Natural attenuation processes include a variety of physical, chemical, orbiological processes that, under favorable conditions, act without human intervention

to reduce the mass, toxicity, mobility, volume, and concentration of contaminants ingroundwater Depending on the geologic conditions, types of contaminants, and con-taminant mass and distribution at a given contaminated site, MNA could emerge asthe preferred choice of remediation approach Natural attenuation relies on the assim- ilative capacity of the ecosystem for the reduction of contaminant concentration andmass This approach has been utilized by environmental engineers for a long time tocontrol industrial and municipal wastewater discharges into surface waterbodies andmaintain acceptable water quality standards

3.1.1 DeÞnitions of Natural Attenuation

A variety of organizations have espoused the following definitions of naturalattenuation due to the emerging popularity and preference of MNA as the remedi-ation method of choice at many contaminated sites across the country.1

Environmental Protection Agency 2 : This policy directive defines monitored natural attenuation as the reliance on natural attenuation process (within the context of a carefully controlled and monitored site cleanup approach) to achieve site-specific remediation objectives within a time frame that is reasonable com- pared to that offered by other more active methods The “natural attenuation processes” that are at work in such a remediation approach include a variety of physical, chemical, or biological processes that, under favorable conditions, act without human intervention to reduce the mass, toxicity, mobility, volume, or concentration of contaminants in soil or groundwater These in situ processes include biodegradation; dispersion; dilution; sorption; volatilization; radioactive decay; and chemical or biological stabilization, transformation, or destruction of contaminants.

American Society for Testing and Materials (ASTM) 3 : Its document titled

Standard Guide for Remediation of Groundwater by Natural Attenuation at Petroleum Release Sites defines natural attenuation as the “reduction in mass or concentration of a compound in groundwater over time or distance from the source of constituents of concern due to naturally occurring physical, chemical, and biological processes, such as biodegradation, dispersion, dilution, adsorption, and volatilization.”

Air Force 4 : The first document, published in 1995, defines the process as resulting

“from the integration of several subsurface attenuation mechanisms that are classified as either destructive or nondestructive Biodegradation is the most important destructive attenuation mechanism Nondestructive attenuation mech-

anisms include sorption, dispersion, dilution from recharge, and volatilization.”

Army 5 : Its report defines natural attenuation as “the process by which

contam-ination in groundwater, soils, and surface water is reduced over time…[via] natural processes such as advection, dispersion, diffusion, volatilization, abiotic and biotic transformation, sorption/desorption, ion exchange, complexation, and plant and animal uptake.”

In the past, the first question to be asked in consideration of the potential fornatural attenuation at a contaminated site was whether biodegradation of the chem-ical contaminant had been reported Oftentimes the question was, “Does the bio-geochemistry exist for ongoing degradation?” due to the assumption that the respon-sible microorganisms are ubiquitous in the subsurface However, in this chapter theterm “natural attenuation” will include all the processes that contribute towards thedecrease in contaminant concentrations

3.2 APPROACHES FOR EVALUATING NATURAL ATTENUATION

Documenting that contaminant concentration has become very low or detectable

in groundwater samples is an important piece of evidence that natural attenuation

is working However, such documentation is not completely sufficient to show thatnatural attenuation is protecting human health and the environment, for three primaryreasons:

• Monitoring of contaminant concentration reductions is not always precise due to the complex nature of groundwater systems In some cases the total contaminant mass may have decreased, but the contaminant may have transformed to another, more hazardous chemical form.

• In a few instances reactions that initially cause contaminants to attenuate may not

be sustainable until reasonable cleanup goals are achieved.

• Another situation of concern occurs when natural biogeochemical parameters, such as electron acceptors and electron donors that support attenuation, are used

up before the treatment of contamination is complete.

For these reasons, environmental regulators and others should not rely on simplerules of thumb (such as maximum contaminant concentration data or trends in thesedata over a relatively short time) in evaluating the potential success of naturalattenuation

The decision to rely on natural attenuation and the confirmation that it willcontinue to work depend on linking monitoring data to a site conceptual model and

“footprints” of the underlying mechanisms Footprints are mappings of concentrationchanges in reactants (contaminant(s), electron acceptors, and donors) or products ofthe biogeochemical processes (such as Cl– ion, dissolved Fe2+) that degrade or

immobilize the contaminants (Figures 3.1a, b, and c) Footprints can be measured

to document that these transformation or immobilization processes are active at thesite An observation of the loss of a contaminant, coupled to observation of a fewfootprints, helps to establish which processes are responsible for the decrease incontaminant mass and concentrations The three basic steps to document naturalattenuation are as follows:

1. Develop a conceptual model of the site: The model should show where and how fast the groundwater flows, where the contaminants are located and at what concentrations, and which types of natural processes could theoretically affect the contaminants (Figures 3.2a and b).

2. Analyze site measurements: Samples of groundwater should be analyzed cally to look for footprints of the natural attenuation processes and to determine whether these processes are sufficient to control the contamination.

chemi-3. Monitor the site: The site should be monitored until regulatory requirements are achieved to ensure that documented attenuation processes continue to occur.

Although the basic steps are the same for all sites, the level of effort needed tocarry out these steps varies substantially with the complexity of the site When sitecharacteristics or the controlling mechanisms are uncertain, it will be difficult todevelop the site conceptual model; thus, a large amount of data will be required todocument natural attenuation In these complex situations, computer modeling may

be necessary, and data on footprints and site characteristics will have to be morethan adequate to develop the model

Figures 3.1a Initial vinyl chloride plume at a landÞll site in Maryland with radial groundwater

ßow from the center of the landÞll.

Figures 3.1b Natural attenuation effects on the vinyl chloride plume Note: The signiÞcant

reduction in vinyl chloride concentration and mass due to natural attenuation.

Figures 3.1c Effects of the primary electron acceptor dissolved oxygen on the attenuation of

VC and Mn along a North-South transect through the middle of the landÞll.

Three-dimensional perspective plot

of observed vinyl choride concentrations

in groundwater -1996

Landfill boundary

500 200 150 100 20 5 1 0

2+

Saprolite

Bedrock

Sand/Gravel

Figures 3.2a A general site conceptual exposure model (adapted from ASTM, 1997).

Current Domestic Water Supply Well

Future Domestic Water Supply Well

Residual NAPL

Abandoned Well?

Current Municipal Water Supply Well Shallow Water Table

©2001 CRC Press LLC

Figures 3.2b Site conceptual exposure models.

