ASSESSMENT OF INTRINSIC BIOREMEDIATION OF 

GASOLINE CONTAMINATION IN THE SHALLOW AQUIFER, 

LAUREL BAY EXCHANGE, MARINE CORPS AIR STATION 

BEAUFORT, SOUTH CAROLINA

By James E. Landmeyer, Francis H. Chapelle, and Paul M. Bradley

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations Report 96-4026

Prepared in cooperation with the

DEPARTMENT OF THE NAVY, SOUTHERN DIVISION,

NAVAL FACILITIES ENGINEERING COMMAND and

MARINE CORPS AIR STATION BEAUFORT, SOUTH CAROLINA

Columbia, South Carolina 

1996
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

For additional information write to:

District Chief 

U.S. Geological Survey 

Stephenson Center-Suite 129 

720 Gracern Road 

Columbia, SC 29210-7651

Copies of this report can be purchased from:

U.S. Geological Survey 

Earth Science Information Center 

Open-File Reports Section 

Box 25286, Mail Stop 517 

Denver Federal Center 

Denver, CO 80225
CONTENTS

Page

Abstract .................................................................... 1

Introduction ................................................................. 2

Purpose and scope ....................................................... 2

Description of study area .................................................. 3

Framework for assessing intrinsic bioremediation .............................. 3

Field and laboratory data-collection methods ....................................... 5

Field sample collection and analysis ......................................... 5

Laboratory experiments ................................................... 6

Quantification of hydrologic, geochemical, and microbial parameters.................... 7

Hydrologic and hydrogeologic framework..................................... 7

Ground-water chemistry.................................................. 10

Adsorption coefficient.................................................... 24

Microbial degradation.................................................... 24

Assessment of intrinsic bioremediation........................................... 27

Approach to simulating solute transport...................................... 27

Solute-transport simulations of toluene migration .............................. 29

Solute-transport simulations of benzene migration ............................. 39

Conclusions ................................................................ 49

References cited............................................................ 50
ILLUSTRATIONS

Page

Figure 1. Map showing location of Laurel Bay Exchange, Marine Corps Air

Station Beaufort, S.C., study area ...................................4

2. Diagram showing lithologic description from a borehole located near

LB-EX-14......................................................5

3-5. Maps showing:

3. The water table in the shallow aquifer system, Laurel Bay Exchange,

Marine Corps Air Station Beaufort, S.C., April 17, 1993 ...............8

4. The water table in the shallow aquifer system, Laurel Bay Exchange,

Marine Corps Air Station Beaufort, S.C., March 10, 1994..............9

5. Location and geometry of the benzene, toluene, ethylbenzene, 

total xylene, and methyl-tert-butyl-ether plumes, Laurel 

Bay Exchange, Marine Corps Air Station Beaufort, S.C., 

March 11,1994 ..............................................13

6-11. Graphs showing:

6. Relation between hydrocarbon concentrations and distance from

the source area, March 11, 1994..................................14

7. Relation between dissolved inorganic carbon and dissolved oxygen 

in contaminated and uncontaminated ground water, Laurel Bay 

Exchange, Marine Corps Air Station Beaufort, S.C., March 11, 1994 ... .20

8. Relation between dissolved inorganic carbon and dissolved 

ferrous iron in contaminated and uncontaminated ground 

water, Laurel Bay Exchange, Marine Corps Air Station 

Beaufort, S.C., March 11, 1994 ..................................21

9. Relation between hydrocarbon and dissolved oxygen concentrations

for toluene and benzene, March 11, 1994...........................23

10. Relation between adsorbed and dissolved concentrations of benzene 

and toluene for aquifer sediments from Laurel Bay Exchange, 

Marine Corps Air Station Beaufort, S.C............................25

11. Mineralization rates of carbon 14-toluene under aerobic and

anaerobic aquifer conditions.....................................26

12-26. Maps showing:

12. The locations of the Saturated and Unsaturated TRAnsport

model area ...............................................31

13. Simulated 1994 distribution of toluene for values of the

Standard Model at Laurel Bay Exchange, Marine Corps

Air Station Beaufort, S.C. ...................................32

14. Simulated toluene plume in 1995, 1996,2001, and 2011 at Laurel Bay 

Exchange, Marine Corps Air Station Beaufort, S.C., using 

the Standard Model ........................................33

iv
ILLUSTRATIONS-Continued

Page

12-26. Maps showing:

15. Simulated 1994 distribution of toluene released in 1991 

assuming the Conservative Model at the Laurel Bay 

Exchange, Marine Corps Air Station Beaufort, S.C................35

16. Simulated 1994 distribution of toluene for minimum and

maximum values of hydraulic conductivity at Laurel Bay

Exchange, Marine Corps Air Station Beaufort, S.C................36

17. Simulated 1994 distribution of toluene for minimum and

maximum values of biodegradation at Laurel Bay Exchange,

Marine Corps Air Station Beaufort, S.C.........................37

18. Simulated 1994 distribution of toluene for minimum and

maximum values of toluene adsorption capacity at Laurel

Bay Exchange, Marine Corps Air Station Beaufort, S.C............38

19. Simulated 1994 distribution of toluene for minimum and

maximum values of hydrologic, geochemical, and microbial

parameters at Laurel Bay Exchange, Marine Corps Air

Station Beaufort, S.C. ......................................40

20. Simulated 1995,1996, 1998, 2000, and 2001 distributions of 

toluene after source removal in 1994, at Laurel Bay 

Exchange, Marine Corps Air Station Beaufort, S.C................41

21. Simulated 1993 distribution of benzene for values of the 

Standard Model at Laurel Bay Exchange, Marine Corps 

Air Station Beaufort, S.C. ...................................42

22. Simulated 1994 distribution of benzene for values of the 

Standard Model after source removal in 1993, at Laurel 

Bay Exchange, Marine Corps Air Station Beaufort, S.C............43

23. Simulated 2006, 2011, and 2044 distributions of benzene

assuming Standard Model transport parameters, at Laurel

Bay Exchange, Marine Corps Air Station Beaufort, S.C............45

24. Simulated 1994, 2022, and 2050 distributions of benzene 

assuming Conservative Model transport, at Laurel Bay 

Exchange, Marine Corps Air Station Beaufort, S.C................46

25. Simulated 1994 and 2022 distributions of benzene using a 

minimum value of hydraulic conductivity, at Laurel Bay 

Exchange, Marine Corps Air Station Beaufort, S.C................47

26. Simulated 1994 and 2022 distributions of benzene using a 

maximum value of hydraulic conductivity, at Laurel Bay 

Exchange, Marine Corps Air Station Beaufort, S.C................48

v
TABLES

Page

Table 1. Concentrations of benzene, toluene, ethylbenzene, total xylenes, 

and methyl-tert-butyl-ether in ground water from monitoring 

wells at the Laurel Bay Exchange, Marine Corps Air Station 

Beaufort, S.C., March 11, 1994.......................................11

2. Solubilities of benzene, toluene, ethylbenzene, o-xylene, and

methyl-tert-butyl-ether in ground water at 25 degrees Celsius...............12

3. Concentrations of dissolved oxygen, ferrous iron, and sulfide in

ground water, and field parameter values of pH and temperature

of ground water from monitoring wells at the Laurel Bay Exchange,

Marine Corps Air Station Beaufort, S.C., March 11, 1994.................. 16

4. Concentrations of cations in ground water from monitoring wells at 

the Laurel Bay Exchange, Marine Corps Air Station Beaufort, 

S.C., March 11, 1994...............................................17

5. Concentrations of anions in ground water from monitoring wells at 

the Laurel Bay Exchange, Marine Corps Air Station Beaufort, 

S.C., March 11, 1994...............................................18

6. Concentrations of dissolved inorganic carbon and methane in

ground water from monitoring wells at the Laurel Bay Exchange,

Marine Corps Air Station Beaufort, S.C., March 11, 1994................. .19

7. Most probable values and best estimates for hydraulic gradient, 

hydraulic conductivity, microbial degradation, and adsorption 

for site conditions at the Laurel Bay Exchange, Marine Corps 

Air Station Beaufort, S.C............................................30

VI
CONVERSION FACTORS, VERTICAL DATUM, AND ABBREVIATIONS

Multiply By To Obtain

Length

inch (in.) 25.4 millimeter

foot (ft) 0.3048 meter

foot per foot (ft/ft) 0.3048 meter per meter

mile (mi) 1.609 kilometer

Area

square mile (mi2) 2.590 square kilometer

Flow

foot per day (ft/d) 0.3048 meter per day 

foot per year (ft/yr) 0.3048 meter per year

Volume

gallon (gal) 3.785 liter

Temperature: In this report, temperature is given in degrees Celsius (°C), which can be converted 

to degrees Fahrenheit (°F) by the following equation:

°F = (1.8x°C) + 32

Sea Level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929~a 

geodetic datum derived from a general adjustment of the first-order level nets of the United States 

and Canada, formerly called Sea Level Datum of 1929.

Chemical concentration: In this report, chemical concentration in water is expressed in metric 

units as milligrams per liter (mg/L) or micrograms per liter (ng/L).

Radiochemical activity: In this report, radioactivity units are given in microcuries (|iCi).

vii
CONVERSION FACTORS, VERTICAL DATUM, AND ABBREVIATIONS-Continued

Biodegradation rate constant: In this report, first-order kinetics are used to model biodegradation 

rates of petroleum hydrocarbons. First-order rate constants (K^j0) are expressed in units of percent 

of compound degraded per day (pct/d). Because percent is dimensionless, this reduces to units of 

inverse days (d ).

Adsorption coefficient: In this report, the adsorption coefficient (Kad) is expressed as feet cubed 

per milligram (fr/mg), which are equivalent to units of liters per kilogram (L/kg).

Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Other units used in report:

gram = g 

micrograms = jig 

micrometer = jim 

milliliter = mL 

parts per million = ppm

Abbreviations used in this report:

BTEX - benzene, toluene, ethylbenzene, and total xylenes

DIG - dissolved inorganic carbon

FID - flame-ionization detection

GC - gas chromatography

MCAS - Marine Corps Air Station

MCL - maximum contaminant level

MSGL - Microbial Studies Group Laboratory

MTBE - methyl-tert-butyl-ether

POC - point(s) of contact

SCDHEC - South Carolina Department of Health and Environmental Control

SUTRA - Saturated - Unsaturated TRAnsport

TEAP - terminal electron-accepting process

USGS - U.S. Geological Survey

viii
ASSESSMENT OF INTRINSIC BIOREMEDIATION OF GASOLINE

CONTAMINATION IN THE SHALLOW AQUIFER, LAUREL

BAY EXCHANGE, MARINE CORPS AIR STATION

BEAUFORT, SOUTH CAROLINA

By James E. Landmeyer, Francis H. Chapelle, and Paul M. Bradley

ABSTRACT

Laboratory, field, and digital solute-transport-modeling studies demonstrate that 

microorganisms indigenous to the shallow ground-water system at Laurel Bay Exchange, Marine 

Corps Air Station Beaufort, South Carolina, can degrade petroleum hydrocarbons in gasoline 

released at the site. Microorganisms in aquifer sediments incubated in the laboratory under 

aerobic and anaerobic conditions mineralized radiolabeled carbon 14-toluene to 14C-carbon 

dioxide with first-order rate constants of Kbio - -0.640 per day and Kbio = -0.003 per day, 

respectively. Digital solute-transport modeling using the numerical code SUTRA revealed that 

anaerobic biodegradation of benzene occurs with a first-order rate constant near Kbio = -0.00025 

per day. Sandy aquifer material beneath Laurel Bay Exchange is characterized by relatively high 

hydraulic conductivities (K^ = 8.9 to 17.3 feet per day), average ground-water flow rate of about 

60 feet per year, and a relatively uniform hydraulic gradient of 0.004 feet per foot. The sandy 

aquifer material also has low adsorptive potentials for toluene and benzene (both about K^ = 2.0 

x 10"9 cubic feet per milligram), because of the lack of natural organic matter in the aquifer. The 

combination of this ground-water-flow rate and absence of significant adsorptive capacity in the 

aquifer permits toluene and benzene concentrations to be detected downgradient from the source 

area in monitoring wells, even though biodegradation of these compounds has been demonstrated. 

Solute-transport simulations, however, indicate that toluene and benzene will not reach the Broad 

River, the nearest point of contact with wildlife or human populations, about 3,600 feet west of 

the site boundary. These simulations also show that contamination will not be transported to the 

nearest Marine Corps property line about 2,400 feet south of the site. This is primarily because 

the source of contaminants has essentially been removed, and the low adsorptive capacity of the 

aquifer sediments has prevented the occurrence of an adsorbed, continuous source of petroleum 

hydrocarbons. Therefore, digital simulations of toluene and benzene transport at Laurel Bay 

Exchange indicate that intrinsic bioremediation could be a successful remediation alternative for 

prohibiting transport of dissolved toluene and benzene to the Broad River.
INTRODUCTION

An unknown volume of gasoline from the underground storage tank system at the Laurel 

Bay Exchange, Marine Corps Air Station (MCAS) Beaufort, S.C., was detected in the subsurface 

in September 1991. A contamination assessment made during March and April 1993 indicated 

that petroleum hydrocarbon concentrations in soils and ground water exceeded maximum 

contaminant levels (MCL's) for ground water established by the South Carolina Department of 

Health and Environmental Control (SCDHEC) (U.S. Navy Department, 1993). 

Recommendations were made to reduce these concentrations to compliable levels and included 

(1) tank excavation (source removal) and thermal treatment (incineration) of the removed soil, (2) 

ground-water extraction and reinjection, and (3) enhancement of naturally occurring 

biodegradation through engineering processes at the site (U.S. Navy Department, 1993). The 

tanks and surrounding sediments were excavated and removed in 1992 pursuant to SCDHEC 

regulations (F. Araico, Marine Corps Air Station Beaufort, written commun., 1994).

The U.S. Geological Survey (USGS), in cooperation with the Southern Division, Naval 

Facilities Engineering Command, U.S. Navy, and MCAS Beaufort quantified hydrologic, 

geochemical, and microbial data in sediments from Laurel Bay Exchange to determine if 

contaminant transport will occur to pre-defined points-of-contact (POC) such as adjacent surfacewater bodies. The objective of this study is to establish a framework for assessing intrinsic 

bioremediation or a remedial alternative consistent with recent guidance documents prepared by 

the SCDHEC.

Purpose and Scope

The results presented in this report summarize a laboratory-, field-, and numericalmodeling-based study focused on the assessment of intrinsic bioremediation of gasoline in the 

shallow aquifer at Laurel Bay Exchange, MCAS Beaufort, S.C. (hereafter referred to as either 

Laurel Bay or the site). This study provides the necessary criteria to support intrinsic 

bioremediation as a feasible remedial strategy compared to the more costly, engineered 

alternatives offered in the Navy's original assessment report. This study consisted of (1) 

delineating the 1994 location of two different hydrocarbon plumes at the site (benzene, toluene, 

ethylbenzene, and total xylenes BTEX, and the gasoline additive methyl-tert-butyl-ether ~ 

MTBE); (2) assessing the hydrologic conditions in the shallow aquifer system, and identifying 

potential points of ground-water discharge that coincide with points-of-contact (POC) or MCAS 

property boundaries; (3) determining the rate of toluene degradation under aerobic and anaerobic 

aquifer conditions using radiolabeled toluene, and the degree of toluene adsorption onto aquifer 

sediments; (4) using a digital solute-transport model to estimate the anaerobic degradation rate of 

benzene, and a laboratory study to assess the degree of benzene adsorption onto aquifer 

sediments; and (5) conducting a digital solute-transport analysis designed to estimate rates of 

toluene and benzene transport in the aquifer in order to estimate the extent of toluene and benzene 

transport to delineated POC or property lines by incorporating data gathered in steps 1-4.
Description of Study Area

The Laurel Bay Exchange, MCAS, in Beaufort, S.C., is located on Port Royal Island, in the 

Sea Islands of the Atlantic Coastal Plain physiographic province (fig. 1). The service station at 

the site handled gasoline that leaked from underground storage tanks into the silty sand sediments 

that comprise the Pleistocene to Holocene surficial terrace deposits on Port Royal Island. The 

surface sediments at the site, to a depth of around 40 ft, consist of Holocene (<7,000 years Before 

Present) fine-grained sand and represent a back-barrier island depositional fades. The lithology 

of these sediments is shown in a representative lithologic log of a boring near LB-EX-14 (fig. 2). 

The uncontaminated sediments at the site are comprised of fine-to-coarse grained sands and some 

silt coated with ferric oxyhydroxides. This indicates the relatively shallow depth of the ground 

water and its saturation with dissolved oxygen. The amount of ferric oxyhydroxide coating 

diminishes, however, as depth and petroleum hydrocarbon concentrations increase.

Framework for Assessing Intrinsic Bioremediation

The efficiency of intrinsic bioremediation to limit contaminant migration in ground-water 

systems can be assessed by quantitatively comparing rates of contaminant transport to rates of 

biodegradation. If rates of transport by ground water are fast relative to rates of biodegradation, 

then contaminants may migrate with ground water and potentially reach POC with human or 

wildlife populations. Conversely, if transport rates are slow relative to biodegradation rates, 

contaminant migration will be more confined and be less likely to reach POC. In either case, the 

efficiency of intrinsic bioremediation can be assessed at a particular site by evaluating the 

presence or absence of contaminant transport to pre-determined POC. Thus, this assessment 

includes determining the hydrologic (rates of ground-water flow), geochemical (adsorption 

properties of the aquifer material), microbial (rates of degradation), and demographic (POC) 

constraints in the system.

The U.S. Geological Survey (USGS), in cooperation with the Southern Division, Naval 

Facilities Engineering Command, U.S. Navy, and MCAS Beaufort, has developed a framework 

for assessing the efficiency of intrinsic bioremediation that is based on these components. In this 

framework, hydrologic, geochemical, and microbial data are integrated using a digital solutetransport model to determine rates of contaminant transport from a source area to pre-defined 

POC such as adjacent surface-water bodies. This framework is consistent with recent guidance 

documents prepared by the SCDHEC and is designed for application at sites where ground-water 

contamination is the principle issue of concern.
LB-EX-5A

STORAGE TANK AREA

LB-EX-RW

BUILDING

p'

LB-EX-16

!

N

Blue Ridge

Piedmont

00

SOUTH CAROLINA!

Lower Coastal Plain

Upper Coastal

Plain

[STUDY AREA

50 100 150 200 FEET

50 METERS

EXPLANATION

ROAD

.......... FENCE LINE

LB-EX-16 MONITORING WELL

AND IDENTIFICATION NUMBER

Figure 1. Location of Laurel Bay Exchange, Marine Corps Air Station Beaufort, S.C., study area.
ILU 

LU

Q. 

LU 

Q

o5- 

10-

15i \j

LITHOLOGY

...;... ; ; :.

.-.._,

END OF HOLE

DESCRIPTION

Red, fine sand

Yellow sand

White, moist sand

White to brown sand

Figure 2. Lithologic description from a borehole 

located near LB-EX-14.

FIELD AND LABORATORY DATA-COLLECTION METHODS

Ground-water samples were collected and analyzed for BTEX and MTBE concentrations to 

document the 1994 extent of petroleum hydrocarbons in ground water at the Laurel Bay site. 

Laboratory experiments were performed to determine the capacity of the aquifer material to 

adsorb toluene and benzene, and to estimate rates of toluene degradation, whereas benzene 

degradation was determined using an iterative digital solute-transport modeling approach. 

