GROUNDWATER MODELING WORK PLAN

POPILE, INC. SUPERFUND SITEEl Dorado, Arkansas

Prepared For:

U.S. Army Corps of Engineers,New Orleans District

New Orleans, Louisiana

Prepared By:

Morrison Knudsen CorporationEnglewood, Colorado

Under Contract To:

U.S. Army Corps of EngineersTulsa District

Tulsa, Oklahoma

Total Environmental Restoration ContractContract No. DACA56-94-D0021

October 1998

GROUNDWATER MODELING WORK PLAN

POPILE, INC. SUPERFUND SITEEl Dorado, Arkansas

REVIEWS AND APPROVALS

Steve Roe, MK Program Manager

Joel Siegel, MK Project Manager

Acceptance:

Ted EiltsU.S. Army Corps of EngineersNew Orleans

Shawn GhoseU.S. Environmental Protection Agency

Table of Contents

1.0 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-11.1 Remediation by Natural Attenuation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-11.2 Biodegradability of Creosote and Pentachlorophenol . . . . . . . . . . . . . . . . . . . . 1-21.3 Evidence of Natural Attenuation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-31.4 Modeling Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-4

2.0 Site Conditions ........................................................ 2-12.1 Geology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-12.2 Hydrogeology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-12.3 Nature and Extent of Contamination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2

3.0 Code Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

4.0 BIOPLUME m M o d e l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-14.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-14.2 Theory of Hydrocarbon Biodegradation in Ground-water . . . . . . . . . . . . . . . . . 4-1

4.2.1 Electron Acceptors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-14.2.2 Biodegradation Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-2

4.3 Model Applicability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-3

5.0 Model Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-15.1 Application Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-15.2 I n p u t D a t a . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-3

5.2.1 Grid Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-35.2.2 Initial Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-45.2.3 Boundary Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-45.2.4 Contaminant Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-45.2.5 Advection, Dispersion, Retardation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-55.2.6 Electron Acceptors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-6

5.3 Validation, Verification, Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-65.4 Sensitivity Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-7

6.0 Quality Assurance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-16.1 Quality Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-16.2 Quality Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1

7.0 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1

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List of Tables

Table

3-1 Comparison of Important Features Between RT3D and BIOPLUME m . . . . . . . . . . . 3-2

List of Figures

5-1 Popile Site Groundwater Modeling Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-2

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1.0 Introduction

Momson Knudsen Corporation (MK) is submitting this Groimd Water Modeling Work Plan to the

U. S. Army Corps of Engineers (USAGE) - New Orleans District to describe the concept,

procedures, and procedural controls that will employed in modeling the groundwater flow and

transport of contaminants of concern at the Popile site in El Dorado, Arkansas. This work plan

follows a ground water modeling data collection field program conducted by MK in July through

October 1998, aimed at completing the definition of the residual sources of contamination, the

pattern of contamination in the ground water (i.e., nature and extent), and in obtaining soil physical,

chemical, physicochemical, biological, and biogeochemical information on the aquifer materials and

the contaminants' interaction with them. Results of other studies dating to 1989 factor into this work

plan.

The Data Quality Objectives (DQOs) for the project have posed among other questions, whether

ground water contamination is entering the Bayou de Loutre, and if not, whether it has a reasonable

chance to do so in the future. The preliminary data indicate that site contaminants are not entering

the Bayou. The ground water model will therefore address the possibility that contaminants may

reach the Bayou in the future.

To answer this question, the ground water model will simulate processes that collectively cause

natural attenuation. In context of the modeling to be done and of this work plan, reference to the

term "Remediation by Natural Attenuation" is only for purposes of assessing whether conditions are

favorable for site contaminants to be immobilized. The modeling does not presume that complete

natural attenuation is synonymous with "remediation" as that term is customarily used, or that

USACE or any other agency is seeking to "remediate" the site by groundwater modeling of the

attenuation process.

1.1 Remediation by Natural Attenuation

Over the past several years, the high cost and poor performance of many pump-and-treat systems

has led scientists to consider natural attenuation as an alternative technology for groundwater

remediation. Studies at over 30 sites around the country reported in Wiedemeier et al., (1995) and

others (Stauffer et al., 1994; Stroo et al., 1988; Borden, 1986; and Mikesell and Boyd, 1986) have

shown that naturally occurring biological processes can significantly enhance the rate of hydrocarbon

removal from contaminated aquifers.

"FAWHITBTERCVDOSlS.PPLYworkplan.wpd 1-1 (Rev. a, November 5,1998)

Remediation by natural attenuation (RNA) is the reduction in concentration, mass or mobility of a

chemical of concern (COC) with distance and time due to naturally occurring processes in the

groundwater system. These processes can be classified as physical (such as dispersion, diffusion,

and dilution), sorption, and biodegradation. The physical and sorption processes result in reduction

of concentration and/or mobility of a chemical but not its total mass, and are referred to as

"nondestructive" mechanisms. The chemical and biological reactions result in the reduction of the

total mass in the aquifer, and are referred to as "destructive" mechanisms. For hydrocarbons in thesubsurface, biological degradation is found to be the most important process in the reduction of mass

(ASTM, 1998).