Primary Sources

Secondary Sources

Transport Mechanisms

Exposure Routes

Receptors

Chemical Storage

Piping / Distribution

Operations

Waste Management Unit

Soil or Waste Piles

Lagoons or Ponds

Other

ResidentialCommercial/IndustrialConstruction WorkerRelevant Ecological Recepto

ResidentialCommercial/IndustrialConstruction WorkerRelevant Ecological Recepto

ResidentialCommercial/Industrial

Affected Subsurface Soils (>3 ft depth)

Affected Surface Soils (<3 ft depth)

Dissolved Groundwater Plume

Non-Aqueous

Volatilization and Atmospheric Dispersion

AIR Inhalation of Vapor

or Particulates

GROUNDWATER Potable Water Use

SOIL Dermal Contact

or Ingestion

Wind Erosion and Atmospheric Dispersion

Dissolved Groundwater Plume

Leaching and Groundwater Transport Mobile

©2001 CRC Press LLC

3.3 PATTERNS VS PROTOCOLS 3.3.1 Protocols for Natural Attenuation

Within the past few years, many organizations have issued documents providingguidance on evaluating natural attenuation.1 Among the 14 documents developed by

a range of organizations from federal and state agencies to private companies andindustry associations, the available technical protocols address two classes of organiccontaminants only: fuel hydrocarbons and chlorinated solvents (with the exception

of the Department of Energy (DOE) document) A large body of empirical evidenceand scientific and engineering studies in recent years has been developed to supportunderstanding of natural attenuation of these contaminants — mostly fuel hydro-carbons under certain conditions However, the natural attenuation of polycyclicaromatic hydrocarbons, polychlorinated biphenyls, explosives, and other classes ofpersistent organic contaminants is not addressed in any protocol.1 Furthermore,although the DOE document proposes a method for assessing natural attenuationprocesses for inorganic contaminants such as metals, such processes are extremelycomplex, and this document does not adequately reflect this complexity.6

A recent effort was made to compare the guidelines currently available on naturalattenuation against a list of characteristics of a comprehensive protocol.1 The con-sensus was that a comprehensive protocol should cover three broad areas:

• Community concerns: The protocol should describe a plan for involving the affected community in decision making, maintaining institutional controls to restrict use of the site until cleanup goals are achieved, and implementing contin- gency measures if natural attenuation fails to continue as expected.

• Scientific and technical issues: The protocol should describe how to document which natural attenuation processes are responsible for observed decreases in contaminant concentrations, how to assess the site for contaminant source and hydrogeologic characteristics that affect natural attenuation, and how to assess the sustainability of natural attenuation over the long term.

• Implementation issues: The protocol should be easy to follow and should describe the monitoring frequency and various monitoring procedures, in addition to the training and expertise required for the personnel carrying out the field implemen- tation.

None of the current documents fulfills all the criteria defined above.1 To someextent, this reflects the various, and sometimes limited, purposes for which thesedocuments were prepared Some are detailed technical guides; others are intended

to help ensure consistency in site evaluation within a particular organization (such

as a private corporation or a branch of the military), and others are intended to guidepolicy Nonetheless, key gaps in the existing body of protocols have to be addressed.The existing protocols provide little or no discussion of when and how to involvethe public in site decisions and when and how to implement institutional controls

In the few instances where these matters are mentioned, the discussion is typicallybrief, almost in passing Although most environmental regulatory agencies haveseparate policies that specify procedures for community involvement and

institutional controls, these procedures may be inadequate in cases where naturalattenuation is selected as the remedy Discussion of when and how to implementcontingency plans in case natural attenuation does not work is also inadequate inmany of the protocols Further, the protocols do not provide sufficient guidance onwhen and how engineered methods to remove or contain sources of contaminationbenefit natural attenuation.

A major shortcoming of some of the protocols relates to scoring systems used forinitial screening to determine whether a site has potential for treatment by naturalattenuation Such scoring systems yield a numeric value for the site in question If thisvalue is above a certain level, the site is judged an eligible candidate for naturalattenuation Frequently, such scores are used inappropriately as the key factor indeciding whether natural attenuation can be a successful remedy at the site Moreover,these scores often lead to erroneous conclusions about whether natural attenuation will

or will not succeed, due to the complexity of the processes involved and the tendency

of scoring systems to oversimplify them In addition, the scoring systems developedfor evaluating natural attenuation at petroleum sites are erroneously used to evaluatesites with chlorinated solvents by many practitioners of remediation

In summary, the existing body of natural attenuation protocols is limited inseveral important areas.1 Where and how existing protocols can be used to meetregulatory requirements for documenting site cleanup — and whether such protocolsare required at all — is also unclear Guidance on the use of natural attenuation forremediation has to be developed to cover topics not addressed in existing protocolsand to provide for the use of protocols in regulatory programs

3.3.2 Patterns of Natural Attenuation

Instead of relying on protocols and scoring systems, an educated screening toolshould be to observe the patterns in reduction of contaminant concentrations Nat-urally attenuating contaminant plumes can take a variety of forms: they might beexpanding, stable, or shrinking, depending on the trends in the spatial variations ofcontaminant concentrations with time (Figures 3.3a, , and c) Common patterns inall attenuating plumes are a decline in the dissolved contaminant mass with time,and a decline in contaminant concentrations downgradient from the source Oncethese patterns are observed initially, the following list of questions should be devel-oped to collect additional data to develop a platform demonstrating that MNA is anongoing and continuing process to meet the site cleanup objectives:

• What chemical, physical, and biological processes are in effect to support natural degradation of the site-specific contaminants?

• What site biogeochemical conditions are needed for these chemical, physical, and biological processes to work? Which types of site conditions are optimal? Which conditions inhibit natural attenuation?

• What level of information is needed to characterize the site fully?

• What breakdown products that may be more toxic, persistent, or mobile are created when the contaminants degrade? How does one prove that contaminants are degrading into harmless substances?

• What kinds of specific monitoring and testing are needed to determine that the site and the contaminants are suitable for natural attenuation? Is extensive mon- itoring necessary?

• How long is it reasonable to monitor to ensure that natural attenuation is working?

• How viable are institutional controls? Can they be enforced?

• Is stabilization by natural attenuation irreversible for metals or other substances?