Hydrologic, geochemical, and microbial values were evaluated within a framework based on 

digital solute-transport modeling to determine contaminant transport relative to containment at 

Laurel Bay.

Field Sample Collection and Analysis

Sampling of existing monitoring wells at the site for water-chemistry data, BTEX and 

MTBE concentrations, and aquifer sediment samples for laboratory degradation experiments was 

conducted on March 10 and 11, 1994. An initial trip to the site was made on February 10, 1994, 

to collect aquifer sediments for laboratory determination of adsorption coefficients (K^) for 

toluene and benzene. A synoptic water-level measurement was made on all existing monitoring 

wells on March 10, 1994, using a steel tape. All water levels were corrected to sea level.
During well purging, measurements of water temperature (in degrees Celsius) and pH (in 

standard units) were made until stable values were obtained. Dissolved-oxygen concentrations 

were quantified using the Winkler titration method (Hach Company, 1989). Dissolved ferrous 

iron [Fe(II)], and sulfide concentrations were determined using the Hach colorimetric method 

(Stookey, 1970). Ground-water samples to be analyzed for water-chemistry data and BTEX and 

MTBE concentrations were collected following the purging of the standing-well volume using a 

peristaltic pump (about 5 gallons, equivalent to 3 well volumes). Ground-water samples to be 

analyzed for BTEX and MTBE concentrations were collected in a Teflon bailer, transferred to 

40 mL, pre-preserved (with HC1) amber glass vials using a bottom-discharge stopcock on the 

bailer, sealed with Teflon-lined septa screw-caps, and placed on ice. All water samples were 

collected in duplicate for each well sampled, and were delivered to a contract, SCDHEC-certified 

laboratory (#40111) within 3 days of sample collection. Samples for dissolved inorganic carbon 

(DIG), an indicator of hydrocarbon degradation, and methane (CH4) analyses were collected in 

the field by passing 5 mL of ground water through a 0.45-jim filter attached to a 10-mL syringe 

into separate, sealed vials. Methane and DIG concentrations were quantified in the USGS 

Microbial Studies Group Laboratory (MSGL) by flame ionization detection (FID) gas 

chromatography (GC). Samples for inorganic cation and anion analyses in ground water were 

collected in polyethylene bottles after filtration with a 0.45-micron filter. All anions were 

quantified in the USGS MSGL by ion-conductivity detection. Contaminated aquifer sediment 

was collected near LB-EX-8 (fig. 1) using an alcohol-rinsed, stainless-steel hand auger, and 

placed in pre-sterilized, wide-mouthed glass mason jars. These jars were stored on ice in the field 

and in the refrigerator in the MSGL until analysis.

Laboratory Experiments

The primary processes that can affect the migration of organic hydrocarbon contaminants 

from a source area in the subsurface are adsorption and microbial degradation. Both processes 

were quantified in the USGS MSGL. The extent to which these factors interact to affect 

contaminant migration are then examined in a framework using a predictive digital solutetransport and ground-water-flow model.

Adsorption coefficients for toluene and benzene for aquifer sediments from the site were 

determined using a modification of the method described in Schwarzenbach and Westall (1981). 

Approximately 5 g of dried, sterilized (to avoid potential microbial degradation affects) aquifer 

sediment collected near LB-EX-14 (fig. 1) was placed in septated serum vials, and amended with 

solutions of either 0, 5,10, and 50 jig of toluene dissolved in a solution of ground water and 

MTBE to represent in situ conditions. All vials were equilibrated by shaking on a vibrating table 

for 1 hour. Samples of the liquid fraction were analyzed for toluene using FID with GC. The 

amount of compound adsorbed to aquifer material was the difference between the amount added 

and the amount present in the liquid fraction after equilibration. Concentrations of the toluene 

measured were regressed against concentrations of toluene added to obtain a linear adsorption 

coefficient. The same procedure was used to determine the adsorption coefficient for benzene 

onto aquifer sediments from the Laurel Bay site.
The microbial degradation of toluene was determined by quantifying the rate of production 

of radiolabeled 14CO2 from a known amount of radiolabeled toluene added to a microcosm 

containing saturated aquifer sediment from near LB-EX-8 (fig. 1). Approximately 428,000 

disintegrations per minute of 14C-toluene (specific activity of 9.7 milliCuries per millimole) was 

added to triplicate 40-mL glass microcosms containing 5 mL of a sediment/ground-water slurry 

and 2.5 mL of ground water collected from a nearby contaminated well (LB-EX-RW, fig. 1). For 

aerobic and anaerobic treatments, dead controls (to account for abiotic evolution of 14CO2) were 

made by sterilizing the prepared microcosm in an autoclave (121 °C for 1 hour) and adding 

8 mM (millimolar) HgCl2 . Aerobic treatments for determining degradation rates of toluene were 

made without further modification.

Anaerobic degradation rates were estimated using vials prepared as above, but made 

anaerobic by purging the vials with helium for 3 minutes upon capping. At specified times during 

the experiment, vials were sacrificed by acidification with 45% H3PO4, the evolved 14CO2 

captured in a 3 M (molar) KOH solution suspended in the microcosm in a plastic bucket 

assembly, and the 14C activity measured through liquid scintillation counting after addition to 

scintillation cocktail using a 1600TR liquid scintillation counter. The 14C activities were 

corrected with dead controls, background radiation, activity at time point t=0, and counting 

efficiency. Activities are reported as a percentage recovered of the added label.

The microbial degradation rate of benzene was not measured in the laboratory using the 

radiolabel approach described above. Rather, the digital solute-transport model SUTRA 

(Saturated and Unsaturated TRAnsport) (Voss, 1984) was used to estimate an anaerobic benzene 

biodegradation rate by performing iterative calibration simulations to reproduce benzene 

concentrations observed in 1993 and 1994.

QUANTIFICATION OF HYDROLOGIC, GEOCHEMICAL, 

AND MICROBIAL PARAMETERS

The results of the hydrologic, geochemical, and microbial investigations are presented in 

this section. The hydrology and hydrogeologic framework and ground-water chemistry data are 

presented, as well as the adsorption and microbial degradation rates as determined in field and 

laboratory studies.

Hydroloqic and Hydrogeologic Framework

The position of the water table in the shallow aquifer system was previously documented 

during contaminant assessment activities conducted in March and April 1993 (U.S. Navy 

Department, 1993) (fig. 3). In addition, the elevation of the top of each well casing was 

determined by survey. Water levels were measured using a steel tape on March 10,1994 (fig. 4). 

In general, the water table is between 10 and 13 ft below land surface, and in a few places, as 

much as 17 ft below land surface. The generalized direction of ground-water flow in the shallow 

aquifer system is toward the south and west, away from the study site. The area of potential 

ground-water discharge from the shallow aquifer system is the Broad River (fig. 1), located 

approximately 3,600 ft west of the site boundary.
FORMER 

UNDERGROUND 

STORAGE TANK AR 28.88 

28.86 A LB-EX-4 

LB-EX-3A ,s

LB-EX-RW \,\

28.65 BUILDING ^- 

N

DIRECTION OF 

GROUND-WATER FLOW

0h0

50 

1

100 

I

150 200 FEET 

I I

I 

50 METERS

 26.80 

A LB-EX-16 

25.39

EXPLANATION

ROAD

FENCE LINE

WATER LEVEL CONTOUR-- Shows altitude in which water would have stood in tightly cased wells. 

Contour interval is 0.2 Feet. Datum is sea level.

MONITORING WELL, IDENTIFICATION NUMBER AND WATER ELEVATION ON 4/17/93, IN FEET 

ABOVE SEA LEVEL.

Figure 3. The water table in the shallow aquifer system, Laurel Bay Exchange,

Marine Corps Air Station Beaufort, S.C., April 17, 1993

(from U.S. Navy Department, 1993).
FORMER 

UNDERGROUND 

STORAGE TANK AREA

t

N

DIRECTION OF 

GROUND-WATER FLOW

0

0

50

I

100 150 200 FEET 

I I I

I 

50 METERS

- 26.80 

ALB-EX-16 

25.39

EXPLANATION

ROAD

FENCE LINE

WATER LEVEL CONTOUR- Shows altitude in which water would have stood in tightly cased wells. 

Contour interval is 0.2 Feet. Datum is sea level.

MONITORING WELL, IDENTIFICATION NUMBER AND WATER ELEVATION ON 3/10/94, IN FEET 

ABOVE SEA LEVEL.

Figure 4. The water table in the shallow aquifer system, Laurel Bay Exchange, 

Marine Corps Air Station Beaufort, S.C., March 10, 1994.
The surficial aquifer system beneath the study site is comprised of fine-to-coarse grained 

sands with very few clay lenses, and is characterized by relatively high hydraulic conductivities. 

Rising-head aquifer tests conducted on four individual monitoring wells during the contamination 

assessment indicated an average hydraulic conductivity of 11.1 ft/d (U.S. Navy Department, 

1993).

The rate of ground-water movement can be determined using Darcy's equation:

v = K(dh/dx)/n, (1) 

where

v is the average velocity of ground-water flow, in feet per day;

K is the average hydraulic conductivity, in feet per day;

dh/dx is the hydraulic gradient determined between two wells, dimensionless; and,

n is the aquifer sediment porosity, dimensionless.

A conservative estimate of the rate of ground-water flow can be determined by comparing the 

flow at the steepest hydraulic gradient with the flow at the shallowest hydraulic gradient. At the 

site, however, hydraulic gradients are uniform over the study area. The water level north of the 

site at LB-EX-4 (fig. 4) was 27.66 ft above sea level on March 10,1994, and the water level south 

of the site at LB-EX-11 (about 300 ft from LB-EX-4) was 26.27 ft above sea level. A 

representative hydraulic gradient of 0.0046 ft/ft was determined between these two wells. 

Assuming a porosity of 30 percent for fine-grained sand, and using the average hydraulic 

conductivity of 11.1 ft/d, an estimate of the average ground-water flow of 0.170 ft/d, or about 62 

ft/yr was calculated.

