Groundwater Pollution

150507 GW 10a Enhanced Natural Attenuation

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These lectures were adopted from “ENHANCED ATTENUATION: A REFERENCE GUIDE ON AP-PROACHES TO INCREASE THE NATURAL TREATMENT CAPACITY OF A SYSTEM” August 2006 Washington Savannah River Company. Prepared for the U.S. Department of Energy

www.cluin.org/download/contaminant-focus/tce/DOE_EA_doc.pdf

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Monitored natural attenuation (MNA) and enhanced attenuation (EA) are two environmental management strategies that rely on various pro-cesses to degrade or immobilize contaminants and are used at sites where contaminant plumes have low risk and are not growing.

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Sometimes the natural attenuation capacity of the system may not be enough to allow MNA to be included in the remediation train and “active” treatments alone will not allow the remediation goal to be met.

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Enhanced attenuation is when an enhancement is added as part of a remedy to increase the system attenuation capacity.

“… the introduction of an enhancer of any type is no longer considered to be ‘natural’ attenuation.”

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Sustainable EA processes, when added to the remediation treatment train, may result in an increase in attenuation capacity sufficient for meeting the remediation goal.

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Enhanced attenuation processes fall into two classes:

a) those that reduce mass loading from the source to the plume, and

b) those that augment existing attenuation processes occurring within the plume and at the compliance point.

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The treatment process must have the ability to reduce flux to the point that the natural processes can meet remedial goals at the compliance point within a reasonable timeframe. In addition, the enhancements must be sustainable.

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A sustainable enhancement is an intervention that continues until such time that the enhancement is no longer required to reduce contaminant concentrations or fluxes. Ideally, a single phase of intervention will be enough to produce a sustainable enhancement.

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Enhancements that can meet the requirements of sustainability and reducing contaminant mass flux include the following:

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Reduce mass loading from a source• Engineered structures to intercept and divert

surface runoff, stormflow, and groundwater flow (also including diversion of competing electron acceptors)

• Methods for reducing infilitration in source areas (e.g. engineered covers; plant based methods)

• Waste encapsulation (e.g. grouting; diffusion barriers using non-toxic vegetable oil)

• Techniques for lowering the hydraulic gradient in source areas (e.g. french drains, plant-based methods)

• Passive reduction of source mass (e.g. soil vapor extraction by barometric pumping)

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Reduce mass flux within a plume

• Biological processesBiostimulation

Bioaugmentation

Wetlands (natural and constructed)

Plant-based methods

• Abiotic processesContaminant degradation by biologically-

enhanced abiotic reactions

Permeable reactive barriers

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Many of these enhancements represent mature technologies that enjoy a firm theoretical basis, an extensive history of full-scale field deployment, and long-term performance data under a variety of site conditions (e.g. engineered diversion structures and covers).

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In contrast, some enhancements are still under development with few or no examples of field implementation (e.g. diffusion barriers, biologically-enhanced abiotic processes). Case studies will give examples where enhancements have been used and their impact on the physical properties of the system (e.g. hydraulic gradient) or the concentration of contaminants.

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Each category of enhancement along with an assessment of any technical and regulatory issues that might be involved.

Technical issues frequently include concern about the sustainability of the enhancement.

Regulatory issues tend to focus on acceptability.

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The decision process by which enhancements are chosen depends on

• site conditions (e.g. geology, hydrology, depth of contaminants),

• potential locations for deploying an enhancement (source, plume, receptor), and

• cost factors.

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We will look at major classes of enhancements information for some of the key criteria that will affect the selection of enhancements for a site. It is useful for comparing the features of different types of enhancements and to what part of the source-plume system they are applicable.

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There are different areas of research on enhancements. Current research activities include the following projects:

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• Use of electron shuttles to biologically enhance abiotic dechlorination

• Enhanced passive remediation using bioaugmentation with aerobic bacteria for cis-

1,2-DCE

• Developing an MNA modeling tool based on RT3D (RT3D is a software package for simulating three-dimensional, multi-species, reactive transport of chemical compounds (solutes) in groundwater. http://bioprocess.pnnl.gov/

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Future research will help in understanding the science behind enhancements, improved deployment strategies, better methods to address subsurface heterogeneities, sustainability issues, and cost-effectiveness.