3.3.2.1 Various Patterns of Natural Attenuation

Removal of Contaminant Sources: At most contaminated sites, the bulk of thecontaminant mass is in what remediation professionals call “source zones.” Examples

of source zones include landfills, areas of chemical spills, buried tanks that containresidual chemicals, deposits of tars, etc Some of these sources can be easily located

Figures 3.3a Expanding plume.

Cross Sectional View

Contaminated Zone

and complete or partial removal or containment may be possible However, othercommon types of sources often are extremely difficult to locate and remove orcontain One example of a source in this category is chemicals that have sorbed tosoil particles but have the potential to dissolve later into groundwater that contactsthe soil Another extremely important example is the class of organic contaminantsknown as “nonaqueous-phase liquids” (NAPLs) There are two types of NAPLs:those that are more dense than water (dense nonaqueous-phase liquids, or DNAPLs),and those that are less dense than water (light nonaqueous-phase liquids, orLNAPLs) When released to the ground, these types of fluids move through thesubsurface in a pattern that varies significantly from that of the water flow becauseNAPLs have different physical properties than water As shown in Figure 3.4a, b,and c, LNAPLs can accumulate near the water table, DNAPLs can penetrate thewater table and form pools along geologic layers, and both types of NAPLs canbecome entrapped in soil pores These NAPL accumulations contaminate the

Figures 3.3b Stable groundwater plume.

Cross Sectional View

Contaminant plume is almost stationary over time and concentrations at points

within the plume are relatively constant over time with a slight declining trend.

Contaminated

Zone

Monitoring Well

groundwater that flows by them by slow dissolution Common LNAPLs includefuels (gasoline, kerosene, and jet fuel) and common DNAPLs include industrialsolvents (trichloroethene, tetrachloroethene, and carbon tetrachloride) and coal tar.Once they have migrated into the subsurface, NAPLs are often difficult or impossible

to locate in their entirety Normally, the total mass of a contaminant within sourcezones is significantly larger compared to the mass dissolved in the plume Therefore,the source usually persists for a very long time The rate at which contaminantsdissolve from a typical NAPL pool is so slow that many decades to centuries may

be needed to dissolve the NAPL completely by dissolution without any intervention.The potential for success of natural attenuation of various dissolved organic andinorganic compounds is presented in Table 3.1

Given the persistent nature of contaminant sources, removing them would seemlike a practical way to speed natural attenuation of the contaminant plume (Figure3.4) In many cases, environmental regulators require source removal or containment

Figures 3.3c Shrinking groundwater plume.

Cross Sectional View

Contaminant plume is receding back toward the source area over time and the

concentrations at points within the plume are declining over time.

Contaminated

Zone

Monitoring Well

Table 3.1 The Potential for Success of Natural Attenuation for Various Compounds

(adapted from NCR, 2000)

Likelihood of Success Organic

Hydrocarbons

Oxygenated Hydrocarbons

Alcohols, ketones, esters

Chlorinated Aliphatics

transformation

Moderate to High

as part of a natural attenuation remedy Although requiring source control or removal

is good policy for many sites, expert opinions conflict on whether source removal

is advisable when using natural attenuation as a remedy, even when such removal

is technically feasible

Nonmetals

Oxyanions

Figures 3.4 Various possibilities of source zone contamination.

Table 3.1 The Potential for Success of Natural Attenuation for Various Compounds

(adapted from NCR, 2000) (continued)

Likelihood of Success

a) Not Enough Mass Spilled to Form an NAPL

t1

t3

t4t2

DNAPL Release

MNAPL MDISS t1

t3

t4t2

Goals of source removal should be the following:

• Remove as much contaminant mass as practical to reduce the mass flux of taminants emanating from the source zone, thus reducing the concentration of the contaminant plume rapidly and also reducing the longevity of the required mon- itoring period; and

con-• Avoid any changes that would reduce the effectiveness of natural attenuation, such

as disturbing the natural dissolution equilibrium from an NAPL source by drilling through it and thus increasing the mass flux.

In theory, if one can delineate the source completely and succeed in removing most

of the mass, then a significant benefit may be achieved There are many case studiesavailable in the literature even for compounds like polycyclic aromatic hydrocarbons(PAHs) plumes in which it appears that, after removal of the source, the plumesattenuated rapidly However encouraging this example might be, this kind of successmay not always be realized Particularly, DNAPL sources in fractured bedrock envi-ronments cannot be delineated completely and/or cannot be removed to any significantdegree at a reasonable cost Hence, source removal options may be rejected becausenone are anticipated to be able to warrant the expense and risks of the removal effort

by removing all of the source mass without leaving a significant level of residual mass

In some cases, source removal efforts may directly and adversely affect naturalattenuation Most of the negative impacts will be caused mainly by the disturbance

of the equilibrium between the moving groundwater and the quiescent mass ofNAPL, particularly DNAPL As a precautionary measure, an outside-in approach toinvestigating the source zone is recommended in contrast to an inside-out approach.Consideration should be given when looking at removal of the source of onetype of contaminant which may adversely affect natural attenuation of another typeand thus result in minimal or no overall benefit A good example is the removal of

a petroleum hydrocarbon source zone serving as a nutrition source for microbesinvolved in degrading a chlorinated solvent plume Such an action could slow down

or completely shut off natural attenuation of the chlorinated solvent

Natural Attenuation Capacity (NAC): The manner in which natural attenuationand active remediation measures (such as source removal, pump and treat, chemicaloxidation, or enhanced bioremediation) are combined depends on the natural atten- uation capacity (NAC) of the system If the NAC is small, for example, activeremediation measures will need to remove or degrade a high proportion of thecontaminant source to protect downgradient receptors Conversely, if the NAC islarge, less source removal may be required to protect downgradient receptors Ineither case, it is necessary to quantify the NAC of the biogeochemical system tocombine contaminant source-removal methods with natural attenuation effectively.Natural attenuation capacity is a concept that refers to the capacity of a bio-geochemical system to lower contaminant concentrations along aquifer flow paths.The NAC of groundwater systems depends on hydrogeologic (dispersion and advec-tion) and biological (biodegradation rates) factors for organic contaminants andprecipitation potential also for heavy metals

The concept of NAC is useful because it illustrates those characteristics andparameters of a groundwater system that affect the efficiency of natural attenuation.7

For example, if the biodegradation rate constant is small (@ 0.001 d–1) relative tothe groundwater velocity (~3 ft/day) and aquifer dispersivity (30 feet), the NAC ofthe system also will be small Because of this small NAC, contaminants will betransported relatively long distances downgradient of the source area (Figure 3.5a).Conversely, if the biodegradation rate is high relative to groundwater velocity andaquifer dispersivity, the NAC will be proportionally higher, and the transport ofcontaminants will be restricted closer to the source area

Quantitative mathematical techniques in addition to empirical methods are able to estimate NAC In addition to NAC, the distance that contaminants aretransported in a groundwater system also depends on the contaminant concentrations

avail-at the source area (Figure 3.5b)

Figure 3.5a The effect of natural attenuation capacity on contaminant transport 7

Figure 3.5b The effect of source area concentrations on the distance required to reach

cleanup standards.

