Environmental Fluid Mechanics - Chapter 16 (end) doc

16.2 SOIL AND AQUIFER REMEDIATION16.2.1 General Aspects In cases of soil and aquifer contamination, the source of contaminant is usuallylocated above ground, or in a shallow depth below

In general, the initial phase of remediation involves controlling inant disposal into the particular environment under consideration This maymean complete cessation of all source loadings or at least reduction of loads

contam-to a suitable level contam-to allow recovery of the system, either by natural or byengineered processes According to the type of environment and the timeand length scales of the problem, the remediation may include containmentand treatment Most phases of environmental remediation require an under-standing of fluid flow and transport phenomena by advection and diffusion,

as discussed in previous chapters of this text Remediation strategies alsomay incorporate the use of chemical agents and biodegradation of contami-nants Methodologies that have been developed and used for many decadesfor water and wastewater treatment can often be adapted for environmentalremediation

16.2 SOIL AND AQUIFER REMEDIATION

16.2.1 General Aspects

In cases of soil and aquifer contamination, the source of contaminant is usuallylocated above ground, or in a shallow depth below the ground surface The

contaminant penetrates first to the vadose zone, and later it reaches the

ground-water Therefore the issue of aquifer and groundwater contamination is usuallyassociated with site contamination On the other hand, if the contaminant pene-tration is limited to the vadose zone, then site contamination is not necessarilyaccompanied with aquifer and groundwater contamination.Figure 16.1shows

a schematic description of site contamination, which originates from a typicallandfill

In addition to landfills, other sources of groundwater contaminationinclude spills of soluble substances, which become completely sorbed in thevadose zone and then are gradually released by percolating runoff water

An important category of potentially spilled materials includes a variety ofhydrocarbons, such as oils and fuels, and these are collectively known as

nonaqueous phase liquids (NAPLs) When a NAPL has a density less than

that of water, it is referred to as a light nonaqueous phase liquid, or LNAPL When NAPL is denser than water, it is called dense nonaqueous phase liquid

(DNAPL) When LNAPLs are released at the soil surface, they percolate

through the vadose zone and eventually float on top of the groundwater tableand the capillary zone, while gradually releasing dissolved hydrocarbon intothe flowing groundwater DNAPLs sink through the water layer and rest onthe bottom of the aquifer, except for material that may be adsorbed onto soils,

Figure 16.1 A typical site contamination originating from a landfill.

either in the vadose zone or in the water layer itself LNAPLs also may sorbonto soils.

Assuming that any ongoing source of contamination can be removed,there are different alternatives to consider in deciding how to remediate agiven site Sometimes removal of the contaminated vadose zone is desir-able and feasible In cases of relatively small oil spills it is quite common

to excavate the soil contaminated by the spill, to avoid contact between theoil spill and groundwater However, removal of the contaminated soil intro-duces an additional problem of disposal of the removed soil If the amount

of contaminated soil is not too large, then incineration may be appropriate.For large quantities of contaminated soil, a more common reclamation methodinvolves soil excavation and deposition in a bioreactor, where biodegradation

of the contaminant can be achieved in a comparatively short time period Thisrequires controlling the appropriate supply of moisture, oxygen, and nutrientsfor the enhancement of the microorganism development and growth

16.2.2 Containment of the Contaminated Site

If the contaminated site cannot be excavated economically or technically,then it may be appropriate to contain it and to apply technologies of in-situremediation Containment of the contaminated site is obtained by surrounding

the contaminated site by cutoff walls, or vertical barriers, as shown inFig 16.2.For the analysis of the vertical barrier performance, it is possible toadopt a one-dimensional conceptual model as shown inFig 16.3.The barrierconsists of a porous medium with very low permeability and it separates the

Figure 16.2 Containment of the contaminated site by vertical barriers.