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Development of enhanced attenuation will help in the accelerated development of cost effective and environmentally protective technical solutions to address the real-world plumes at groundwater pollution sites.

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Enhanced attenuation processes - two classes:

a) those that reduce mass loading from the source to the plume, and

b) those that add to existing atten-uation processes occurring within the plume.

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For an existing source or active treatment to be classified as an EA technology, the treatment process must be able to re-duce pollution to the point that the natural processes can re-mediate within a reasonable time.

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Enhancements include:Reduce mass loading from a

source• Engineered structures to

change surface runoff, storm-flow, and groundwater flow (also including competing electron ac-ceptors)

• Methods for reducing infiltra-tion in source areas (e.g. engi-neered covers; plant-based methods)

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Enhancements include:Reduce mass loading from a source• Waste encapsulation (e.g.

grouting; diffusion barriers using non-toxic vegetable oil)

• Lowering the hydraulic gra-dient in source areas (e.g. french drains, plant-based meth-ods)

• Passive reduction of source mass (e.g. soil vapor extraction by pumping)

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Reduce mass flux within a plume• Biological processes

o Biostimulationo Bioaugmentationo Wetlands (natural and constructed)o Plant-based methods

• Abiotic processeso Contaminant degradation by biologi-

cally-enhanced abiotic reactionso Permeable reactive barriers

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Which enhancements are chosen for application at a site depends on: –site conditions (e.g. geology, hy-

drology, depth of contaminants), –potential locations for using an

enhancement (source, plume, re-ceptor), and

–cost factors.

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Is it possible to sustainably change natural attenuation processes so that they are more effective and increase the reduction in mass flux of contaminants?

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An enhancement is anything we might do to a source-plume system that in-creases the amount of contaminant degradation or immobilization more than that which occurs naturally with-out our work.

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Examples of enhancements to physical atten-uation processes include:

• Hydraulic change to reduce advective trans-porto Reduction of infiltrating precipitation (e.g., caps,

plants, etc.)o Interception and diversion of up-gradient

groundwater before it can reach the source (e.g., drains)

• Source containment to reduce mass loadingo Physical source containment strategies (e.g.,

slurry walls; sheet piling)o Introduction of vegetable oil into the source area

to create a diffusion barrier to the contaminants• Increase source removal by barometric

pumping

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Example of enhancements to chemical attenuation processes that degrade contaminants include:

• Addition of substances that result in increased in situ production of re-duced iron and sulfur phases (e.g. FeS) that can abiotically degrade pol-lutants

• Installation of a reactive zone (per-meable reactive barrier with zero-va-lent iron or other reactive media, biobarrier, etc.)

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Examples of enhancements to biologi-cal attenuation processes include:

• Addition of substances that stimulate naturally occurring bacteria to in-crease the rate or extent of degrada-tion

• Adding additional bacterial species that can live in the plume environ-ment and will increase the overall degradation of contaminants

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An enhancement does not have to re-main effective indefinitely.

It only must operate as long as neces-sary to maintain a favorable balance between mass flux and system at-tenuation capacity.

Eg, a constructed wetland might be an important enhancement for achiev-ing mass balance early in the treat-ment cycle of a plume, but the ne-cessity for the wetland to operate at top efficiency declines as the pollu-tant disappears.

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The best place to use enhancements is as important to the success of EA as identifying the enhancements them-selves.