Depending upon the dissolution of contaminants and the properties of the aquifer, a plume willexpand until it reaches equilibrium when the rate of contaminant release from the source is balancedwith the rate of natural attenuation. At equilibrium the plume stabilizes. When the source area is

depleted to the point that the rate of natural attenuation exceeds the source input, the result will bea shrinking plume over time.

RNA is a generally acceptable remedy if a plume is shrinking or stable (primary line of evidence)

and there are no impacts to receptors. If a plume is expanding, but at a rate lower than the

groundwater velocity, the risk reduction and performance goals may still be met depending on the

presence and location of receptors. Secondary lines of evidence, such as estimates of naturalattenuation rate and favorable biological conditions may also be used to demonstrate RNA (ASTM,1998).

1.2 Biodegradability of Creosote and PentachlorophenolCreosote is an oily distillate from wood tars, consisting mainly of phenolic cmpd, guaiacol, cresol,

and methlycresol (Micromedex, 1997). It is a highly insoluble dense non-aqueous phase liquid

(DNAPL) that tends to collect in pools within the soil matrix. Water tends to flow around these

accumulations because the oil is highly hydrophobic, and as a result only a small fraction of the total

contained polycyclic aromatic hydrocarbon (PAH) mass is actually exposed to the groundwater.

These PAH compounds have solubilities in water ranging from a few milligrams per liter (mg/1) toless than 1 gram per liter (g/1). Total PAH concentrations in groundwater exposed to creosote rarely

exceeds 5 mg/1 (Stroo et al, 1997). Although some other hydrocarbons are also present, the total

oxygen demand (TOD) in groundwater even immediately downgradient of a source is very low.

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Pentachlorophenol (PCP) is more water soluble (14 mg/1 at 20 °C) than the PAH compounds, but

when PCP is used in wood preserving, it is applied in a carrier oil to assist in penetration into the

wood. The PCP does not readily partition from the oil into the groundwater, so PCP can slowly

leach into the groundwater for long periods of time.

Creosote contains several PAH compounds, which vary in their physical and chemicalcharacteristics. The lower molecular-weight PAHs (2-4 rings, e.g., naphthalene etc.) are more

^biodegradable, volatile, and water-soluble than the heavier compounds. PAHs are biodegradable,especially under aerobic conditions (Sims and Overcash, 1983). Several of the lower molecular-weight PAHs are also biodegraded under anaerobic conditions (Mihelcic and Luthy, 1988).

PCP in an aqueous phase can be rapidly biodegraded though both aerobic and anaerobic processes

(Stroo et aL, 1997; Edgehill and Finn, 1983).

The adoption ofRNA as the remedial alternative at three wood treating facilities with creosote andPCP contamination have been discussed in Stroo et al. (1997), so there is precedent to consider RNA

at Popile.

1.3 Evidence of Natural AttenuationAs of issuance of this plan, the complete data set containing information on all the measured

bioparameters has not yet been evaluated. However, preliminary observations can be made from thedissolved oxygen levels and total microbial population counts for upgradient, source, and

downgradient locations (MK, 1998). Generally, the data show little to no degrader activityupgradient, little to no degrader activity within the source (with the exception of one location whichshowed moderate activity) and moderate degrader activity downgradient. Dissolved oxygen levelswere highest upgradient, depressed mid-source, and moderately low downgradient. These

observations indicate a scenario where no degrader populations exist upgradient outside of the

contaminant zone, no degrader populations exist within the source area where contaminant levels

are highest and likely toxic, and moderate numbers ofdegraders are active downgradient where theirgrowth is probably limited by the solubility and transport of the contaminant from the source. These

observations support the premise that biodegradation is a contributor to disappearance of organiccontaminants in the groundwater, at least as far as it having to be accounted for in code selection.The site conceptual model will continue to be refined during the early stage of setup and calibrationof the ground water model.

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1.4 Modeling ObjectivesAs discussed in the previous sections, the primary line of evidence for RNA at the Popile Site is that

long-term groundwater flow and contaminant transport have not resulted in a widespread dissolved

contaminant plume. Although the secondary lines of evidence provided by the Phase II study (MK,

1998) support the postulate that both aerobic and anaerobic biodegradation are taking place, these

evidences are inconclusive in determining whether the contaminant plumes will migrate to theBayou de Loutre in the future. In addition, it is unknown whether the adoption of RNA as the soleremedial action or the "no action" alternative is adequate to achieve the remedial goal of plumeconfinement due to the processes of sorption and biodegradation.

The primary objective of this groundwater modeling is to evaluate whether RNA at the Popile Siteis preventing contaminants now, or in future, from entering the Bayou de Loutre. Under this scope

ofwork, MK will perform the following specific tasks:

(1) Review and compile data from the previous site investigations for input to the BIOPLUME

III model. Per plan, literature values will be used ifin-situ or laboratory measurements were

not obtained.