Figure 16.3 Conceptual model of flow with a vertical barrier.

contaminated groundwater from fresh groundwater According to Eq (11.3.1),the one-dimensional differential equation of contaminant transport through thebarrier is

Cen may gradually decrease if the contained area is subject to remediationtreatment However, for a conservative calculation, we may assume that Cen

is constant At a downstream cross section the contaminant concentration is

Cex The value of Cexincreases with time due to the contaminant flux throughthe barrier The increasing value of Cexhas no effect on the advective contam-inant flux through the barrier, but it may lead to a decreasing diffusive flux,due to a smaller concentration gradient Therefore, again for a conservativecalculation, we consider that CexD 0, and its value is kept constant Such

an assumption has no effect on the advective contaminant flux through thebarrier, but it maintains the maximum possible diffusive flux of the contami-nant The instantaneous contaminant flux F, at any cross section of the barrier,

We may refer to differences between values of F at the entrance crosssection, where x D 0, and values of F at the exit cross section, where x D

L At the entrance cross section, the contaminant is subject to advection,represented by the first term on the right-hand side of Eq (16.2.2), as well

as dispersion, which is represented by the second term on the right-hand side

of Eq (16.2.2) At the exit cross section, due to the assumption of CexD 0,the advective contaminant flux vanishes, and the contaminant is transportedsolely by dispersion

Even under unsteady-state conditions, the advective flux is assumed to

be kept constant at the entrance cross section of the barrier On the other hand,the dispersive flux gradually decreases, as noted above Initially it is very large,when the contaminant concentration gradient is large On the other hand, atthe exit cross section the dispersive flux gradually increases from an initialvalue of zero Therefore calculation of steady-state conditions may provide anestimate of the maximum contaminant flux that can be expected at the exitcross section of the barrier Of course, from a practical view point, it is alsoappropriate to provide an estimate of the time period needed to develop themaximum steady-state flux For a conservative contaminant, under steady stateconditions the flux F is constant at every cross section of the barrier Undersuch conditions, Eq (16.2.2) is obtained by direct integration of Eq (16.2.1)

A further integration of Eq (16.2.2) then gives

ln

K

F

v
C D

vx

where K is an integration constant Applying the boundary conditions of C D

Cen at x D 0, and C D 0 at x D L, then shows that

For large values of Peb, this expression reduces to

16.2.10Also, if Peb is large, then steady-state conditions of contaminant transportthrough the barrier are established after an approximate time period of

T ³ L

If Peb is small, then the time period required for the establishment of state conditions can be estimated using analytical solutions of the diffusionequation

steady-16.2.3 Pump-and-Treat of Contaminated Groundwater

Following containment of a contaminated site, appropriate treatment nologies are generally needed to bring the site to full reclamation Ground-water of the contaminated site can be pumped into a treatment plant andlater reinjected into the aquifer Sometimes the treated groundwater can beused directly, mainly for irrigation purposes A variety of treatment methods

tech-are classified as in-situ treatment methods These can sometimes be applied

without physical barriers A common approach is to apply hydrodynamicisolation approaches, rather than physical barriers, to contain the contami-nated portion of the aquifer Hydrodynamic isolation applies various types

of injection and extraction well combinations that do not allow the tion of groundwater from the contaminated site to neighboring aquifers InFig 16.4,schematics of two common options of hydrodynamic isolation areshown

migra-Calculation of flow conditions in the two examples ofFig 16.4can bedone using potential flow theory and well hydraulics, as detailed inChap 11.Each of these examples is associated with the separation of the aquifer flowinto two regions One region incorporates mainly the fresh groundwater Theother region incorporates a comparatively small portion of the fresh ground-water flow and also the flow of contaminated groundwater A well-definedline of separation represents the interface between these two regions Theschematic of Fig 16.4 shows two examples of pumping of contaminatedgroundwater for its possible treatment by conventional methods of waste

Figure 16.4 Hydrodynamic isolation of a contaminated site: (a) isolation by a single pumping well; and (b) isolation by a combination of a pumping well and an injection well.