We can think of a plume as divided into three enhancement zones:

• Source enhancement zone• Plume enhancement zone• Discharge enhancement zone

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In general, the source zone is the most effective region in which to apply en-hancements because:

• The source usually is relatively small• The source depth often is not great• There many enhancement choices

that together can be very effective• Many source enhancement technolo-

gies have a long record of success

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The discharge zone is another area where enhancements can be cost-ef-fective if:

• Contamination releases occur in a relatively restricted region

• Contaminants go to the surface

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The main body of the plume is the most challenging zone in which to apply enhancements because:

• The scale of the plume area is greater than for source and discharge zones

• The depth tends to be greater than for the other zones

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The mass flux of contaminants coming from a source zone is the mass of contaminants crossing through a unit area of a plane (e.g. per m2 ) oriented normal to the plume axis per unit time (e.g. per sec.). The integrated mass flux can be thought of as the total mass of contaminant pass-ing through the plane per unit time (i.e. encompassing the entire cross sectional area of the plume). One or more en-hancements to a source – plume system will result in a decrease in the contaminant mass flux.

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Also the enhancement(s) must last for a time to give a sustainable reduc-tion in flux for as long as necessary to achieve regulatory requirements. The objective of such a reduction is to achieve a sustainable balance be-tween source loading (the integrated mass flux from the source) and plume attenuation capacity (the in-tegrated reduction of mass flux due to all natural and enhanced attenua-tion).

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• Actions that reduce source load-ing:o Hydraulic manipulation o Passive residual source reduction

• Actions that increase the attenu-ation capacity of the system:o Biological degradation of contami-

nantso Abiotic degradation of contami-

nants

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ENHANCEMENTS TO REDUCE PLUME LOADING: HYDRAULIC MANIPULATION

REDUCE INFILTRATION THROUGH THE SOURCE ZONE

Intercept and divert surface runoff and stormflow water

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REDUCE INFILTRATION THROUGH THE SOURCE ZONE

Intercept and divert surface runoff and stormflow water

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HYDRAULIC MANIPULATION REDUCE MASS TRANSFER OF CON-

TAMINANTS TO GROUNDWATER IN A SOURCE ZONE eg

Source Containment Modifying the Hydraulic Gradient

with Drainage StructuresModifying the Hydraulic Gradient

Through Phytotranspiration

53Modifying the Hydraulic Gradient Through Phytotranspiration

54Modifying the Hydraulic Gradient Through Phytotranspiration

55Depression of the Water Table under a Tree Plantation

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Engineered Phytotranspiration System

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HYDRAULIC MANIPULATION ELECTRON ACCEPTOR DIVERSION

To accelerate the natural dechlorination process use methods to increase the sup-ply of electron donors to dechlorinating bacteria.

Add complex electron donors (such as lac-tate, molasses, mulch, etc.) that ferment in-situ to release hydrogen.

There is another approach to increasing the electron donor supply to the dechlorina-tors.

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The presence of electron acceptors (primar-ily dissolved oxygen, nitrate, and sulfate) in a source zone will result in biodegrada-tion reactions that compete with beneficial dechlorination reactions for electron donor.

This competition occurs in cases where the electron donor is in the source zone before remediation or if the electron donor supply is enhanced by adding fermentation sub-strates or hydrogen directly.

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It should be possible to permanently divert the transport of competing electron acceptors (oxygen, nitrate, and sulfate) away from chlorinated solvent plumes so that more electron donor (i.e., organic substrates and/or dissolved hydrogen) is preserved for beneficial reductive dechlorination reactions.

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Electron Acceptor Diversion

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ENHANCEMENTS TO REDUCE PLUME LOADING: PASSIVE RESIDUAL SOURCE REDUCTION

62PASSIVE VAPOR EXTRACTION OF VOCS FROM THE VADOSE ZONE

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INCREASE SYSTEM ATTENUATION CAPACITY: BIOLOGICAL PRO-CESSES

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INCREASE SYSTEM ATTENUATION CAPACITY: BIOLOGICAL PRO-CESSESMICROBIAL DEGRADATION METHODSBiostimulation using Long-Lived Elec-

tron Donors, Electron Acceptors and Nutrients

Enhancing the in-situ biodegradation pro-cesses rely on the continuous or periodic addition of one or more chemical reagents including electron acceptors (oxygen, nitrate, sulfate), electron donors (carbohydrates, fatty acids, and H2), and nutrients (nitrogen, phosphorus, trace minerals and vitamins).