High NAC Moderate NAC

Very Low NAC

Distance Along Flow Path

Distance Along Flow Path

High concentration, low NAC

3.4 PROCESSES AFFECTING NATURAL ATTENUATION OF

COMPOUNDS 3.4.1 Movement of Contaminants in the Subsurface

Even in the absence of biotic and/or abiotic transformations of a contaminant,the contaminant always is subject to transport processes — meaning that physicalprocesses cause it to move All important transport processes for subsurface con-taminants can be categorized as dilution, advection, dispersion, or “phase transfer”(from one type of physical medium to another, such as from an NAPL to groundwater

or from water to the soil matrix)

3.4.1.1 Dilution (Recharge)

Recharge is the amount of water entering the saturated zone of the water table

at the water table surface, made available mainly by precipitation events In rechargeareas, flow near the water table is generally downward Recharge defined in thismanner may therefore include not only precipitation that infiltrates through thevadose zone, but also water entering the groundwater system via discharge fromsurface water bodies Where a surface water body is in contact with or is part of thegroundwater system, the definition of recharge is stretched slightly However, suchbodies often are referred to as recharging lakes or streams.8 The recharge of thewater table aquifer has two effects on the natural attenuation of a dissolved contam-inant plume: 1) additional water entering the system due to infiltration of precipita-tion or from surface water will contribute to dilution of the plume and 2) the influx

of relatively fresh, electron-acceptor-charged water will alter the geochemical cesses and in some cases, facilitate additional biodegradation.8,9

pro-Recharge from infiltrating precipitation is the result of a complex series ofprocesses in the unsaturated zone Description of these processes is beyond the scope

of this chapter; however, it is worth noting that the infiltration of precipitation throughthe vadose zone brings the water into contact with the soil and thus may allow theintroduction of electron acceptors (such as NO3 and SO 42– ) in addition to the DO inthe recharge water and also dissolved organic carbon (electron donor) Infiltrationtherefore provides fluxes of water, inorganic species, and possibly organic speciesinto the groundwater In the case of surface water it may be connected as part ofthe groundwater system, or it may be perched above the water table In either case,the water entering the groundwater system will not only aid in dilution of a con-taminant plume, but it may also add electron acceptors and possible electron donors

to the groundwater

An influx of electron acceptors will tend to increase the overall assimilationcapacity of the groundwater system In addition to the introduction of electronacceptors that may be dissolved in the recharge (e.g., dissolved oxygen, nitrate, orsulfate), the infiltrating water may also foster biogeochemical changes in the aquifer.For example, Fe2+ will be oxidized back to Fe3+ and will be precipitated out Thisreprecipitation of Fe3+ could be again available for reduction by microorganisms.Such a shift may be beneficial for biodegradation of contaminants utilized as electron

donors, such as fuel hydrocarbons or vinyl chloride However, these shifts can alsomake conditions less favorable for reductive dechlorination.

Evaluating the effects of recharge can be difficult The effects of dilution might

be estimated if one has a detailed water budget for the system in question However,

if a plume has a significant vertical extent, it cannot be known with any certaintywhat proportion of the plume mass is being diluted by the recharge In addition,separating the effects of dilution from other processes of mass reduction may bedifficult After recharge, the effects of the addition of electron acceptors may beapparent due to elevated electron acceptor concentrations, differing patterns in elec-tron acceptor consumption, or by-product formation in the area of recharge How-ever, the effects of short-term variations in such a system (which are likely due tothe intermittent nature of precipitation events in most climates) may not easily bequantified Where recharge is from surface water, the influx of mass and electronacceptors is more steady over time In this scenario, quantifying the effects of dilutionmay be less uncertain, and the effects of electron acceptor replenishment may bemore easily identified (although not necessarily quantified)

In some cases the effects of recharge-diluting contaminant plumes can be mated with a simple relationship based on the specific discharge of groundwaterpassing through the point of interest and the amount of recharge entering the plumearea It is imiportant to note that at most sites, recharge will not actually mix withgroundwater in an aquifer but will form a stratified layer on top due to the very lowamount of vertical dispersion characteristic of aquifer systems Mixing can beassumed in some cases, such as a very thin, unconfined aquifer: the aquifer dis-charges into a surface water body, and the groundwater associated with the recharge

esti-is assumed to be mixed with the original groundwater flowing past a source zone.8-10

The relationship for estimating the amount of dilution caused by recharge is

(3.1)

Eliminating the width and rearranging gives:

(3.2)

where

CL = concentration at distance L from origin assuming complete mixing of

recharge with groundwater (mg/L)

C0 = concentration at origin or at distance L = 0 (mg/L)

R = recharge mixing with groundwater (ft/yr)

W = width of area where recharge is mixing with groundwater (ft)

L = length of area where recharge is mixing with groundwater (ft)

dcdt

dcdt

VD = Darcy velocity of groundwater (ft/yr)

Th = thickness of aquifer where groundwater flow is assumed to mix

com-pletely with recharge (ft)

3.4.1.2 Advection

Transport of a contaminant molecule occurring with the groundwater movement

is called advection or convection or bulk flow Advection occurs in any moving fluid.Thus, contaminants can advect when they are in air in soil pores or in a movingNAPL, as well as in water Advection transport is illustrated simply by considering

a contaminant that does not react biotically or abiotically (also known as conservativecompound or tracer) in the subsurface and that moves at the average velocity of thegroundwater Figures 3.6a and b describe this phenomenon The contaminant moves

at exactly the same velocity as the water and does not change from its initialconcentration of C0¢ at the injection point.9

Figure 3.6a Dispersion of a pulse of a tracer substance in a sand column experiment.

Figure 3.6b Concentration curves showing plug ßow with an instantaneous source from

advection only and from a combination of advection, dispersion, and sorption.