treatment Such an approach is called pump-and-treat Hydrodynamic

isola-tion, incorporated with pump-and-treat, can be a comparatively inexpensivemethod, which leads to gradual reclamation of the contaminated portion ofthe aquifer

To obtain high efficiency of the systems shown inFig 16.4,it is tant to avoid conditions of significant dispersion and mixing between the freshand contaminated groundwater However, problems arise if contamination ofthe aquifer is associated with significant sorption–desorption capacity onto thesoil of the aquifer For example, in cases of soil contaminated with LNAPL,then in the case described byFig 16.4a,groundwater is continuously contam-inated by the residual adsorbed or entrapped material It should be noted that

impor-in cases of NAPL entrapment, different agents to enhance the remediation,such as surfactants and nutrients for microbial activity, can be added to thewater injected into the aquifer However, such materials should be chosen so

as not to cause other types of aquifer pollution

If the contaminated site of Fig 16.4b is rich with adsorbed orentrapped contaminant, then injected water is subject to contaminationprior to its pumping by the pumping well Figure 16.5 shows a schematic

of a pump-and-treat system, in which the aquifer is contaminated byNAPL Figure 16.5a illustrates a problem of contamination by LNAPLwhere, due to seasonal and annual fluctuations of the groundwater, somequantities of the LNAPL are entrapped within the top layers of theaquifer The flow induced by the pump-and-treat system is associatedwith dissolution and solubilization of the entrapped NAPL, as well aswith penetration of the dissolved constituents into the deeper portions ofthe aquifer In the case described by Fig 16.5b, DNAPL is entrappedthroughout the entire thickness of the aquifer Induced groundwater flow

of the pump-and-treat system is associated with the dissolution of theentrapped DNAPL

Calculations of the performance of the pump-and-treat system shown

in Fig 16.5 can be done using a conceptual one-dimensional flow model.Under such conditions, the process of NAPL dissolution and mass transfer

from the entrapped NAPL ganglia to the flowing aqueous phase can be

calcu-lated using the approach presented in Sec 11.5 The pump-and-treat system

of Fig 16.5athen appears to be inefficient, as most of the induced water flow cannot be in contact with the entrapped LNAPL Furthermore,the induced groundwater flow enhances transverse dispersion, which leads topenetration of dissolved constituents into deeper layers of the aquifer

ground-As an alternative, the pump-and-treat system ofFig 16.6is based on the

use of a single pumping well The discharge of the well causes a drawdown

of the groundwater table and an associated cone of depression The cone of

depression contains the lens of LNAPL and avoids the uncontrolled migration

Figure 16.5 Examples of pump-and-treat systems: (a) aquifer contamination by LNAPL; and (b) aquifer contamination by DNAPL.

of NAPL The floating lens of LNAPL flows towards the pumping well in theregion of the cone of depression Various techniques can then be applied tocollect the floating LNAPL in that region, by various types of membranes andfloating pumps

Following the pumping of the contaminated groundwater, it must betreated The appropriate treatment of the extracted groundwater depends onthe type of contaminant In cases of inorganic contaminants, precipitation is anattractive treatment method Precipitation is governed by the pH value, which

Figure 16.6 Containment of the LNAPL lens by a single pumping well.

may be adjusted by adding lime to the treatment stream Sometimes aeration ofmetals creates salts with faster precipitation Dissolved organic materials can

be removed by air stripping Organic compounds, which have low volatility,cannot be removed efficiently by air stripping Instead, they can be sorbedonto activated carbon Other compounds can be treated by biological methodssimilar to the treatment of domestic wastes