(2) Develop a conceptual model with the latest data available to describe the hydrogeologic and

geologic conditions of the aquifer system, contaminant source and release, as well as the

transport and biodegradation processes.

(3) Demonstrate pathways and final products of the aerobic and anaerobic biological reactionsbetween the hydrocarbon contaminants and the electron acceptors.

(4) Simulate the transport and biodegradation of PCP and the seven B(a)P equivalenthydrocarbon contaminants. These include:

(a) to determine the state of the dissolved plume, e.g., if the dissolved hydrocarbon plumeis expanding, stable, or shrinking;

(b) to determine the future impacts of the dissolved plume to the Bayou de Loutre; and

(c) to calculate the rate of natural attenuation.

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The model application will follow the procedures discussed in Section 5.0, including theapplication process, input data, calibration, and sensitivity analysis.

(5Y Use simulation results to evaluate the adequation of current monitoring well arrangement.

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2.0 Site Conditions

2.1 GeologyThe near surface geology beneath the Popile Site consists of Quaternary age alluvium and the

underlying Tertiary age Cockfield and Cook Mountain Formations. The Quaternary age alluvium

consists of stream sediments and floodplain deposits located within and adjacent to the Bayou DeLoutre. Stream sediments are confined to within the Bayou channel and typically grade from sands

and silts in the upper part to sand and gravel in the lower section and seldom exceed 10 feet inthickness. Floodplain deposits consist of thin beds of silts and clays and may cover the low lying

areas adjacent to the Bayou.

The Tertiary age Cockfield Formation is the surface unit that covers the majority of the Popile Site.The Formation consists ofunconsolidated to weakly consolidated fluvial deposits which exhibit afining upward sequence. The unit ranges in thickness across the site from 20 to 66 feet but istypically about 45 to 50 feet thick. The lower portions of the Formation consist of well sorted cleanmedium- to fine-grained sands containing zones of laminated or thinly bedded carbonaceous matter(e.g., decomposed wood and vegetable matter). Thin layers of gravel occur occasionally within these

sands but are not laterally continuous across the site. These Cockfield sands grade upward into siltysands and silts. In places, fat silty clays exist within this upper fine grained unit. This upper finegrained unit ranges in thickness from 5 to 16 feet thick across the site.

The underlying Tertiary age Cook Mountain Formation consists of clays and silty clays and mayrange up to 160 feet in thickness. It is regionally the lower confining layer for the overlying

Cockfield aquifer and also the upper confining layer for the underlying Sparta Sand aquifer.

2.2 HydrogeologyThe uppermost aquifer beneath the Popile Site occurs within the sands of the Cockfield Formation

which overly the Cook Mountain Formation. Groundwater within this aquifer appears to be under

unconfined to semi-confined conditions. Unconfined conditions appear to exist along the westernboundary of the site but quickly change to semi-confined condition eastward across the site.

Confined conditions are most noticeable at monitoring well MW-34 where the water level in the well-is above ground surface. The upper fine-grained zone of the Cockfield appears to act as the upperconfining layer. Groundwater flow beneath the site is eastward toward the Bayou De Loutre.Beneath the former process impoundment area, flow is due eastward, while flow directions in the

F:\WHITBTERCVDO#18.PPL\workplan,wpd 2-1 (Rev. a, November 5,1998)

vicinity of the former sludge pit and northern portions of the site are more northeasterly and

northerly, respectively. The horizontal gradient toward the Bayou varies from 0.01 ft/ft to 0.03 ft/ft.

2.3 Nature and Extent of ContaminationContaminants of concern (COCs) at the Popile Site consist ofPCP, and the following polynuclear

aromatic hydrocarbons (PAHs): benzo(a)pyrene, dibenz(a,h)anthracene, benzo(a)anthracene,benzo(b)fluoranthene, benzo(k)fluoranthene, chrysene, and indeno(l,2,3-cd)pyrene.

Contaminant sources include the DNAPL retained in the unsaturated and saturated zones beneaththe former process impoundment and sludge pit. Free phase products appear to be confined to within

the upper portions of the Cockfield (less than 16 feet bgs). Free phase products have not migrateddownward to the Cook Mountain Formation and have not migrated laterally off-site. Soluble

hydrocarbons from the DNAPLs continue to partition into the aqueous phase as infiltration and freshgroundwater enter the source areas. The dissolved phase plume extends approximately 200 feet

downgradient from the source areas. The spreading of the dissolved phase plumes is controlled bythe processes of advection, dispersion, sorption, and biodegradation.

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3.0 Code Selection

Technical criteria for selecting codes for general groundwater flow and transport modeling have been

formulated by U.S. EPA (1988). The code selection process is in essence the matching of project

modeling objectives with features and capabilities of existing computer models. Selecting an

appropriate model includes analysis of me specified modeling needs and available commercial and

public domain models.