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• The treatment reagent should be a non-toxic material.

• The treatment reagent should be relatively immobile to prevent it from being washed out of the treatment zone by flowing groundwater.

• The treatment reagent should be relatively resistant to biological or chemical attack so that it will last for several years be-tween applications.

• The treatment reagent should be suffi-ciently reactive to support the desired chemical or biological reaction at rates that meet treatment objectives.

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The carbon source/electron donors are often organic amendments delivered as aqueous solutions (containing lac-tate, molasses, or similar com-pounds), proprietary polymerized or-ganics (such as Hydrogen Release Compound - HRC), nontoxic oils (such as soybean oil, lard oil, etc.), and blended amendments (such as wa-ter-oil emulsions).

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INCREASE SYSTEM ATTENUATION CAPACITY: BIOLOGICAL PRO-CESSESMICROBIAL DEGRADATION METHODS

BioaugmentationThere are many sites where microbial

analyses show that chlorinated solvent degrading microorganisms are not present or are at low population num-bers so their activity does not give enough biodegradation capacity.

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Bioaugmentation involves seeding aquifers with microorganisms depending on the mix of chlorinated solvents present and the environment.

For example, viable approaches include the addition of halorespiring microorganisms in anaerobic plumes that contain electron donors and chlorinated solvents, or adding microorganisms that can aerobically biodegrade lower chlorinated solvents where these compounds may discharge or mix with aerobic waters.

After seeding the microorganisms should spread and grow and increase the natural attenuation capacity.

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INCREASE SYSTEM ATTENUATION CAPACITY: BIOLOGICAL PRO-CESSES

Wetland SystemsStudies over the past 10 years of natu-

ral wetlands into which cVOC-con-taminated groundwater discharges show that a wide variety of chlori-nated compounds can be treated in this type of environment.

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ABIOTIC AND BIOLOGICALLY MEDI-ATED ABIOTIC ATTENUATION METHODS

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ABIOTIC AND BIOLOGICALLY MEDI-ATED ABIOTIC ATTENUATION METHODSABIOTIC REACTIONS WITH REDUCED

IRON AND SULFUR PHASES WITH OR WITHOUT BIOLOGICAL MEDIATION

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ABIOTIC AND BIOLOGICALLY MEDI-ATED ABIOTIC ATTENUATION METHODSABIOTIC REACTIONS WITH REDUCED

IRON AND SULFUR PHASES WITH OR WITHOUT BIOLOGICAL MEDIATION

Abiotic Reactions with Reduced Min-eral Phases

Minerals found in the shallow subsurface can cause the abiotic degradation of chlorinated solvents. Natural reductants are soil minerals that contain reduced forms of iron and sulfur.

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ABIOTIC AND BIOLOGICALLY MEDI-ATED ABIOTIC ATTENUATION METHODSABIOTIC REACTIONS WITH REDUCED

IRON AND SULFUR PHASES WITH OR WITHOUT BIOLOGICAL MEDIATION

Biologically Mediated Abiotic Reac-tions

Biogeochemical reductive dechlorination (BiRD) involves both biological and chemical reactions for abiotic reduction of chlorinated solvents, such as PCE and TCE. Microbes may produce reductants that help reactions with minerals in the aquifer.

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ABIOTIC AND BIOLOGICALLY MEDI-ATED ABIOTIC ATTENUATION METHODS SORPTIONThere is a correlation between the

amount of natural organic matter (NOM) which is present in soils and aquifer ma-terials and its capacity to sorb com-pounds with elevated KOW values.

The octanol-water partition coefficient (KOW) measures how much an organic compound will partition into octanol in comparison to water.

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ABIOTIC AND BIOLOGICALLY MEDI-ATED ABIOTIC ATTENUATION METHODS REACTIVE BARRIERS

example

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Different Configurations of the Funnel and Gate Technology.Groundwater is directed into a reactive barrier.

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a Reactive Barrier Using Emulsified Oil

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