Distance (x)t

Advection and Dispersion

Advection, Dispersion, and Sorption

Advection Only Advection

The mass flux rate at which a dissolved contaminant moves across a vertical

plane in the subsurface is the product of the contaminant concentration and the

velocity of groundwater Groundwater velocity is governed by three key factors

specific to each site:

• The hydraulic gradient includes gravity and pressure components and is the

driving force for water movement Water always moves in the direction of higher

hydraulic head (which can be thought of qualitatively as elevation) to lower head.

• Hydraulic conductivity is the ability of porous rocks or soil sediments to transmit

fluids and is measured from field tests or samples Hydraulic conductivity values

for common rocks and sediments vary over ten orders of magnitude from almost

impermeable crystalline rocks to highly permeable gravels; the hydraulic

conduc-tivity values for fractured rocks, sand, and clay are between these extremes A

contaminant plume moving with the groundwater will travel faster through sand

layers, which have high hydraulic conductivity, than through clays of low

hydrau-lic conductivity, under the same hydrauhydrau-lic head gradient.

• Porosity is a measure of the volume of open spaces in the subsurfaces relative to

the total volume Like hydraulic conductivity, it depends on the type of geologic

material present and can be determined from field tests or samples.

The equation for describing the rate of groundwater flow from one location to

another is known as Darcy’s equation:

(3.3)where

KH = hydraulic conductivity (in units of distance per time)

= hydraulic gradient

VD = Darcy velocity (in units of distance per time)

To determine the seepage velocity of a contaminant that travels at the same speed

as the groundwater, the Darcy velocity must be divided by the effective porosity e:

(3.4)

KH and e can be estimated using various field test methods or laboratory evaluations

of cores taken from the subsurface Uncertainty is inherent in all such measurements,

and this uncertainty must be acknowledged by developing a range of possible flow

3.4.1.3 Dispersion

Spreading of contaminants from the main direction of groundwater flow takes

place as the groundwater moves, altering concentrations from those that would occur

if advection were the only transport mechanism This mixing is called hydrodynamic

dispersion The mechanisms causing dispersion within the plume include molecular

diffusion, different water velocities within individual pores, different water velocities

between adjacent pores, and tortuosity of the subsurface flow path (Figure 3.7)

Mixing caused by local variations in velocity is also known as mechanical dispersion

Groundwater scientists quantify the combined mixing effect using a hydrodynamic

dispersion coefficient DH Except at very low water velocities, DH increases linearly

with the average speed of groundwater

The curve labeled “dispersion” in Figure 3.6 a and b illustrates the effects of

dispersion for a conservative contaminant that travels precisely with the water

mol-ecules The solute is detected at the observation well before it would be if advection

were the only process affecting its movement Dispersion causes the solute to spread,

rather than moving as an unchanged “plug.”

Molecular Diffusion: Molecular diffusion takes place as a result of the

contam-inant gradients created within the zones of contamination It is significant only when

the groundwater velocities are low, and the diffusive flux of a dissolved contaminant,

at steady state, can be described by Fick’s first law

(3.5)where

F = mass flux of solute per unit area of time

D = diffusion coefficient

C = solute concentration

= concentration gradient

Figure 3.7 Seemingly random variations in the velocity of different parcels of groundwater

are caused by the tortuous and variable route the water must follow.

Flow Direction Average Water C'

B' A'

For systems where the dissolved contaminant concentrations are changing with

time, Fick’s second law must be applied The one-dimensional expression of Fick’s

second law is

(3.6)

where, is the change in concentration with time

The process of diffusion is slower in porous media than in open water because

the contaminant molecules must follow more tortuous flow paths To account for

this, an effective diffusion coefficient D* is used Fetter estimates a range of 1 ¥

10–9 to 2 ¥ 10–9 m2/S for D* has been estimated.9(a)

The effective diffusion coefficient is expressed quantitatively as

D* = wD (3.7)where w is the empirical coefficient determined by laboratory experiments The

value of w ranges greatly from 0.01 to 0.5.9

Mechanical Dispersion: Mechanical dispersion occurs due to variations in flow

velocity because of varying pore throat sizes and tortuosity caused by variations in

flow path lengths An additional cause of mechanical dispersion is variable friction

within an individual pore, thus allowing the groundwater flowing in the center of

the pore to move faster than groundwater flowing next to the soil particle itself

The component of hydrodynamic dispersion contributed by mechanical

disper-sion can be described as:

where

µx = dispersivitiy

V = seepage velocity

Advection dispersion equation: The advection-dispersion equation, which

includes hydrodynamic dispersion, can be described as:8,9

(3.9)where

2 2

3.4.2 Phase Transfers

Contaminants will be added or removed from the groundwater when they transferbetween phases The relevant phases in the subsurface are groundwater (dissolved),soil grains (adsorbed), NAPLs (liquid), and soil gas (air) in the vadose zone Phasetransfers can increase or decrease the contaminant concentration within the ground-water plume, depending on the transfer mechanism, the contaminant, and thegeochemistry Although the basic concepts of phase transfer are straightforward,quantification of these transfers often is not easy

3.4.2.1 Sorption

Many contaminants, including chlorinated solvents, BTEX and dissolved metals,are removed from solution by sorption onto the aquifer matrix, thus slowing themovement of contaminants This slowing of contaminant transport is called retardation

of the contaminant relative to the average seepage velocity of groundwater and results

in a reduction in dissolved organic concentrations in groundwater Sorption can alsoinfluence the relative importance of volatilization and biodegradation Figure 3.6b

illustrates the effects of sorption on an advancing dissolved contaminant front.Sorption is a dynamic and reversible reaction; thus, at a given solute concentration,some portion of the contaminant is partitioning out of solution onto the aquifer matrix,and some portion is desorbing and reentering solution As solute concentrations change,the relative amounts of contaminant that are sorbing and desorbing will change Forexample, as solute concentrations decrease due to other factors such as biodegradationand dilution, the amount of contaminant reentering solution will probably increase.The affinity of a given compound for the aquifer matrix will not be sufficient to isolate

it permanently from groundwater, although for some compounds the rates of desorptionmay be so slow that the adsorbed mass may be considered as permanent residualwithin the time scale of interest Sorption, therefore, does not permanently removesolute mass from groundwater; it merely retards migration