The Popile site specific criteria used in the computer model selection are:

(1) the code is suitable for simulating advection, dispersion, and sorption;(2) the code can simulate biodegradation with various reaction kinetics;(3) the code is reliable;

(4) the code can be applied efficiently.

The reliability of a computer model is defined by the level of quality assurance applied during itsdevelopment, its verification and field validation, and its acceptance by users. A model's efficiencyis determined by the availability of its program and documentation, and its usability, portability,modificability, and economy with respect to human and computer resources required.

Two well-known public domain models were examined during the code selection process and thecomparisons of their important features and capabilities are presented in Table 3.1.

Based on the Popile project modeling objectives discussed in Section 1.4, and the model capabilities,

reliabilities, and efficiencies discussed in Table 3.1 and in Section 4, the BIOPLUME in program(U.S. EPA, 1998) is proposed.

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Table 3.1 - Comparison of Important Features between RT3D and BIOPLUME III

Features

Developer

Geometry

Dispersion

Sorption

Biodegradation

-

Graphical User

Interface (GUI)

Geostatistics

Simulated vs.Observed

RT3D

U.S. DOE

3-D

Yes

Yes

Provides 7 modules for pre-defmed problems:

1) Instantaneous aerobic decay of BTEX;2) Instan. decay of BTEX using aerobic &

anaerobic electron acceptors;3) Kinetic-limited decay of BTEX using

aerobic & anaerobic electron acceptors;4) Rate-limited sorption reactions;5) Double Monod model;6) PCE chain reaction;7) Aerobic/anaerobic model for PCE to TCE

degradation.

Provides a user-defined reaction modulewhich requires the user to incorporate thereaction kinetics and hydrocarbon to electronacceptor stoichiometric ratio to a FORTRANsubroutine.

In summary, both the electron acceptor typesand reaction kinetics are pre-defined in amodule.

RequiresGMS/MODFLOW

No

No-

BIOPLUME ffl

U.S. EPA

2-D

Yes

Yes

Both the electron acceptor typesand reaction kinetics areflexible inputs to the program.Depending on the hydrocarbonbeing simulated, reactionstoichiometric ratios to theelectron acceptors can beinputted. Also, three reactionkinetics can be specified frominput, e.g.. InstantaneousReaction; First Order DecayFunction; and Monod Function.

Built-in

Excellent Integration

Built-in

Built-in

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4.0 BIOPLUMEm Model

4.1 Overview

BIOPLUME in was developed through collaboration between the Air Force Center for

Environmental Excellence (AFCEE), the EPA's National Risk Management Research Laboratory

(NRMRL), and EPA's Robert S. Kerr Environmental Research Center (RSKERC). The program

is a two-dimensional, finite difference model, specifically designed for simulating the natural

attenuation of organic contaminants in groundwater due to the processes ofadvection, dispersion,

sorption, and biodegradation. The model simulates the aerobic and anaerobic biodegradation of

organic contaminants using oxygen, nitrate, iron (III), sulfate, and carbon dioxide as electron

acceptors. The model solves the transport equation six times to determine the fate and transport of

the hydrocarbons and the electron acceptors (see section 4.2.1).

Three different kinetic expressions can be used to simulate the aerobic and anaerobic biodegradation

reactions. These include:

(1) instantaneous reaction;

(2) first order decay; and

(3) Monod kinetics.

The principle of superposition is used to combine the hydrocarbon plume with the electron acceptor

plumes to determine the resulting concentrations of the reactants. Consumption of the hydrocarbon

and the electron acceptors is governed by the stoichiometric ratio in the reactions.

4.2 Theory of Hydrocarbon Biodegradation in Groundwater4.2.1 Electron Acceptors

Biologically mediated degradation are reduction-oxidation (redox) reactions, involving the transfer

of electrons from the organic contaminant compound to an electron acceptor. Oxygen is the electron

acceptor for aerobic metabolism whereas nitrate, ferric iron, sulfate and carbon dioxide serve aselectron acceptors for alternative anaerobic pathways.

In the presence of organic substrate and dissolved oxygen, microorganisms capable of aerobic

metabolism will predominate over anaerobic forms. However, dissolved oxygen is rapidly

consumed in the interior of contaminant plumes, converting these areas into anoxic (low oxygen)

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zones. Under these conditions, anaerobic bacteria begin to utilize other electron acceptors to

metabolize dissolved hydrocarbons. The principal factors influencing the utilization of the various

electron acceptors include:

• The relative biochemical energy provided by the reaction.• The availability of individual or specific electron acceptors at a particular site.• The kinetics of the microbial reaction associated with the different electron acceptors.

The transfer of electrons during the redox reaction releases energy which is utilized for cellmaintenance and growth. The biochemical energy associated with alternative degradation pathwayscan be represented by the redox potential of the alternative electron acceptors: the more positive the

redox potential, the more energetically favorable is the reaction utilizing that electron acceptor. Witheverything else being equal, organisms with more efficient modes of metabolism grow faster and

therefore dominate over less efficient forms.