Ground-Water Chemistry

Concentrations of BTEX and MTBE were measured in ground-water monitoring wells at 

Laurel Bay (table 1). Concentrations of benzene in ground water ranged from below detection 

limits (BD) to 9,450 j^g/L. Concentrations of toluene ranged from BD to 22,500 j^g/L. 

Concentrations of ethylbenzene and total xylenes ranged from BD to 3,100 ng/L and from BD to 

19,350 ng/L, respectively. Concentrations of MTBE in ground water ranged from BD to 

251,OOO^g/L.

10
Table 1 .-Concentrations of benzene, toluene, ethylbenzene, total xylenes, and methyl-tert-butyl-ether in 

ground water from monitoring wells at the Laurel Bay Exchange, Marine Corps Air Station Beaufort, 

S.C., March 11, 1994

, micrograms per liter; BD, below detection limit; MTBE, methyl-tert-butyl-ether; <, less than]

Monitoring well 

number 

(fig. 4)

LB-EX-1

LB-EX-3

LB-EX-4

LB-EX-5

LB-EX-7

LB-EX-8

LB-EX-9

LB-EX-10

LB-EX-11

LB-EX-1 2

LB-EX-1 3

LB-EX-14

LB-EX-1 5

LB-EX-1 6

LB-EX-RW

Benzene 

(Hg/L)

8,450

6.1

BD

BD

9,450

9,180

36.6

BD

BD

BD

BD

BD

6.7

6.5

8,650

Toluene 

(l^g/L)

20,600

22.2

BD

BD

22,300

22,500

132

BD

BD

BD

BD

BD

6.1

BD

16,100

Ethylbenzene 

(W?/L)

<5

<5

BD

BD

3,100

2,400

11.8

BD

BD

BD

BD

BD

5.9

BD

BD

Xylene 

(total) 

(Mg/L)

13,850

258

BD

BD

15,100

19,350

60.7

BD

BD

BD

BD

BD

11.4

BD

7,900

MTBE 

(Mg/L)

15,400

22.2

BD

BD

251,000

26,700

54.4

BD

BD

BD

BD

BD

BD

800

70,000

Based on the solubility differences between BTEX compounds compared to MTBE (table 

2), two overlapping, elliptically shaped plumes can be delineated (fig. 5). The plumes of BTEX 

and MTBE developed simultaneously from a common source but have moved differentially in 

ground water in response primarily to differences in characteristic adsorption, solubility, and 

degradation properties. The BTEX plume is delineated by concentrations exceeding detection in 

monitoring wells LB-EX-1, 3, 7, 8, 9, 15, 16, and RW. By March 1994, this plume has advanced 

approximately 350 ft from the source area, and is about 180 ft wide. The plume of MTBE is 

delineated by concentrations exceeding detection in monitoring wells LB-EX-1, 3, 7, 8, 9, 16, and 

RW. The plume has advanced approximately 350 ft from the source area, and is about 180 ft 

wide. Wells upgradient from the source area (LB-EX-4 and 5) had no detectable BTEX or 

MTBE. Wells adjacent to the plume boundaries (LB-EX-10, 11,12,13, and 14) had no detectable 

BTEX or MTBE concentrations.

11
Table 2.--Solubilities of benzene, toluene, ethylbenzene, o-xylene, and 

methyl-tert-butyl-ether in ground water at 25 degrees Celsius

[mg/L, milligram per liter]

~ , Solubility in ground water Compound , x

(mg/L)

Benzene 1,800

Toluene 535

Ethylbenzene 152

o-Xylene 170

Methyl-tert-butyl-ether 20,000

The 1994 BTEX plume has migrated about 350 ft from the source area since detection in 

September 1991. Given that the ground-water flow rate is roughly 70 ft/yr, and if no adsorption 

or microbial degradation were retarding the movement of BTEX from the source area, then the 

plume should have moved about 210 ft if the plume was released at the time of detection. 

Therefore, the plume may have been released 1 to 2 years before September 1991.

Evidence of the plume moving away from the source area is apparent when comparing the 

1994 water-chemistry sampling results with those from March and April 1993 (U.S. Navy 

Department, 1993). In the most contaminated well, LB-EX-1, benzene and toluene 

concentrations were 16,000 and 43,000 ng/L, respectively, in 1993. One year later, they were 

8,450 and 20,600 j^ig/L, respectively. This reduction does not represent depletion by degradation 

only; rather, it also represents ground-water transport as observed by increases in benzene and 

toluene concentrations in wells downgradient of LB-EX-1. For example, in LB-EX-8, benzene, 

toluene, and MTBE increased in one year from 1,700 to 9,180 |^g/L, 4,000 to 22,500 ^ig/L, and 

18,000 to 26,700 |^g/L, respectively. This apparent movement from the source area is exhibited 

by increasing toluene concentrations away from the underground storage tanks to about 150 ft, 

beyond which no toluene is observed in monitoring wells farther downgradient (fig. 6).

12
FORMER

UNDERGROUND

STORAGE TANK AREA

t

N

o

0h0

50

1

100 

1

150

I

200 FEET

I

l 

50 METERS

.LB-EX-16

EXPLANATION

BTEX > 5 |ig/L

MTBE > DETECTION

FENCE LINE

MONITORING WELL AND 

IDENTIFICATION NUMBER

Figure 5. The location and geometry of the benzene, toluene, ethylbenzene, total

xylene (BTEX), and methyl-tert-butyl ether (MTBE) plumes, Laurel Bay Exchange,

Marine Corps Air Station Beaufort, S.C., March 11, 1994.

13
25000

20000

<

cc

UJ

o

o 

o

z

o

ffl 

cc 

< 

o 

o

cc 

o

15000

10000

"

5000

0

RW

8

EXPLANATION

15* TOLUENE AND WELL IDENTIFICATION

15B BENZENE AND WELL IDENTIFICATION

RW

8

0 50 100 150 200 250 300 

DISTANCE FROM SOURCE AREA, IN FEET

350

Figure 6. Relation between hydrocarbon concentrations and distance 

from the source area, March 11, 1994.

Detectable concentrations of MTBE, an anti-knock compound added to gasoline after 1979, 

have spread a considerable distance since leak detection in September 1991. Compared to BTEX 

compounds, MTBE is extremely soluble (table 2), is not adsorbed by aquifer material, and is 

believed to remain undegraded by microorganisms (Mormile and others, 1994). As such, it is 

often detected beyond BTEX plumes or at higher concentrations at contaminated sites. At Laurel 

Bay, a higher concentration (800 i^g/L) than benzene (6.5 ng/L) is detected in the well farthest 

from the plume source. Because MTBE is currently an unregulated compound, no degradation 

studies using MTBE were performed.

14
Concentrations of dissolved oxygen (DO), Fe(II), and sulfide were determined in ground 

water, as well as pH and temperature (table 3). The ground water sampled in the shallow 

subsurface beneath the site was generally aerobic in wells not affected by petroleum 

hydrocarbons (average dissolved-oxygen concentration of 4.16 mg/L), and contained very little 

sulfide. In wells contaminated with petroleum hydrocarbons, however, dissolved oxygen was 

low, and dissolved Fe(II) concentrations were relatively high. These high concentrations of 

dissolved Fe(II) associated with the presence of organic contaminants result from the anaerobic 

microbial Fe(ni) reduction of petroleum hydrocarbons in anoxic ground water (Lovley and 

others, 1989).

The concentrations for all cations analyzed in ground water beneath the study site are 

generally low and were not diagnostic of significant geochemical reactions (table 4). These 

concentrations primarily reflect the composition of rainwater at the site that has not had sufficient 

time following infiltration to the water table to react with the aquifer material, which itself has 

been highly leached with time.

Concentrations of anions in ground water were also determined (table 5). Concentrations of 

chloride do not seem to have any correlation with the presence of hydrocarbon contamination. 

Concentrations of bromide, although low, are higher at the contaminated wells than in 

uncontaminated wells. A possible source of this bromide could be the bromine used as a catalyst 

during the manufacture of gasoline. Concentrations of nitrate are low at the site, and would 

generally tend to limit the use of nitrate reduction for microbial degradation of hydrocarbons in 

the anaerobic part of the aquifer. The highest concentrations of nitrate are in water from two 

wells drilled in a grassy area of the site, and could be explained by the use of fertilizers during sod 

emplacement. Comparison of the low concentrations of nitrate (table 5) with the relatively higher 

concentrations of ammonium (table 4) reveals that most of the dissolved organic nitrogen in this 

shallow aquifer system is present as reduced ammonium. There is a correlation between 

increasing hydrocarbon contamination and lower sulfate values, but it is not uniform across all 

wells. The lowest concentrations of sulfate (0.82 and 0.99 mg/L for LB-EX-RW and LB-EX-7, 

respectively), in water from these contaminated wells may reflect sulfate reduction of petroleum 

hydrocarbons by the microbial community. However, sulfate-reduction by-products such as 

sulfide were below detection at these wells (table 3). Concentrations of phosphate were below 

detection in ground water from the monitoring wells. This lack of an essential nutrient for 

microbial protein synthesis may limit microbial metabolism.

Concentrations of dissolved inorganic carbon (DIC) and methane (CH4) in ground water 

were determined at the site (table 6). DIC concentrations in ground water tended to increase as 

petroleum hydrocarbon contamination increased and dissolved-oxygen concentration decreased 

(fig. 7). Because DIC is comprised of inorganic forms of carbon, such as CO2, the high level of 

DIC in contaminated areas reflects the microbial mineralization of organic compounds to CO2. 

Concentrations of DIC also show a direct relation to an increase in dissolved Fe(II) in ground 

water (fig. 8). This provides an additional line of evidence that microbial Fe(III) reduction is a 

predominant terminal electron-accepting process (TEAP) in the shallow, anaerobic part of the 

contaminated aquifer. No methane was detected in any of the samples analyzed. The lack of 

methane suggests that although anaerobic conditions prevail in the contaminated part of 

theaquifer, methanogenic pathways of degradation are not being utilized by the microbial 

community.