The various mechanisms that cause sorption effects to take place within theaquifer matrix are described in detail in Chapter 2 Because of their nonpolarstructure, hydrocarbons most commonly exhibit sorption through the process ofhydrophobic bonding When the surfaces comprising the aquifer matrix are lesspolar than the water molecule, as is generally the case, there is a strong tendencyfor the nonpolar contaminant molecules to partition from the groundwater and sorb

to the aquifer matrix This phenomenon, referred to as hydrophobic bonding, is an

important factor controlling the fate of many organic pollutants in soils As described

in Chapter 2, two components of an aquifer have the greatest effect on sorption:organic matter and clay minerals In most aquifers, the organic fraction tends tocontrol the sorption of organic contaminants

Sorption Models and Isotherms: Regardless of the sorption mechanism, it is

possible to determine the amount of sorption to be expected when a given dissolvedcontaminant interacts with the materials comprising the aquifer matrix Bench-scaleexperiments are performed by mixing water-contaminant solutions of various con-centrations with aquifer materials containing various amounts of organic carbon and

clay minerals The solutions are then sealed with no headspace and left until librium between the various phases is reached (True equilibrium may require hun-dreds of hours of incubation, but 80 to 90% of equilibrium may be achieved in one

equi-or two days.) The amount of contaminant left in solution is then measured.The results are commonly expressed as a plot of the concentration of chemicalsorbed (mg/g) vs the concentration remaining in solution (mg/L) The relationshipbetween the concentration of chemical sorbed (Ca) and the concentration remaining

in solution (Cs) at equilibrium is referred to as the sorption isotherm because the

experiments are performed at constant temperature (Figure 2.11) Sorption isothermsgenerally exhibit one of three characteristic shapes, depending on the sorptionmechanism: the Langmuir isotherm, the Freundlich isotherm, and the linear isotherm(a special case of the Freundlich isotherm)

Retardation: As mentioned earlier, sorption tends to slow the transport velocity

of contaminants dissolved in groundwater When the average velocity of a dissolvedcontaminant is less than the average seepage velocity of the groundwater, the con-

taminant is said to be retarded The coefficient of retardation, R, is used to estimate

the retarded contaminant velocity The variation between the velocity of the water and that of the contaminant is caused by sorption and is quantified by thecoefficient of retardation, defined as:

ground-(3.10)where

R = coefficient of retardation

V = average seepage velocity of groundwater parallel to groundwater flow

Vc = average velocity of contaminant parallel to groundwater flow

The ratio (V/Vc) describes the relative velocity between the groundwater and thedissolved contaminant When Kd = 0 (no sorption), the transport velocities of thegroundwater and the solute are equal (V/Vc) If it can be assumed that sorption isdescribed adequately by the distribution coefficient (valid when the fraction oforganic carbon (foc) > 0.001), the coefficient of retardation for a dissolved contam-inant is described by the following equation:9

(3.11)where

The bulk density, rb, of a soil is the ratio of the soil mass to its field volume.Bulk density is related to particle density by the following equation:

where n is the total porosity and rs is the density of soil grains comprising theaquifer In sandy soils, rb can be as low as 1.81g/cm3 In aggregated loams andclayey soils, rb can be as low as 1.1g/cm3

The sorption relationship shown above expresses the coefficient of retardation

in terms of the bulk density and effective porosity of the aquifer matrix and thedistribution coefficient for the contaminant Substitution of this equation into Equa-tion 3.10 gives

Two methods are used to estimate the distribution coefficient and amount ofsorption (and thus retardation) for a given aquifer-contaminant system The firstmethod involves estimating the distribution coefficient by using Koc for the contam-inants and the fraction or organic carbon comprising the aquifer matrix The secondmethod involves conducting batches of column tests to determine the distributioncoefficient Because numerous authors have conducted experiments to determine Kocvalues for common contaminants, literature values are reliable, and it generally isnot necessary to conduct laboratory tests.9

VV

Knc

3.4.2.2 Stabilization

The transfer of an organic compound from an NAPL source to the surroundingwater increases the contaminant concentration in groundwater The rate of transfervaries depending on the type of NAPL Computation of this transfer rate can becomplex because the transfer rate depends on chemical properties of the contaminantand the NAPL, as well as on resistance at the interface between the water and theNAPL.11 Diffusion of the contaminant within the NAPL itself also can affect thetransfer rate for viscous NAPLs

DNAPLs: Dense nonaqueous phase liquids (DNAPLs) present in the form of

residual (held under capillary forces) or free phase (mobile) product may result incontinued long-term contamination of the surrounding groundwater The marginallysoluble organic contaminants can partition into the aqueous phase at rates slowenough to continue to exist as a nonaqueous phase, yet rapid enough to causesignificant groundwater contamination DNAPLs can migrate to depths well belowthe water table As they migrate, they can leave behind trails of microglobules inthe pore spaces of the soil matrix, which effectively serve as long-term sources ofgroundwater contamination

Current conceptual DNAPL transport models suggest that, when sinking freephase DNAPL encounters a confining layer (e.g., competent clay or bedrock zone),

it can accumulate, or “pool,” and spread laterally until it encounters a fracture or analternative path of relatively low flow resistance towards deeper zones.11 In addition,globules can enter pores and be held as a residual phase in capillary suspension.This complex mode of subsurface transport results in unpredictable heterogeneousdistribution of nonaqueous product that is difficult to delineate

The current lack of appropriate methods for detecting and delineating widelydispersed microglobules of DNAPL has been identified as one of the most significantchallenges today Investigative techniques that have been used to identify DNAPLsource zones are listed below It should be noted that some of those techniques arewell proven and extensively field tested, while others are considered relatively new.12

• Soil gas surveys

• Visual evidence of soil, rock and/or groundwater samples

• Chemical analyses of soil, rock and/or groundwater samples

• Enhanced visual identification — shake tests

• Enhanced visual identification — UV fluorescence with portable light, dye tion with Sudan IV or Oil Red O

addi-• Accumulation within monitoring wells at target locations

• Partitioning interwell tracer tests

• Backtracking using dissolved concentrations in wells (the 1% rule)

• Surface geophysics

• Subsurface geophysics

• Cone penetrometer testing (CPT) methods:

• Permeable membrane sensor, membrane interface probe (MIP)