BIOPLUME DDL simulates, in time and space, biodegradation sequentially in the following order:

Oxygen -> Nitrate -> Iron (in) -> Sulfate -> Carbon Dioxide

The biodegradation of a hydrocarbon in a given location using nitrate, for example, can only occur

if oxygen has been depleted to its threshold concentration at that location.

4.2.2 Biodegradation CapacityTo apply an electron acceptor-limited kinetic model, the amount ofbiodegradation that groundwater

moving through the source zone can support must be determined. The conceptual model is:

« Groundwater upgradient of the source contains electron acceptors.

• As the upgradient groundwater moves through the source zone, hydrocarbons in the NAPL phaseand contaminated soil release dissolved hydrocarbons (in the case ofPopile site, PCP and theseven B(a)P equivalents).

• The biological reactions continue until the available electron acceptors are consumed.

• The total amount of available electron acceptors available for biological reactions can be

estimated by calculating the difference between upgradient wells and source zone wells foroxygen, nitrate, and sulfate.

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Using stoichiometry, a utilization factor can be developed to convert the mass of oxygen, nitrate,

and sulfate consumed to the mass of dissolved hydrocarbon that are used in the biodegradation

reactions.

For a given background concentration of an individual electron acceptor, the potential

contaminant mass removal or "biodegradation capacity" depends on the "utilization factor" for

that electron acceptor.

4.3 Model ApplicabilityThe BIOPLUME in model has been developed mainly to simulate the natural attenuation of

hydrocarbons due to dispersion, sorption, and biodegradation using oxygen, nitrate, iron (DI),sulfate, and carbon dioxide as electron acceptors. The model is generally used to answer the

following questions regarding natural attenuation:

• How large will the plume extend if no engineered/source controls are implemented?• How long will the plume persist until natural attenuation process completely dissipate the

contaminants?

The BIOPLUME III model is applicable to the Popile Site, as the objective of groundwater modelingis to evaluate the remedial goal of contaminant plume confinement by natural attenuation due to thediscussed physical and biological processes.

In addition, the BIOPLUME III program is reliable as validations have been performed by the model

development team and by others against analytical solutions, laboratory results, and field studies assummarized in Rifai et al (1998). The program is a public domain computer program and runs ina Windows95/NT environment. It has been integrated with the Environmental Information System

(EIS), a user friendly graphical user interface (GUI) by contractors of the Air Force.

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5.0 Model Application

5.1 Application Process

Effective application of the BIOPLUME m model to the Popile Site will comprise qualitative and

quantitative procedures which require the knowledge of groundwater hydrology, contaminant

transport mechanisms, biodegradation, and numerical methods. Figure 5.1 is the flow diagram of

the proposed model application processes.

Preparation of an operational model for the Popile Site can be divided into three distinct stages:

(1) initialization and preparation;

(2) calibration;

(3) problem solving or scenario analysis.

Often, results from certain steps within a phase are used as feedback in previous steps, resulting in

an iterative procedure.

The modeling process is initiated with the formulation of the modeling scenarios, derived from

analysis of the site characterization. Within this context, compilation, inspection, and interpretation

of available data have resulted in the first conceptualization of the groundwater system as discussed

in Section 2.

Calibration, starts with the design or improvement of the model grid and the preparation of an input

file by assigning initial and boundary conditions and other data pertinent to the execution of

BIOPLUME III computer code. The actual model runs then take place, followed by the

interpretation of simulation results and comparison with observed data. The results of these

simulations are used to further improve the concepts of the groundwater system and the values of

the hydraulic parameters. Sensitivity runs are performed to assist in the calibration procedure.

After the calibration stage has .been concluded satisfactorily, it is followed by the scenario analysis,

in which the calibrated model is used to obtain answers to the questions posed in the modeling

objectives. The use of uncertainty analysis in this stage of the modeling process provides insight

into the reliability of the computed predictions.

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Figure 5-1 Popile Site Groundwater Modeling Processes

Formulation of Objectives and Scenarios

Compile and Interpret Available Data

1——^

L

Compile and Interpret Available Data

^

Conceptualiza

CodeS

^

Input Data

^

Perform Mod<

i

Interpret Results and Compare with Observed Data

^

Calibration and S

^

Scenario Sim

^

Uncertain!

i

Verification of Scenario Analysis

r

tion of System

election

r

Preparation

'

e\ Simulations

r

r

ensitivity Analysis

r

nulation Runs

r

y Analysis

r

4———|

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Verification of Scenario Analysis

The final stage of the model application involves checking of simulation results with respect to

feasibility and completeness.

5.2 Input Data5.2.1 Grid DesignIn application of numerical models, one of the elements most critical to the accuracy of thecomputational results is the spatial and temporal discretization chosen. Spatial discretization isrepresented by the grid overlaying the aquifer and formed by cells defined by interconnected nodes.The discretization in time is represented by the sequence of time steps selected for the simulations.