15
Table 3.--Concentrations of dissolved oxygen, ferrous iron, and sulfide in ground water, and field values 

ofpH and temperature of ground water from monitoring wells at the Laurel Bay Exchange, Marine 

Corps Air Station Beaufort, S.C., March 11, 1994

[mg/L, milligrams per liter; °C, degrees Celsius; BD, below detection limit; 

ND, no data collected; <, less than]

Monitoring well number 

(fig. 4)

LB-EX-l

LB-EX-3

LB-EX-4

LB-EX-5

LB-EX-7

LB-EX-8

LB-EX-9

LB-EX-10

LB-EX-l 1

LB-EX-12

LB-EX-13

LB-EX-14

LB-EX-15

LB-EX-16

LB-EX-RW

Dissolved 

oxygen3 

(mg/L)

0

2.18

5.34

5.07

0

0

4.11

2.93

4.66

4.43

5.91

5.30

.90

4.92

0

Ferrous 

ironb 

(mg/L)

19.8

.7

.6

BD

20.0

6.8

BD

BD

BD

BD

ND

ND

2.8

BD

9.8

Sulfidec 

(mg/L)

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

ND

<.02

pHd

6.12

4.90

6.97

4.63

5.98

5.78

4.76

5.75

4.77

5.30

7.56

7.30

5.53

4.50

5.97

Temperatured 

(°O

19.7

18.6

16.1

17.9

20.2

17.7

17.2

17.8

17.2

16.3

17.0

17.1

16.3

16.8

20.8

a. Dissolved oxygen was measured using the Winkler titration method (Hach Company, 1989). 

b. Ferrous iron was measured using a Hach colormetric kit (Stookey, 1970). 

c. Sulfide was measured using a Hach colormetric kit (Hach Company, 1989). 

d. pH and temperature were measured using a Beckmann pH/temperature probe.

16
Table ^.--Concentrations of cations in ground water from monitoring wells at the Laurel Bay Exchange, 

Marine Corps Air Station Beaufort, S.C., March 11, 1994

[mg/L, milligrams per liter; R, replicate sample; BD, below detection limit]

Monitoring well 

number 

(fig- 4)

LB-EX-1LB-EX-3LB-EX^LB-EX-5LB-EX-7 LB-EX-7R

LB-EX-8LB-EX-9

LB-EX-10LB-EX-11LB-EX-12LB-EX-13LB-EX-14LB-EX-1 5

LB-EX-1 6LB-EX-RW

Magnesium (Mg2+) 

(mg/L)

0.67

1.27

.96

1.66

.69.72

1.85

1.36

1.861.781.06

2.07

1.831.711.19

.99

Calcium 

(Ca2+) 

(mg/L)

35.14

3.77

1.88

1.16

7.49 7.22

10.56

4.17

6.31

4.57

4.81

3.57

1.91

12.34

1.36

14.46

Sodium 

(Na+) 

(mg/L)

7.053.72

3.375.823.51 3.545.693.795.63

10.57

3.88

23.02

4.09

18.18

8.14

4.00

Potassium 

(K+) 

(mg/L)

4.54

.50

.27.16

1.48 1.37

.83.25.29.61.54.50.38

1.741.441.86

Ammonium 

(NH+) 

(mg/L)

9.25

BD

BD

BD

5.18 

4.79

1.35

BDBD

Bd

BDBD

BD

BD

BD

2.64
oo

Table ^.-Concentrations ofanions in ground water from monitoring wells at the Laurel Bay Exchange, 

Marine Corps Air Station Beaufort, S.C., March 11, 1994

[mg/L, milligrams per liter; R, replicate sample; BD, below detection limit]

Monitoring well 

number 

(% 4)

LB-EX-1LB-EX-3LB-EX^LB-EX-5LB-EX-7LB-EX-7RLB-EX-8

LB-EX-9LB-EX-10LB-EX-1 1LB-EX-12

LB-EX-13

LB-EX-14LB-EX-15

LB-EX-16LB-EX-RW

Chloride

(CD

(mg/L)

4.47

8.19

5.539.265.555.449.047.01

11.11

16.14

6.87

24.45

7.06

22.26

12.02

2.89

Bromide 

(Br-) 

(mg/L)

0.12

BDBD

BD

.27

.17.23

BDBD

.15

.08

.15

BD

BD

.11

.08

Nitrate 

(N03 -) 

(mg/L)

0.19

.39

.15.05

BD

.04

BD

.08.07.04

.07

6.36

2.66

.67

BD

BD

Phosphate (P043 -) 

(mg/L)

BD

BD

BDBD

BDBDBD

BDBDBD

BDBD

BD

BD

BDBD

Sulfate 

(S042-) 

(mg/L)

8.14

6.755.607.45

.99

.95

9.82

12.12

14.75

14.8412.13

24.43

9.93

27.88

15.50

.82
Table ^--Concentrations of dissolved inorganic carbon and methane in ground water from 

monitoring wells at the Laurel Bay Enchange, Marine Corps Air Station Beaufort, S.C., 

March 11, 1994

[mg/L, milligrams per liter; R, replicate sample; BD, below detection limit]

Monitoring well number 

(% 4)

LB-EX-1

LB-EX-3LB-EX-4

LB-EX-5LB-EX-7LB-EX-7RLB-EX-8LB-EX-9LB-EX-10

LB-EX-1 1

LB-EX-1 2LB-EX-1 3LB-EX-14

LB-EX-1 5LB-EX-16LB-EX-RW

Dissolved inorganic carbon 

(mg/L)

208.1

113.7

52.9

18.7

158.0

220.2

132.6

31.0

34.4

1.9

44.246.7

33.367.3

28.8

251.1

Methane 

(mg/L)

BDBDBDBD

BDBDBDBDBD

BDBDBDBDBD

BDBD
I

o o

o

CO CO

o

300

2 200 

DC

en o

< 

O 

DC

OS 100oLU

>

... 1

RW

8

en o

0

0

EXPLANATION

WELL IDENTIFICATION

/

is

/

10

12 3

, 6 4

12345 

DISSOLVED OXYGEN, IN mg/L

Figure 7. Relation between dissolved inorganic carbon and dissolved

oxygen in contaminated and uncontaminated ground water, Laurel Bay

Exchange, Marine Corps Air Station Beaufort, S.C., March 11, 1994.
300'

E 250

Z

O

g 200

Oo

< 150 

O

cc 

o

ZQ 100

111

oV)

1 1 I

RW

EXPLANATION

WELL IDENTIFICATION

0

15 CONTAMINATED GROUND WATER

UNCONTAMINATED GROUND WATER

5 10 15 20 

DISSOLVED FERROUS IRON, IN mg/L

25

Figure 8. Relation between dissolved inorganic carbon and dissolved 

ferrous iron in contaminated and uncontaminated ground water, 

Laurel Bay Exchange, Marine Corps Air Station Beaufort, S.C.,

March 11, 1994.
The ground-water concentrations of particular anions and cations provide several lines of 

evidence that the petroleum hydrocarbons are undergoing intrinsic bioremediation in the shallow 

aquifer at Laurel Bay. The initial line of evidence is the lack of DO in the most contaminated 

monitoring wells (fig. 9). Oxidation of hydrocarbons through the microbial reduction of oxygen 

is the most efficiently used TEAP in the subsurface, and as such, rapidly decreases the DO 

concentration when petroleum hydrocarbons are released. There is an inverse relation between 

concentrations of toluene and benzene dissolved in ground water and DO (fig. 9); namely, high 

toluene and benzene concentrations result in low concentrations of DO and the same is true for 

the reverse. This relation has been observed at many sites involving the release of BTEX 

compounds (Chiang and others, 1989; McAllister and Chiang, 1994; and Chapelle and others, 

1995).

Low concentrations of DO allow further oxidation of hydrocarbons to occur through the 

reduction of ferric iron as Fe(III). Ferric iron (and its hydroxides) forms a ubiquitous reddish 

coating on sand grains in shallow aerobic aquifers similar to the aquifer at Laurel Bay. The 

microbial reduction of the Fe(III) on sand grains accompanying the oxidation of petroleum 

hydrocarbons results in the release of dissolved Fe(II) into the ground water. Therefore, the 

relatively high concentrations of Fe(II) measured in wells lacking DO is an indication of 

microbial iron reduction (table 3).

The presence of DO and Fe(II) at the site indicate that at least two processes of aerobic and 

anaerobic microbial degradation are proceeding at the site. Tf DO is present, aerobic degradation 

will proceed until the DO is consumed. When DO levels are depressed, the Fe(ni) present on the 

sand grains that comprise the shallow aquifer is reduced anaerobically. The fact that there is little 

geochemical evidence that the less efficient TEAPs of sulfate-reduction or methanogenesis are 

being utilized indicates the relatively young age of the plume (about 3 years). Additionally, iron 

reduction probably reverts to aerobic degradation after low concentrations of DO are replenished 

following infiltration of DO-charged rainwater.

Concentrations of DIG can also provide a geochemical line of evidence that microbial 

degradation of petroleum hydrocarbons is occurring at the study site for several reasons. First, 

DIG concentrations are higher in monitoring wells contaminated with petroleum hydrocarbons 

and lower in wells without contamination (fig. 7). This suggests that (1) the uncontaminated 

aquifer has very little sedimentary organic matter, and probably results in a carbon-limited 

microbial community; and (2) once an abundant supply of organic matter (petroleum 

hydrocarbons) is supplied, contaminants are mineralized to CO2. Further examination of DIG 

concentrations implies that the major anaerobic process is iron reduction; concentration increases 

in DIG reflect a concomitant increase in Fe(II) concentrations (fig. 8). This is important from the 

standpoint that as BTEX compounds migrate with ground water, sufficient Fe(III) on the aquifer 

material will not be a limiting factor in the degradation of the contaminants.