• Hydrosparse

• Laser induced fluorescence (LIF) techniques

• GeoVis

• Raman spectroscopy

• Electrochemical sensor probe

• Cosolvent injection/extraction technique

• Precision injection/extraction (PIX) technique

• Flexible liner underground technologies everting (FLUTE) membrane technique

It is important to recognize that each of the methods listed presents specificadvantages and disadvantages and applicability will be determined by technical andeconomic challenges encountered at each site Several methods can be complemen-tary in an overall site management plan, and a hybrid approach could be developed

to exploit the strengths of the different techniques at the most appropriate and logicaltimes in the site management process For example, one can initially screen a sitewith a laser induced fluoroscence (LIF) technique or with geophysical techniques,then analyze confirmation soil samples in the field visually, with Sudan IV dye, and

in the laboratory for chemical constituents After determining the location of theDNAPL source zone, discreetly screened or multilevel wells can be installed formonitoring and remediation

CPT and/or geophysical techniques, integrated with minimally intrusive directpush technologies, can provide the framework for development of the conceptualsite model Then the refined conceptual site model integrated with hydrogeologicconsiderations can be used for guidance on a sampling plan to define the spatialextent of the contamination

3.4.2.3 Volatilization

Volatilization reduces the total mass of the contaminant in the groundwatersystem The potential for volatilization is expressed by the contaminant’s Henry’sLaw Constant and described in detail in Chapter 2 Henry’s Law Constants arewidely available for common volatile contaminants (see Appendix A) Although not

a destructive mechanism, volatilization does not remove contaminants from water In addition to Henry’s Law Constant, other factors affecting the volatilization

ground-of contaminants from groundwater include the contaminant concentration, thechange in contaminant concentration with depth, diffusion coefficient of the com-pound, temperature, and sorption Because the soil gas often advects and dispersionalso occurs in the gas phase, contaminants transferred to the soil gas often migrateaway from the location at which they volatilize Volatilization itself does not destroycontaminant mass or permanently immobilize it Volatilized contaminants can bio-degrade in some circumstances but also can redissolve in infiltrating groundwater

or be transported to the surface, where humans may be exposed to the vapors

3.4.3 Transformation Mechanisms

A variety of reactions transform contaminants The possible reactions are calledbiogeochemical: all are chemical (prefix chem) and occur in a geological setting(prefix geo), but some are catalyzed by microorganisms (prefix bio) Some bio-geochemical reactions can degrade or transform a contaminant into benign and

harmless end products or immobilize it permanently A contaminant transformed orimmobilized in these ways no longer contributes to groundwater contamination.Although other reactions do not directly lead to such positive results, they can controlwhether or not the transformation or immobilization reactions take place Often, asuite of chemical reactions (termed a reaction network) leads to contaminant trans-formation or immobilization In other instances, the reaction network prevents thecontaminants from being transformed or immobilized and may make natural atten-uation an ineffective remediation strategy.

3.4.3.1 Biodegradation

Microorganisms can cause major changes in the chemistry of groundwater Theirsmall size and adaptability, as well as the diversity of nutritional requirements fordifferent microbes, enable them to catalyze a wide range of reactions that often arethe basis for natural attenuation Chemical changes brought about by microorganismscan directly or indirectly decrease the concentrations of certain groundwater con-taminants Microorganisms use enzymes to accelerate the rates of certain biochem-ical reactions The most important reactions are reductions and oxidations, togetherknown as REDOX reactions The reactions involve transfer of electrons from onemolecule to another, which allow the microorganisms to generate energy and grow(Figure 3.8) More discussions on REDOX reactions and microbial electron transfersare provided in Chapters 2 and 4

Microorganisms reproduce by organizing chemical reactions that create daughtercells composed of cellular components (e.g., membranes, proteins, deoxyribonucleicacid [DNA], cell walls) derived from building blocks that they synthesize or scavengefrom the environment.1 The chemical reactions are made possible by enzymes —protein molecules that bring together the chemicals in a way that allows them toreact quickly (Figure 3.9) The reactions are driven to completion by the expenditure

of cellular energy in the form of a chemical known as adensoine triphosphate (ATP),

Figure 3.8 Conceptual description of microorganisms gaining energy and utilizing the

sub-strate for growth.

Energy

Electrons Electrons and Carbon

2

Electron Acceptor (e.g., O )

New Cells

+

Organic

Contaminant

which can be thought of as a cellular fuel Like all living organisms, microorganismsgenerate ATP by catalyzing redox reactions: they transfer electrons from electron-rich chemicals to electron-poor chemicals The technical term for the electron-richchemical is electron donor substrate As an analogy, human metabolism involvestransfer of electrons from chemicals derived from ingested food (the donor substrate)

to oxygen (the acceptor substrate) inhaled from the air.1

When cells remove electrons from the donor substrate, they do not transfer theelectrons directly to the acceptor substrate Instead, they transfer the electrons tointernal electron carriers as shown in Figure 3.9 Although electrons held by thecarriers can be used for many purposes, the major purpose is to generate ATP through

a process called respiration In respiration, the electrons are passed from carrier tocarrier until they reach the electron-acceptor substrate Since this is the last molecule

to receive the electrons, it is called the terminal electron acceptor The need for ATPproduction forces all microorganisms to have one or more electron-donor and elec-tron-acceptor pairs, and these materials largely define the metabolism of individualmicroorganisms The amount of energy yielded varies depending on the electrondonor and electron acceptor used

Figures 3.9 Conceptual diagram of microbial activity to derive energy for growth and

multiplication (adapted from NRC, 2000).

Reduced Acceptor Product

Synthesis and Maintenance

Ideally, all biologically mediated reactions produce energy for microbial growthand reproduction Biologically mediated electron transfer results in oxidation of theelectron donor, reduction of the electron acceptor, and the population of usableenergy (quantified by the Gibbs free energy of the reaction DGvo ) Table 3.2 presents

a few select electron acceptor and electron donor reactions and calculated DGyvalues.9 Negative values indicate an energy-producing reaction, otherwise called anexothermic reaction, and will proceed from left to right The value of DGyo can beused to estimate how much free energy is consumed or produced during the reaction.Positive values indicate an endothermic reaction; for the reaction to proceed fromleft to right energy must be put into the system Microorganisms will not investmore energy into the system than can be released and must couple an endothermicwith an exothermic reaction to derive energy and grow