A number of general principles will be applied to the Popile project. Among these, the grid will be

designed to optimally represent:

• External and internal boundaries such as recharge, no-flow, geologic formation boundaries• External and internal stresses such as stream stages, contaminant sources, and recharge rates• Aligning where possible the main grid orientation with the principal direction of flow and

transport

• Using a coarse grid where data are scarce or for parts of the model area away from the area ofinterest

• Spacing nodes close together in areas having large changes in transmissivity or hydraulic head,or where concentration gradients are significant

• Where possible, let internal and external boundaries coincide with grid boundaries• Locate "well" nodes near the physical location of the well

There is a direct relationship between numerical accuracy and stability, grid density and time stepsize. Numerical accuracy often can be improved by reducing the grid spacing, as the truncation errorin the numerical approximations is proportional to (Al)2, the square of grid spacing. A related

numerical problem is the occurrence of oscillations. Various methods exist to reduce this problem,such as using the Peclet number (Pg)

P,=v*(At)/D<2

where: v is the groundwater velocity; At is the time step; and D is the diffusivity.

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Oscillations may also be stabilized by reducing grid spacing, or modifying the time step to conform

to the Courant criterion (C,.)

C,=v*(At)/(Al)^ 1

To reduce numerical problems when variable grid spacing is present, the changes in size betweenneighboring cells or elements will be restricted.

5.2.2 Initial ConditionsThe head and concentrations at the start of the simulation period will be specified in the BIOPLUME

III input. The specified variables include: initial water table, initial concentration of contaminants,

dissolved oxygen, nitrate, iron, sulfate, and carbon dioxide.

The initial water table will be developed by contouring water level data measurements. The initialconcentration of contaminants and the initial concentrations of all the electron acceptors will be

generated through kriging of the data from monitoring wells.

5.2.3 Boundary ConditionsThe BIOPLUME III model will be applied to the shallow groundwater aquifer located at the PopileSite, and the Bayou de Loutre. The aquifer is vertically bounded by the water table and the top of

the Cook Mountain Formation. The upstream and downstream limits of the Bayou will extend asfar as necessary to establish the potential for contaminated groundwater inflow. Area boundaries

include the site plus the off site area to the east, up to and including the Bayou de Loutre. If

hydraulic communication is incomplete between the groundwater and Bayou, the model boundarywould be extended to allow underflow. Of particular importance is the aquifer east of the formerprocess ponds, and the Bayou downstream of the potential inflow from the aquifer.

Hydraulic boundary conditions applicable include infiltration recharge, constant head, river, and no-flow boundary conditions.

5.2.4 Contaminant Sources

It is assumed that the non-aqueous phase hydrocarbon within the source zone has been trapped in

localized pools or has reached residual saturation at which DNAPL becomes discontinuous and is

immobilized by capillary forces. This source zone is defined in Section 2.0 and is stable for the

groundwater simulation period. Soluble hydrocarbons from the DNAPL continue to partition into

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the aqueous phase as fresh groundwater enters the source area. Concentrations of the dissolved

hydrocarbons will arrive at their solubilities when reaching equilibrium.

Constant concentrations at the solubilities of the simulated contaminants will be applied to the finite-

difference cells in the BIOPLUME ffl model within the source zone.

5.2.5 Advection, Dispersion, RetardationAdvection is the contaminant migration with the bulk flow of groundwater. Estimation ofadvectionis based on the determination of groundwater flow path and velocity.

Dispersion is the process whereby a plume will spread out in a longitudinal direction along the

direction of groundwater flow, and transversely perpendicular to groundwater flow due to

mechanical mixing in the aquifer and chemical diffusion. Simple estimation techniques based onthe length of the plume are available from a compilation of field test data (Gelhar et al., 1992).Typical dispersivity is a function ofLp (the plume length in ft) as shown below.

• Longitudinal Dispersivity

a, = 3.28 * 0.83 * [log,o (4 / 3.28)]2-414 (Xu and Eckstein, 1995)

• Transverse Dispersivity

a-, = 0.10 * c^ (Gelhar et al., 1992)

The rate atwhich dissolved contaminants moving through an aquifer can be reduced relative to themean flow by sorption of contaminants to the solid matrix. The degree of retardation depends onboth aquifer and constituent properties. The retardation factor (R) is the ratio of the groundwatervelocity to the rate that organic chemicals migrate in the groundwater and can be calculated using:

R = l + K ^ p b / n

^d = "OC ^OC

where: K,, is the distribution coefficient; p,, is the bulk density; n is the porosity; Koc is the organic

carbon partition coefficient; and fycls me fraction of organic carbon in the soil.

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5.2.6 Electron AcceptorsElectron acceptors required for input to the BIOPLUME in model include oxygen, nitrate, iron (III),

sulfate, and carbon dioxide. Concentrations of these constituents in groundwater from wells within

and outside the plume area have been measured as shown in Section 1.3. Values measured from the

upgradient wells will be used for input as constant concentration boundary conditions.