16.2.4 In-Situ Remediation

In various cases, remediation by pump-and-treat is not feasible or is not theoptimal method For example, when a volatile organic compound is spilledinto the unsaturated zone, it partitions between the liquid and vapor state Thevapors may migrate through the vadose zone and accumulate in undergroundstructures like basements, where they pose a threat of fire or explosion In such

cases soil-vapor extraction (SVE) methods can provide an appropriate measure

of in-situ remediation According to these methods, wells are installed in thevadose zone and are used to pump air and vapor Other SVE systems mayincorporate air injection wells and air-vapor extracting wells Such systemscan also be used if the contaminant volatility is comparatively low In suchcases the injected humid air enhances the bioactivity and in-situ bioreme-

diation These kinds of systems are sometimes referred to as air-sparging

systems

16.3 BIOREMEDIATION

As indicated in the previous sections, biological activity may comprise a icant part of the remediation procedure On-site bioremediation is based onexcavation of the contaminated soil, and its placement in bioreactors, intowhich air, water, and nutrients are injected to promote the biological activity.Many engineers and practitioners claim that in most cases the best approachfor bioremediation is to enhance the development of the local indigenousmicroorganism population Others claim that specially acclimated microor-ganisms may produce better results As indicated in the previous section,biological activity can enhance the pump-and-treat approach if the injectionwells provide water rich in air and nutrients, in addition to possible surfactants(emulsifiers), which enhance the solubilization and dissolution of entrappedNAPL Biological activity also can play a significant role in air sparging andother soil vapor extraction procedures

signif-16.3.1 Basic Concepts and Definitions

Bioremediation is based on the biochemical reactions leading to degradation

of the contaminant Usually, bioremediation is considered for application incases of degradation of entrapped NAPL, since removal of the NAPL is very

difficult In general, the organic compound is subject to an oxidation reaction,

in which it loses electrons An electron acceptor, which is subject to a reductionreaction, participates in the oxidation reaction and it gains electrons If theelectron acceptor is oxygen, then the oxidation of an organic compound is

called aerobic heterotrophic respiration If oxygen is not available (anoxic, or

anaerobic conditions), then anaerobic microorganisms use an alternate electronacceptor

Several definitions are useful for the classification of microorganismsinvolved in bioremediation With regard to concentration of organic carbon

in the environment, obligotrophic organisms are most active in cases of low concentration of organic carbon Eutrophic organisms are active in cases of

high concentration of organic carbon Regarding the particular nutritional basis

of the organisms, chemotrophic organisms capture energy from the oxidation

of organic or inorganic materials, autotrophic organisms synthesize their cell

carbon from CO2, and heterotrophic organisms require a source of organic

carbon

The organic compound subject to degradation may be a primary

substrate for the microorganism, provided that it is a source of energy

and carbon In cases where the compound to be degraded is not a primarysubstrate, provision of a primary substrate may be needed Then, the degrading

compound may be considered as a secondary substrate A secondary substrate

usually cannot provide sufficient energy for sustaining the microorganisms,but its degradation, in the presence of the primary substrate, can provide someimportant compounds for microorganism growth.

Biodegradation of a contaminant organic compound can be feasible,provided that the following six basic requirements are satisfied:

1 Presence of the appropriate organisms In general, enhancement

and development of indigenous microorganisms that are capable

of degrading the organic contaminant is recommended, althoughspecialized microorganisms acclimated to a particular contaminant

or environmental condition may be required

2 Primary substrate This is the energy source for the microorganisms.

The organic contaminant can be either the primary or secondarysubstrate The primary substrate is transformed into inorganiccarbon, energy, and electrons

3 Carbon source About 50% of the microorganism dry weight is

composed of carbon Organic chemicals serve as sources of energyand carbon

4 Electron acceptor For the oxidation–reduction process, an electron

acceptor is required Typical electron acceptors are oxygen, nitrate,and sulfate

5 Nutrients Nutrients required for the growth of the microorganisms

include nitrogen, phosphorous, calcium, magnesium, iron, and traceelements These elements are needed for growth of the microor-ganism cell