Collectively, microorganisms can use a wide range of electron donors, includingboth organic and inorganic chemicals Electron acceptors are more limited Commonelectron acceptors include O2, NO3– , NO2– , SO42– , CO2, Fe(III), and Mn(IV) Oxygenhas a special status because of its importance in many environments and reactions.Microbial use of oxygen as an electron acceptor is called aerobic metabolism; micro-bial use of electron acceptors other than oxygen is called anaerobic metabolism.When biotransformation of a particular contaminant leads directly to energygeneration and the growth of more microorganisms, the contaminant is known as aprimary substrate (see Figure 3.8) However, the reactions that lead to microbialmetabolism of contaminants may not be part of cell-building or energy-generatingreactions An important category of such biotransformations is cometabolism Come-tabolism is the fortuitous degradation of a contaminant when other materials areavailable to serve as microorganisms’ primary substrates Cometabolic reactionsoften occur because the enzymes designed for metabolizing primary substratesfortuitously transform the cometabolic substrate

It is important to note the historic debate on the use of the word cometabolism for

the microbially catalyzed process described above.13,14 One school of thought, gated by classical microbiologists, insists that usage of either the term cometabolism

propa-or the term cooxidation to describe conversions of nongrowth substrates by liferating microbial populations in the absence of a metabolizable cosubstrate would

nonpro-be inappropriate The enzymatic conversion of a substrate by a nonproliferating bial population because an enzyme of broad specificity and conversion capability is

micro-in proximity to the substrate might at best be described as bioconversion There is no

co- (with or together) activity concerned with such an event.

First-Order Decay Model: One of the most commonly used expressions for

representing the biodegradation of an organic compound involves the use of anexponential decay relationship:

Table 3.2 Half-Cell Reactions for Some of the Common

Electron Acceptors and Donors (adapted from Wiedemeier et al., 1999)

Half-Cell Reaction

DDDDG r o (kcal/mol e – )

Hydrogen oxidation

1 / 4 CH 2 O + 1 / 4 H 2 O Þ 1 / 4 CO 2 + H + + e – –10.0 Carbohydrate oxidation

Benzene oxidation

14 H 2 O + C 6 H 5 CH 3 Þ 7CO 2 + 36H + + 36e – –6.9 Toluene oxidation

Naphthalene oxidation

4H 2 O + C 2 H 3 Cl Þ 2CO 2 + 11H + + 10e – + Cl – –11.4 Vinyl chloride oxidation

12H 2 O + C 6 H 5 Cl Þ 6CO 2 + 29H + + 28e – + Cl – –8.0 Chlorobenzene oxidation

First-order rate constants are often expressed in terms of half-life for thechemical:

(3.17)

The first-order decay model shown in Equation 3.16 assumes that the solutedegradation rate is proportional to the solute concentration The higher the concen-tration, the higher the degradation rate This method is usually used to simulatebiodegradation of contaminants dissolved in groundwater Modelers using the first-order decay model typically use the first-order decay coefficient as a calibrationparameter and adjust the decay coefficient until the model results match the fielddata With this approach, uncertainties in a number of parameters (e.g., dispersion,sorption, biodegradation) are lumped together in a single calibration parameter.Regression methods are commonly used to obtain approximations of site-specificdegradation rates (first-order) from log-linear plots of concentration vs time Thisinvolves fitting an exponential regression to approximate the trend in the data Thistype of approximation can be used to evaluate trends at an individual well or forseveral wells along a flow path When individual wells are being evaluated, theanalytical data should be used from multiple sampling events, and the time element

in the plot represents the temporal arrangement of the data When multiple wellsalong a flow path are being evaluated, the analytical data from a single samplingevent can be used; the time element in the plot represents groundwater travel timebetween the wells.15

Electron-Acceptor-Limited or Instantaneous Reaction Model: The

electron-acceptor-limited model (traditionally called the instantaneous reaction model) was first

proposed in 1986 for simulating the aerobic biodegradation of petroleum bons.9,16 It was observed that microbial biodegradation kinetics are fast in comparisonwith the transport of oxygen and that the growth of microorganisms and utilization ofoxygen and organics in the subsurface can be stimulated as an electron-acceptor-limited

hydrocar-or instantaneous reaction between the hydrocar-organic contaminant and oxygen

From a practical standpoint, the instantaneous reaction model assumes that therate of utilization of the contaminant and oxygen by the microorganisms is veryhigh, and that the time required to biodegrade the contaminant is very short, almostinstantaneous, relative to the seepage velocity of the groundwater Using oxygen as

an electron acceptor, for example, biodegradation is calculated using the expression:

(3.18)where

DCR = change in contaminant concentration due to biodegradation

O = concentration oxygen

F = utilization factor, the ratio of oxygen to contaminant consumed

tk

The variable F is obtained from the oxidation-reduction reaction involving theorganic and the given electron acceptor.

Biodegradation of Organic Contaminants: Organic contaminants vary widely

in their susceptibility to transformation by microorganisms Some contaminants arehighly biodegradable, while others resist degradation In general, the more degrad-able contaminants have simple molecular structures (often similar to the structures

of naturally occurring organic chemicals), are water soluble and nontoxic, and can

be transformed by aerobic metabolism (Figure 3.10) In contrast, organic nants that resist biodegradation may have complex molecular structures (especiallystructures not commonly found in nature), low water solubility or an inability tosupport microbial growth, or they may be toxic to the organisms

contami-Microorganisms can completely convert some organic contaminants to carbondioxide and water, while they are capable of only partial conversions of others.Complete conversion to carbon dioxide is called “mineralization.” In some cases,the products of partial conversion are more toxic than the original contaminant Vinylchloride is an example of a highly toxic chemical that results from incompletebiodegradation of chlorinated solvents

The following discussion explains how microbial transformations occur for ious organic contaminant classes It describes all of the elements of some metabolicpathways because these illustrate the core concepts of biodegradation Biodegrada-tion pathways for most contaminants are extremely complex, so these pathways arenot described in detail

var-Petroleum hydrocarbons are a highly varied class of naturally occurring chemicalsused as fuels in a variety of commercial and industrial processes Biodegradationpotential varies depending on the type of hydrocarbon

Benzene, Toluene, Ethylbenzene, and Xylene (BTEX): Benzene, Toluene,

Ethylbenzene, and Xylene are components of gasoline Because of their widespreaduse and because BTEX storage tanks commonly leaked in the past, BTEX arecommon groundwater contaminants A large body of scientific research exists onthe biodegradation and natural attenuation of BTEX However, the effectiveness of

Figure 3.10 Schematic diagram describing the mechanisms by which a contaminant

becomes available for biodegradation.