22
ro

25000

20000

Z 

O

DC

15000

LU

O 10000

o o

Z

Q 5000

o 

o

J- I I I I I I I « I 

8

RW

1

RW

EXPLANATION

15" TOLUENE AND WELL IDENTIFICATION

15 ^ BENZENE AND WELL IDENTIFICATION

r I ... i I i i

01234 

DISSOLVED OXYGEN, IN mg/L

Figure 9. Relation between hydrocarbon and dissolved oxygen 

concentrations for benzene and toluene, March 11, 1994.
Adsorption Coefficient

Adsorption coefficients (K^) describe the potential for an organic hydrocarbon to be 

partitioned between an aqueous phase (ground water) and a solid phase (aquifer material). The 

higher the potential to be adsorbed onto aquifer material, the higher the K^ value; the converse 

also holds. Such behavior can be expressed using a linear model of adsorption to predict 

adsorption capacity at a particular concentration. Values of K^, determined for toluene in the 

fine-to-coarse grained, sedimentary organic-poor sand ranged from K^ = 2.0 x 10~9 to 5.0 x 10~9 

fWmg, and were determined by regressing the adsorption data compared to concentration 

(fig. 10). Values of K^ determined for benzene at the site ranged from IC^ = 2.0 x 10~9 to 

1.0 x 10"8 ftVmg. These values are low and suggest that very little toluene or benzene will be 

adsorbed onto the aquifer material and that petroleum-hydrocarbon transport will not be retarded 

relative to ground-water flow. However, these low adsorption data also suggest that contaminated 

aquifer sediments will not be a significant source of contaminants to flowing ground water.

Microbial Degradation

A series of degradation experiments involving radiolabeled 14C-toluene in laboratory 

microcosms were utilized to directly assess the microbial degradation of BTEX compounds in the 

shallow aquifer at the study site. Microcosm conditions were arranged to provide rates of toluene 

degradation under aerobic and anaerobic conditions. This approach allows meaningful 

interpretations to be made for the field conditions observed at the site.

The results of the radiolabeled experiments suggest an active microbial population at the 

site can degrade toluene. Toluene degradation rates under aerobic conditions were 200 times 

faster than rates measured under anaerobic conditions. For example, 14 percent of 14C-toluene 

and 30 percent of 14C-toluene was mineralized after 8 hours and 2 weeks upon starting the 

experiment, respectively (fig. 11 A). Under anaerobic conditions, only 4 percent of the 14Ctoluene was mineralized after a period of 2 weeks (fig. 1 IB).

The microbial degradation of benzene was estimated using a digital solute-transport model. 

The benzene plume measured in April 1993 was simulated using the numerical model SUTRA 

(Voss, 1984). The distribution of benzene in ground water measured in 1994 could only be 

reproduced if an anaerobic microbial degradation rate of Kbio = -0.00025 d' 1 was incorporated into 

the model. Although this indicates low rates of benzene biodegration, it is sufficient to 

measurably attenuate the benzene plume with time.

24
300

250

d 200

BENZENE

Kad = 2.0xl(r9 ft3/mg

600 800 1000

<

cc

LU

a

z

o 

a

a

LU

oo DC 

o

100

75

50

25

TOLUENE

Kad = 2.0xlO-9 ft3/mg

0

0 25 50 75 100 125 150

DISSOLVED CONCENTRATION, IN PARTS PER MILLION

Figure 10. Relation between adsorbed and dissolved concentrations of

benzene and toluene for aquifer sediments from Laurel Bay Exchange,

Marine Corps Air Station Beaufort, S.C.

25
(A)

O 

O

c

UJ

O

UJ

o

UJ

o 

cc

UJ 

Q.

EXPANSION OF EARLY DATA 

. . , , I , , , , I . . .

O

o

UJ

_l

o

UJ 

O

UJ

o 

cc

LU 

Q.

10

9

8

7

6

5

4

3

0 40 80 120 160 200 240 280 320 

TIME, IN HOURS

(B)

0

1 i i i ' 

Kbjo = -0.003 per day

NOT DIAGNOSTIC OF 

TOLUENE DEGRADATION

0 4 6 8 10 

TIME, IN WEEKS

12 14

Figure 11. Mineralization rates of 14C-toluene under 

(A) aerobic and (B) anaerobic aquifer conditions.

26
ASSESSMENT OF INTRINSIC BIOREMEDIATION

The migration of soluble contaminants in ground-water systems is affected by hydrologic, 

geochemical, and microbial processes. The principle hydrologic factors are the dispersion and 

advection of the contaminants in flowing ground water. There are a wide variety of geochemical 

processes that can affect the transport of contaminants including mineral precipitation/dissolution 

and adsorption/desorption reactions. Petroleum hydrocarbons in ground water are not, in general, 

subject to significant mineral precipitation reactions, so the most important geochemical process 

affecting concentrations of such compounds are sorption reactions. Finally, the microbial 

degradation of hydrocarbons can mineralize these toxic compounds into innocuous by-products.

Approach to Simulating Solute Transport

The effects of hydrologic, geochemical, and microbial processes on contaminant transport 

can be quantified using a mass balance equation that includes a component to describe each 

process. Where advection, sorption, and microbial degradation are the principal processes 

affecting transport, a one-dimensional equation may be written as:

DC ^. d C 8C cr,n . _, Ov = D v - SC kC, (2) at dx2 ax

where

iC is concentration of the solute, in milligrams per liter;

t is time, in days;

D is the coefficient of hydrodynamic dispersion, in feet squared per day;

x is distance, in feet;

v is the velocity of ground-water flow, in feet per day;

S and n are adsorption isotherm parameters; and,

k is a first-order microbial degradation rate constant, in per day.

This equation is most readily solved by numerical methods. In this study, the two-dimensional 

solute-transport code SUTRA (Voss, 1984) was used to obtain solutions to equation (2) using the 

most realistic ranges of hydrologic, geochemical, and microbial conditions at Laurel Bay.

27
The hydrologic conditions at the site were simulated by assigning hydraulic conductivities 

measured using aquifer tests in single wells (U.S. Navy Department, 1993) and by using the water 

table observed on March 10, 1994. In comparing this water table with the one for April 1993 

(U.S. Navy Department, 1993), the data indicate the older data showed water levels 2 ft higher 

than those measured in 1994. However, the two sets of data also show that the hydraulic 

gradients are quite similar (figs. 3 and 4). Therefore, no attempt was made to model a particular 

water table, and a constant hydraulic gradient was imposed on the modeled area, which 

significantly simplified the actual hydrologic system.

There are two distinct approaches to modeling complex hydrologic systems. The first, often 

called consolidative modeling, seeks to simulate every possible feature of a hydrologic system to 

obtain a unique solution. While conceptually appealing, consolidative modeling assumes that all 

relevant hydrologic parameters can be measured, and that spatial and temporal variabilities of 

these parameters can be defined. Such an approach comes into conflict with the uncertainties 

inherent to all hydrologic systems under observation. It is not possible, for example, to precisely 

measure the distribution of hydraulic conductivities at a site, so that uncertainties in model results 

are inevitable. When the uncertainties inherent in measuring all of the relevant hydrologic, 

geochemical, and microbial factors are taken into account, it is clear that to arrive at the single 

unique solution envisioned by consolidative modeling is not technically feasible.

The second approach to modeling hydrologic systems, called exploratory modeling, 

explicitly recognizes these inherent uncertainties. Rather than attempting to arrive at a single 

unique solution, exploratory modeling depicts the range of possible solutions based on the 

variability of the revelant parameters. For example, if the hydraulic conductivity (Kaq) is known 

to range two orders of magnitude, an exploratory model will evaluate solute transport for this 

range of Kaq so that the effects of this uncertainty can be described. The results of this approach, 

therefore, are useful for placing reasonable bounds on how a particular system can behave. Thus, 

it could be concluded that for a given range of possible IC^, a particular solute could be 

transported distances dependent upon these ranges. Rather than attempting to define one solution, 

this approach attempts to define the reasonable range of solutions possible in a system.

For the purposes of assessing the efficiency of intrinsic bioremediation to contain 

contaminants, the exploratory modeling approach is most appropriate. In such an assessment, the 

most relevant question to be answered is whether contaminants will reach a point-of-contact 

under any possible conditions that might be feasible at the site. If it can be demonstrated that, 

given the range of parameters for a site, there is no combination that results in solute transport to 

a POC, then it can be concluded with a high degree of confidence that intrinsic bioremediation is 

a feasible remedial alternative. On the other hand, if the given range of parameters suggests that 

solute transport to a POC is possible, then intrinsic bioremediation is not a feasible alternative to 

contain contaminants. For this report, the exploratory modeling approach to solute transport is 

used.

28
Solute-Transport Simulations of Toluene Migration

The range of hydraulic gradients, hydraulic conductivities, adsorption isotherms, and 

microbial degradation rates were determined for the Laurel Bay site (table 7). The parameters of 

greatest uncertainty are K^ and Kbio. The mean value of four K^ measurements was 11.1 ft/d, but 

the standard deviation of 3.3 ft/d indicated significant variability resides in this measurement 

(U.S. Navy Department, 1993). Similarly, the measured microbial degradation rates of toluene 

varied between K^ = -0.003 d' 1 (anaerobic conditions) to -0.640 d' 1 (aerobic conditions) and 

1^^ = -0.00025 d' 1 for the anaerobic degradation of benzene. Given this variability, the modeling 

approach was to calculate the distribution of hydrocarbons for the possible extremes to evaluate 

the potential of solute transport to the nearest POC. To do this, a set of most probable values was 

selected, based on measurements made at the site (table 7), and used to construct a Standard 

Model. This model assumes that the water-table gradients remain constant (that is, the waterlevel distribution is the same as of March 10, 1994); that the aquifer remains anaerobic (that is, 

Kbio = -0.003 d' 1 for toluene; K^ = -0.00025 d' 1 for benzene); and that the aquifer exhibits little 

adsorption capacity for toluene and benzene (K^, = 2.0 x 10~9 ftYmg). The modeled area is 

oriented so that the free-product source area is located on the upgradient side (fig. 12), and the 

nearest of the two POC (the property line to the south of the boundary of the site) is located on the 

downgradient side of the modeled area. The ultimate discharge area for ground water beneath the 

site (the Broad River) is not included in the model because of scale constraints. This model is not 

intended to represent actual values of parameters, but rather to provide a standard against which 

to compare the affects on solute transport from changing the parameters within the measured 

ranges.