In addition to the dissolved oxygen influx from upgradient groundwater, aquifer vertical exchangeof oxygen with the unsaturated zone will also be included. The reaeration coefficient can be

calculated using:

K = 2611 (Dv )°-79 exp[-10.5B/(B+l .04)] (Borden, 1986)

where: B is the saturated thickness of the aquifer; and Dy is the vertical dispersion coefficient.

5.3 Validation, Verification, CalibrationThe Quality Assurance Project Plan (QAPP) for the Groundwater Investigation, Modeling and

Remedial Design (MK, 1998a) stated that the data quality objectives for the modeling results willbe formulated by, and subject to, the validation and calibration procedures in the GroundwaterModeling Work Plan. As a minimum, the model must be demonstrated to function in the following

ways:

• Convergency - A test of whether the model gives a unique answer for a single set of input

parameters, and that this answer is correct when compared to closed-form solutions of the same

problem.• Stability - A test that the model does not break down and fail under any data sets.

• Continuity - A test that the output continuously depends on changes to each input variable, e.g.,different input produces different output all the time.

The above criteria for a numerical simulation normally are warranted when the conditions discussed

in Section 5.2.1 are met. However, MK will perform such criteria verification on all the simulationanalyses.

Calibrating the BIOPLUME HI model is the process of demonstrating that the model is capable of

producing field-measured values of the head and concentrations at the Popile Site. For the case ofgroundwater flow, calibration is accomplished by finding a set of parameters, boundary and initial

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conditions, and stresses that produces simulated values of heads that match measured values to a

satisfactory level.

The procedure for calibrating the BIOPLUME in model is by manual trial-and-error selection of

parameters. The parameters that are adjusted for calibrating the flow at the site include porosity,specific storage, transmissivity, andthickness within the range of field measurements. Recharge will

be determined with an infiltration calculation. The main parameters that can be used to calibrate thetransport and fate of the hydrocarbons include the source definition, dispersivity, distributioncoefficient, and biodegradation parameters.

Due to the lack of long-term historical groundwater level measurements and the unknown releasehistory of contaminants, the model calibration will only be limited to match the steady state

piezometric heads using data from the MK field investigation phase (MK, 1998). Model calibrationto match the dissolved phase contaminant plume will not be performed. However, the plume willbe used to calculate longitudinal and transverse dispersivities as discussed in Section 5.2.5 and toaid in evaluating the biodegradation parameters, such as the half-lives and Monod kinetics

parameters.

The Root Mean Square (RMS) Error will be calculated to assess the head calibration error.

The output from the BIOPLUME in model includes a hydraulic mass balance and a chemical mass

balance assessment. These mass balance assessments will be used to determine how well the

numerical techniques are performing in terms of simulating the specific site conditions. Waterbalances of less than 1% and chemical mass balances of less than 15% are desirable.

5.4 Sensitivity Analysis

MK will perform sensitivity analysis to quantify the effects of uncertainty in the estimates of model

parameters on model results. During a sensitivity analysis, calibrated values for transmissivity,

thickness, recharge, dispersivity, etc. are systematically changed within the range of field

measurements. The magnitude of change in heads and concentrations from the calibrated model isa measure of the sensitivity of the model results to the particular parameter. The results of this

analysis are expressed as the effects of the parameter change on the spatial distribution of heads andconcentrations.

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6.0 Quality Assurance

The Popile project quality assurance program has been discussed in the Quality Assurance Project

Plan (QAPP) (MK, 1998a). The following sections presents the modeling task specific QA

procedures to assure technically and scientifically adequate execution of all the modeling steps, and

to assure that all modeling-based analysis is verifiable and defensible..

6.1 Quality ControlQC refers to the following procedures that ensure the quality of the overall modeling study. Theseprocedures include the use of appropriate methodology, adequate validation, and proper usage of the

selected methods and models.

• Correct and clear formulation of problems to be solved

• Project description and objectives• Type of modeling approach to the problems• Conceptualization of groundwater system and contaminant transport processes• Detailed description of assumptions and simplifications, both explicit and implicit• Data acquisition and interpretation• Model selection process and justification• Model preparation (parameter selection, data entry)• The validity of the parameter values used in the model application• Protocols for parameter estimation and model calibration• Identification of calibration goals and evaluation of how well they have been met

• The role of sensitivity and uncertainty analysis

• Presentation of simulation results

6.2 Quality Assessment

Quality assessment is applied to monitor the implementation of the QC procedures and consists ofauditing and technical review.

Audits are internal administrative procedures designed to assess the degree of compliance with the

QC requirements. Compliance is measured in terms of traceability of records, accountability

(approvals from project manager), and fulfillment of procedures described in the QC plan. Thepaper trail for audits should consist of reports and files that include a description of:

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• Assumptions• Input parameter values and sources

• Boundary and initial conditions

• Grid and time interval justifications

• Changes made in code• Actual input used• Output of model runs and interpretation

• Calibration of model• Sensitivity and uncertainty analysis

All data files, source codes, and executable versions of computer software used in the modelingstudy will be retained for auditing or post-project reuse in hard-copy and at higher levels in digital

form.