6 Acceptable environmental conditions Such conditions include

humidity, temperature, pH, salinity, hydrostatic pressure, radiation,and absence of toxic materials

where x is the size of the population and  is the growth rate coefficient for

the population The growth of microorganisms in a limited environment such

as an aquifer is schematically shown inFig 16.7.This figure indicates that therate of growth in such an environment is subject to changes with the size of thepopulation One of the most useful models used to describe microorganism

population growth in a closed environment is the logistic model, which is

Figure 16.7 Growth curve for microorganisms in a limited environment.

popula-Phases of Microorganism Population Growth

According to Fig 16.7 and Eq (16.3.2), the following phases of populationgrowth can be defined in a limited environment:

1 Lag phase In this phase the microorganism population size

is extremely small and there is no discernible increase inpopulation size

2 Acceleration phase This phase shows the beginning of a gradual

increase in the population rate of growth

3 Exponential phase In this phase the rate of growth obtains its

maximum value According to Eq (16.3.1), the microorganismpopulation size is significantly smaller than its maximum value, andthe population size is subject to exponential growth

4 Retardation growth In this phase the rate of growth of the population

starts to decline, as the maximum population is approached

5 Maximum population phase In this phase the microorganism

popula-tion is metabolically active, but the populapopula-tion size is kept constant.For this phase, x D xf

6 Death phase In this phase the population size decreases due to

lack of substrate, the accumulation of toxins, or other phenomena

It should be noted that the logistic model of Eq (16.3.2) cannotdescribe the death phase, as it considers only effects of the size ofthe population on its rate of growth

Various types of kinetic expressions may be useful for the prediction ofrates of biodegradation of organic contaminants in aquifers Such expressionsrefer to the effect of substrate availability, as well as other limiting factors, on

the rate of growth of the microorganisms A common approach is the Monod,

or Michaelis–Menton kinetic expression,

 D max

Cs

KsC Cs

16.3.3where Csis the concentration of the growth-limiting substrate The coefficient

Kc is called the half-saturation constant and is defined as the limiting substrate concentration that allows the microorganisms to grow

growth-at half the maximum specific growth rgrowth-ate A low value of Kc indicatesthat the microorganism is capable of growing rapidly in an environmentwith low concentration of the growth-limiting substrate If several growth-limiting substrates should be considered, then the concentrations and half-saturation constants of all growth-limiting substrates should be incorporated

in Eq (16.3.3) by products of terms similar to that of Eq (16.3.3) Usually,besides the organic substrate, it is appropriate at least to consider oxygen asanother growth-limiting substrate

In the limiting case of Cs× Kc, Eq (16.3.3) yields

When this occurs, the reaction is called a zero-order reaction Alternatively,

in the limiting case of Cs− Kc, Eq (16.3.3) yields

 Dmax

Kc

This is called a first-order reaction, with the first-order rate constant given by

max/Kc

If we let M represent the microbial mass per unit of groundwater volumeand Y the amount of organism mass formed for each unit of substrate mass

utilized (this is called the yield coefficient ), then the change in substrate

concentration can be expressed as

dCs

dt D maxMCs

The ratio between maxand Y represents the maximum contaminant utilization

rate per unit mass of microorganisms.

Equation (16.3.6) should be accompanied by the equation of biologicalmass transport, growth and decay,

of no limiting supply for all nutrients However, in a real aquifer, there isusually a limiting supply of at least one of the growth nutrients, and the organicsubstrate, as well as the limiting growth nutrient and microorganisms also, aresubject to transport by advection and dispersion Using the expressions forMonod kinetics, all of these effects may be combined as

treatment Such an approach is called pump-and-treat Hydrodynamic

isola-tion, incorporated with pump-and-treat, can be a comparatively... conditions can be estimated using analytical solutions of the diffusionequation

steady -1 6. 2.3 Pump-and-Treat of Contaminated Groundwater

Following containment of a contaminated site,... theentrapped DNAPL

Calculations of the performance of the pump-and-treat system shown

in Fig 16. 5 can be done using a conceptual one-dimensional flow model.Under such conditions, the process