The simulation of toluene transport and concentration distribution in 1994 based on the 

Standard Model (fig. 13) shows the development of a toluene plume from a constant source (free 

product leaching into ground water at a concentration of 50 mg/L) introduced at a known time 

(late 1991) in a sandy aquifer. The model is based on the best estimate for hydraulic conductivity 

(Kgq =11.1 ft/d), predominantly anaerobic rates of toluene degradation (K^ = -0.003 d' 1 ), and 

toluene adsorption (Kad = 2.0 x 10'9 ft3 /mg). Ideally, the use of these parameters should closely 

approximate plume distribution similar to that observed during field sampling. The maximum 

extent of the simulated 1994 toluene plume (delineated by the 0.1 mg/L contour) reaches 

dimensions of 400 ft in length, which is 100 ft farther than the 300 ft that the toluene plume 

migrated in the field (fig. 5). The difference can be explained by the lack of wells in the 

downgradient area of the plume and absence of hydrocarbon concentration data in areas the 

simulation shows as being contaminated.

The Standard Model indicates that the toluene plume reaches steady-state conditions (no 

change in toluene concentrations over time) about 2001, when the rate of toluene migration is 

balanced by the rate of toluene degradation. As a result, the plume stops growing (figs. 14A-D). 

At steady state, toluene concentrations are contained to within 125 ft of the present plume 

boundary.

29
Table 7.-Most probable values and best estimates for hydraulic gradient, hydraulic

conductivity, microbial degradation, and adsorption for site conditions at the Laurel Bay 

Exchange, Marine Corps Air Station Beaufort, S.C.

[ft/ft, foot per foot; ft/d, feet per day; frVmg, cubic feet per milligram; -, no data]

Most probable values

Parameter High estimate

Hydraulic gradient 0.0046 

(ft/ft)

Hydraulic conductivity 17.3 

(ft/d)a

Microbial degradation rate -0.640 

(toluene) (aerobic) 

(per day)b

Adsorption rate 5.0 x 10"9 

(toluene) 

(ft3/mg)b

Microbial degradation rate 

(benzene) (aerobic) 

(per day)

Adsorption rate 1.0 x 10"8 

(benzene) 

(frVmg)b

Low estimate

0.0046

8.9

-0.003 

(anaerobic)

2.0 X10'9

-0.00025 

(anaerobic)

2.0 xlO'9

Best estimate

0.0046

11.1

-0.003 

(anaerobic)

2.0 x 10'9

-0.00025 

(anaerobic)

2.0 xlO'9

a. U.S. Navy Department, 1993.

b. U.S. Geological Survey Microbial Studies Group Laboratory, Columbia, S.C.

30
(NAVAL HOUSING AREA

[MODEL BOUNDARY!

BASE FROM USGS 7.5 MINUTE TOPOGRAPHIC QUADRANGLE, LAUREL BAY. SOUTH CAROLINA. 1962

SCALE 1:24000

0 1 MILE

1000 1000 2000 3000 4000 5000 6000 7000 FEET

1 KILOMETER

CONTOUR INTERVAL 10 FEET

NATIONAL GEODETIC VERTICAL DATUM OF 1929

Figure 12. Locations of the Saturated and Unsaturated TRAnsport (SUTRA) model area.

31
FORMER

UNDERGROUND

STORAGE TANK AREA

t

N

0h050

I

100 

I

150

I

200 FEET 

I

I 

50 METERS

0.1

L LB-EX-16

EXPLANATION

ROAD

LINE OF EQUAL SIMULATED TOLUENE

CONCENTRATION-lnterval,

in mg/L, is variable.

FENCE LINE

MONITORING WELL AND 

IDENTIFICATION NUMBER

Figure 13. Simulated 1994 distribution of toluene for values of the Standard Model at 

Laurel Bay Exchange, Marine Corps Air Station Beaufort, S.C.

32
(A)

1995 1996

IFORMER

UNDERGROUND 

.STORAGE TANK AREA h

(CL

2001 2011

EXPLANATION

ROAD

LINE OF EQUAL SIMULATED 

TOLUENE CONCENTRATIONInterval, in mg/L, is variable.

FENCE LINE

MONITORING WELL AND 

IDENTIFICATION NUMBER

50 

I

100 

I

150 

I

200 FEET

_I

50 METERS

Kaq=11-1 

Kbio = -0.003 d'

ft3/mg

Figure 14. Simulated toluene plume in (A) 1995, (B) 1996, (C) 2001,

and (D) 2011 at Laurel Bay Exchange, Marine Corps Air

Station Beaufort, S.C., using the Standard Model.

33
A worst-case scenario of toluene transport from a constant source in 1991 to 1994 can be 

simulated using no biodegradation or adsorption, and is considered to be the Conservative Model. 

Because there is no retardation of the contaminant by either adsorption or biodegradation 

processes, the simulated plume migrates no slower than the rate of the ground-water velocity 

(fig. 15). Simulations show, however, that the toluene plume migrates farther than that predicted 

by simple advection, as a result of the dispersive effects of the silty sand grains in the aquifer, that 

cause some ground-water flowpaths to be longer than others. The maximum extent of the toluene 

plume, as delineated by the isoconcentration line of 0.1 mg/L, has migrated about 600 ft from the 

source area by 1994 (fig. 15). A transport rate of about 200 ft/yr can be calculated, which would 

suggest toluene transport to the Broad River in about 18 years. This Conservative Model also 

indicates that the toe of the toluene plume would enlarge continuously over time.

The affects of varying hydraulic conductivity (K^) in simulations using the Standard Model 

on the dimensions of the simulated toluene plume were determined. The simulated distribution of 

toluene assuming the aquifer system has uniformly low (K^ = 8.9 ft/d) and high (K^ = 17.3 ft/d) 

hydraulic conductivities are shown in figure 16A and B, respectively. The higher K^ results in a 

larger (by about 150 ft) plume, because of a higher rate of ground-water flow.

Laboratory studies performed on sediments from the contaminated part of the shallow 

aquifer indicate that rates of toluene biodegradation could be as low as Kbio = -0.003 d" 1 

(anaerobic conditions) or as high as Kbio = -0.640 d" 1 (aerobic conditions). The Standard Model 

describes the 1994 distribution observed with a low (Kbio = -0.003 d" 1 ) rate of degradation 

(fig. 17A). Increasing the degradation rate to Kbio = -0.64 d" 1 results in toluene containment in 

1994 to within 50 ft of the source area (fig. 17B).

The similarity of the Standard Model results (with uniformly anaerobic rates of 

biodegradation) with geochemical conditions in the field indicates that there is significantly more 

anaerobic degradation in the plume body than aerobic degradation. There is probably some 

aerobic degradation following delivery of oxygen-charged rainwater to the plume, which would 

become temporarily aerobic until all the DO is consumed. This does indicate, however, that the 

anaerobic plume is encountering oxygenated ground water at its boundaries and in the direction of 

migration toward POC's.

Laboratory studies indicate that the adsorption capacity of the sandy aquifer material with 

respect to toluene is uniformly low (K^ = 2.0 x 10'9 to 5.0 x 10~9 ftVmg). This small range of 

values was determined by varying the value and showing its affect on contaminant migration. 

Increasing the K^ in the Standard Model to the higher value of 5.0 x 10"9 ftVmg did not 

appreciably reduce the migration of contaminants (fig. 18 A) when compared with results using 

Kad = 2.0 x 10~9 ftVmg (fig. 18B). These results indicate that adsorption does not play a major role 

in the retardation of toluene at the site.

34
FORMER

UNDERGROUND

STORAGE TANK AREA

 0.1

k LB-EX-16

EXPLANATION

ROAD

LINE OF EQUAL SIMULATED 

TOLUENE CONCENTRATIONInterval, in mg/L, is variable.

FENCE LINE

MONITORING WELL AND 

IDENTIFICATION NUMBER

0

I 

0

50 

1

100 

1

150 200 FEET 

I I

50 METERS

Kaq=11.1 ft/d 

Kbio = 0 d' 1 

= 0 ft3/mg

Figure 15. Simulated 1994 distribution of toluene released in 1991

assuming the Conservative Model at the Laurel Bay Exchange,

Marine Corps Air Station Beaufort, S.C.

35
(A) (B)

Co O

Kbjo = -0.003 d' 1Kad = 2.0x10'9 ft3/mgKaq = 8.9 ft/d_____

Kbjo = -0.003 d' 1Kad = 2.0x10'9 ft3/mgKaq =17.3 ft/d

EXPLANATION

I ROAD

 0.1' LINE OF EQUAL 

SIMULATED TOLUENE CONCENTRATION-- 

Interval, in mg/L, 

is variable.

FENCE LINE

.LB-EX-16 MONITORING WELL 

AND IDENTIFICATION 

NUMBER

50 100 150 200 FEET

Figure 16. Simulated 1994 distribution of toluene for (A) minimum and (B) maximum values of hydraulic conductivity

at Laurel Bay Exchange, Marine Corps Air Station Beaufort, S.C.
(A)

Go

(B)

EXPLANATION

 0.1 

ROAD

LINE OF EQUAL SIMULATED TOLUENE CONCENTRATION-- 

Interval, in mg/L. 

is variable.