• Though not anticipated, any modifications made to the codes during model application for the

specific problem, will require the code being tested again; all QC/QA procedures for should, again

be applied, including accurate record keeping and reporting. All new input and output files should

be saved for inspection and possible reuse together with existing files, records, codes, and datasets.

Technical review will consist of evaluation of the technical and scientific basis of the modeling studyby USACE, U.S. EPA, and ADPC&E on a 60% completion basis.

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7.0 References

American Society for Testing and Materials (ASTM), 1984. "Standard Practices for Evaluating

Environmental Fate Models of Chemicals", Annual Book of ASTM Standards, E978-E984, Am.

Soc. For Testing and Matls., Philadelphia, PA.

American Society for Testing and Materials (ASTM), 1998. "Standard Guide for Remediation byNatural Attenuation at Petroleum Release Sites", E1943-98, 100 Barr Harbor Dr.,-West

Conshohocken, PA 19428, August.

Borden, R.C., 1986. "Influence of Adsorption and Oxygen Limited Biodegradation on the Transportand Fate of a Creosote Plume: Field Methods and Simulation Techniques", Houston, Texas.

Edgehill, R.H., and R.K. Finn, 1983. "Activated Sludge Treatment of Synthetic Wastewater

Containing Pentachlorophenol", Biotechnology and Bioengineering, 25:2165-2176.

Gelhar, L.W., C. Welty, and K.R. Rehfeldt, 1992. "A Critical Review of Data on Field-ScaleDispersion in Aquifers", Water Resources Research, 28(7); 1955-1974.

Micromedex, Inc., 1998. "Hazardous Substances Data Bank by National Library of Medicine",TOMES CPS™ System.

Mihelcic, J.R., and R.G. Luthy, 1988. "Microbial Degradation ofAcenaphthene and Naphthalene

under Denitrification Conditions in Soil Water systems". Applied Environmental Microbiology,54:1182-1187.

Mikesell, M., and S.A. Boyd, 1986. "Enhancement of Pentachlorophenol Degradation in Soil TroughInduced Anaerobiosis and Bioaugmentation with Anaerobic Sewage Sludge", ES&T ResearchPaper, No. 12710, Michigan Aricultural Experiment Station.

Morrison Knudsen Corp, 1998. Groundwater Modeling Data Collection, preliminary unpublisheddata from Summer 1998 Field Investigation.

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Morrison Knudsen Corp, 1998a. "Quality Assurance Project Plan for Groundwater Investigation,

Modeling and Remedial Design", Final, Prepared for USACE, June.

National Research Council (NRC), 1993. "In-situ Bioremediation - When Does It Works?", National

Academy Press, Washington, D.C.

Sims, R.C., and M.R. Overcash, 1983. "Fate of Poly-Nuclear Aromatic Compounds in Soil-Plant

Systems", Residue Reviews, 88:1 -68.

Stauffer, T.B., et al., 1994. "Degradation of Aromatic Hydrocarbons in an Aquifer During a FieldExperiment Demonstrating the Feasibility ofRemediation by Natural Attenuation", U.S. Air Force

Report No. AL/EQ-TR-1993-0007., Tyndall Air Force Base, Florida.

Stroo, H.F., et al., 1988. "Development of an In-situ Bioremediation System for a CreosoteContaminated Site", pp919-936. hi Progress, International Conference on Physicochemical andBiological Detoxification of Hazardous Wastes, Atlantic City, New Jersey.

Stroo, H., C. Cosentini, T. Ronniny, and M. Larsen, 1997. "Natural Biodegradation of WoodPreservatives", CCC 1051-5658/97/070477-18, John Wiley & Sons, me.

U.S. EPA, 1988. "Selection Criteria for Mathematical Models Used in Exposure Assessments:Groundwater Models", EPA600/8-8 8/075.

U.S. EPA, 1992. "Groundwater Issue - Fundamentals of Groundwater Modeling", EPA/540/S-92/005, April.

U.S. EPA, 1998. "BIOPLUME in-Natural Attenuation Decision Support System, User's Manual",Version 1.0, EPA/600/R-98/010, Office of Research and Development, Washington, D.C. 20460.

Wiedemeier, T.H., R.N. Miller, J.T. Wilson, andD.H. Kampbell, 1995. "Significance of Anaerobic

Processes for the Intrinsic Bioremediation of Fuel Hydrocarbons", National Groundwater

Association, Proceedings of the Petroleum Hydrocarbons and Organic Chemicals in GroundwaterConference, Houston, Texas, November.

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Xu, M., Y. Eckstein, 1995. "Use of Weighted Least-Squares Method in Evaluation of theRelationship Between Dispersivity and Scale", Journal of Ground Water, 33(6):905-908.

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