FENCE LINE

.LB-EX-16 MONITORING WELL 

AND IDENTIFICATION 

NUMBER

50 100 150 200 FEET

Figure 17. Simulated 1994 distribution of toluene for (A) minimum and (B) maximum values of biodegradation at

Laurel Bay Exchange, Marine Corps Air Station Beaufort, S.C.
(A)

Go 00

Kbio = -0.003 d' 1

Kad = 2.0x10'9 ft3/mg

Kaq =11.1 ft/d

(B)

N

FORMER

UNDERGROUND

ISTORAGE TANK AREA

Kbio = -0.003 d' 1Kad = 5.0x10-9 ft3/mgKaq = 11.1 ft/d

EXPLANATION

I ROAD

0.1 LINE OF EQUAL

SIMULATED TOLUENE CONCENTRATIONInterval, in mg/L, 

is variable.

FENCE LINE

A LB-EX-16 MONITORING WELL 

AND IDENTIFICATION 

NUMBER

50 100 150 200 FEET

Figure 18. Simulated 1994 distribution of toluene for (A) minimum and (B) maximum values of toluene adsorption capacity

at Laurel Bay Exchange, Marine Corps Air Station Beaufort, S.C.
The model analyses indicate that toluene concentrations at Laurel Bay are sensitive to the 

aquifer hydraulic conductivity (K^) and biodegradation rate (K^), and relatively insensitive to 

adsorption (Kad). To show extreme conditions, the 1994 distribution of toluene under the 

minimum and maximum values for all parameters was simulated (fig. 19A and B, respectively). 

Results suggest that the biodegradation rate increase (all uniformly aerobic) produces a greater 

affect (smaller plume) even though a high hydraulic conductivity is used. This suggests that 

inducing aerobic conditions at the site could significantly reduce toluene concentrations.

Removal of the contaminant source and its affect on toluene concentrations downgradient 

with time was simulated using the Standard Model for toluene (fig. 20A-E). Starting with the 

Standard Model plume of toluene, a source removal in 1994 was simulated. One year after the 

source is removed (1995), simulated toluene concentrations in the center of the plume decreased 

from 50 mg/L to about 5 mg/L (fig. 20 A) and the center of the plume began to migrate to the 

south away from the old source area. Two years following source removal (1996), toluene 

concentrations again decreased by about 50 percent (fig. 20B). Plume size and length of transport 

was simulated for years 1998, 2000, and 2001 (fig. 20C-E, respectively). By 2001, toluene 

concentrations had reached about 0.1 mg/L.

Solute-Transport Simulations of Benzene Migration

The same approach used to develop and produce simulations of toluene migration was 

utilized to predict benzene migration. A Standard Model was constructed based on a set of most 

probable values (table 7). This model assumes that the water-table gradients remain constant with 

time (the water-level distribution is the same as of March 10, 1994), that the aquifer remains 

anaerobic and that the aquifer exhibits very little adsorption properties for benzene. As was the 

case for toluene transport simulations, this model is not intended to represent actual values of 

parameters, but rather to provide a standard against which to compare the results on solute 

transport from changing the parameters within the measured ranges.

Simulations of benzene distributions based on the Standard Model for 1993 (fig. 21) and 

1994 (fig. 22) shows the development of a benzene plume from a constant source (free product 

leaching into ground water at 20 mg/L) introduced in late 1991. The model assumes the best 

estimate for hydraulic conductivity (K^ =11.1 ft/d), predominantly anaerobic rates of benzene 

degradation (K^ = -0.00025 d' 1 ), and benzene adsorption (K^ = 2.0 x 10~9 fWmg). Ideally, the 

use of these parameters should approximate a simulated benzene plume migration similar to that 

observed during field sampling. A source removal in 1993 was necessary to simulate the 1994 

benzene concentrations. In 1994, the simulated benzene plume (delineated by the 0.001 mg/L 

contour) reaches maximum dimensions of 400 ft, which is 50 ft farther than the 350 ft that the 

benzene plume migrated in the field (fig. 22). As was the case with simulated toluene 

concentrations, the difference can be explained by the lack of wells and available hydrocarbon 

data in areas the simulation depicts as being contaminated.

39
(A) (B)

Kbio = -0.003 d' 1

Kad = 2.0x10'9 ft3/mgKaq = 8.9 ft/d_____

FORMER

UNDERGROUND

STORAGE TANK AREA

LB-EX-9^

Kbio = -0.640 d' 1Kad = 5.0x10'9 ft3/mgKaq =17.3 ft/d_____

EXPLANATION

ROAD

0.1 LINE OF EQUAL

SIMULATED TOLUENE CONCENTRATIONInterval, in mg/L, 

is variable.

------- FENCE LINE

ALB-EX-16 MONITORING WELL 

AND IDENTIFICATION 

NUMBER

50 100 150 200 FEET

Figure 19. Simulated 1994 distribution of toluene for (A) minimum and (B) maximum values of hydrologic, geochemical, 

and microbial parameters at Laurel Bay Exchange, Marine Corps Air Station Beaufort, S.C.
(A)

1995

(D)

2000

(B) (C)

1996

(E)

2001

1998

EXPLANATION

! ROAD

LINE OF EQUAL SIMULATED 

TOLUENE CONCENTRATIONInterval, in mg/L, is variable.

FENCE LINE

MONITORING WELL AND IDENTIFICATION NUMBER

Kbio = -0.003 d'Kaq =11.1 ft/d

ft3/mg

Figure 20. Simulated 1995 (A), 1996 (B), 1998 (C), 2000 (D), and 2001 (E) distribution of toluene 

after source removal in 1994, at Laurel Bay Exchange, Marine Corps Air Station Beaufort, S.C.
N

t

FORMER

UNDERGROUND

STORAGE TANK AREA

LB-EX-5

LB-EX-4 

LB-EX-3A

0

I 

0

50 

1

100 

1

150 

I

200 FEET 

I

I 

50 METERS

Kaq=11.1

Kbio = -0.00025 d'1

Kad = 2.0x10-9 ft3/mg

EXPLANATION

ROAD

LINE OF EQUAL SIMULATED 

 0.1 BENZENE CONCENTRATIONInterval, in mg/L, is variable.

---------- FENCE LINE

I R PY 1R MONITORING WELL AND ALD-CA-IO IDENTIFICATION NUMBER

Figure 21. Simulated 1993 distribution of benzene for values of the

Standard Model at Laurel Bay Exchange, Marine Corps

Air Station Beaufort, S.C.

42
N

t

LB-EX-5

FORMER

UNDERGROUND

STORAGE TANK AREA

LB-EX-9^

0h050 

1

100 

1

150 

I

200 FEET 

1

I 

50 METERS

Kaq =11.1 ft/d 

Kbio = -0.00025 d'1 

Kad = 2.0x10-9 ft3/mg

EXPLANATION

ROAD

LINE OF EQUAL SIMULATED 

 0.1 BENZENE CONCENTRATION--

Interval, in mg/L, is variable. 

._.__.__._. FENCEUNE

.LB-EX-16 MONITORING WELL AND 

IDENTIFICATION NUMBER

Figure 22. Simulated 1994 distribution of benzene for values of the

Standard Model after source removal in 1993, at Laurel Bay Exchange,

Marine Corps Air Station Beaufort, S.C.

43
Simulations for the Standard Model were run until the year 2044 (fig. 23 A-C). The 

contaminants do not migrate to the nearest POC 2,400 ft south of the site boundary by the year 

2044.

The worst-case benzene-transport scenario, a conservative model, of no biodegradation and 

no adsorption was simulated (fig. 24A). Because there is no retardation of the contaminant by 

either adsorption or biodegradation, the plume migrates with the rate of the ground-water 

velocity. The benzene plume migrates farther than that predicted by simple advection, as a result 

of the dispersive effects of the fine sand grains in the aquifer that cause some ground-water 

flowpaths to be longer. The extent of the plume (delineated by the isoconcentration line of 

0.001 mg/L (fig. 24A-C) has migrated about 470 ft from the source area by 1994 since release in 

1991. A transport rate of about 157 ft/yr can be calculated, which simulates some level of 

benzene in the POC in about 23 years. The Conservative Model also indicates that the plume 

does not enlarge continuously over time, as in the case of toluene, because the source of dissolved 

benzene had been removed. Hence, benzene concentrations decrease because of dilution with 

ground water.

The effects of hydraulic conductivity on simulated plume migration were determined using 

the Standard Model. Distributions of benzene using uniformly low (K^ = 8.9 ft/d) and high 

(Kaq = 17.3 ft/d) hydraulic conductivities were simulated for the years 1994 and 2022 (figs. 25 A 

and B and 26 A and B, respectively). From a comparison of the plume dimensions, the higher 

hydraulic conductivity results in a larger plume by about 150 ft in length because of a higher rate 

of ground-water flow.

44
(A)

cn

0

0

0 2006

Kaq =11.1 ft/d Kbio = -0.00025 d' 1 Kad = 2.0x10'9 ft3/mg

EXPLANATION

ROAD

_ 

LINE OF EQUAL SIMULATED BENZENE 

CONCENTRATION- Interval, in mg/L, is variable.

MONITORING WELL AND IDENTIFICATION NUMBER

(B)

2011

Kaq = 11.1 ft/d Kbio = -0.00025 d' 1 

= 2.0x10'9 ft3/mg

_m LINE OF EQUAL SIMULATED BENZENE 

CONCENTRATION- Interval, in mg/L, is variable.

(C)

2044

aq ft/d

Kbio = -0.00025 d' 1 

Kad = 2.0x10-9 ft3/mg

EXPLANATION

ROAD

_m_ LINE OF EQUAL SIMULATED BENZENE 

Ul1 CONCENTRATION- Interval, in mg/L, is variable

MONITORING WELL AND IDENTIFICATION NUMBER

Figure 23. Simulated 2006 (A), 2011 (B), and 2044 (C), distributions of benzene assuming Standard Model transport 

parameters, at Laurel Bay Exchange, Marine Corps Air Station Beaufort, S.C.
(A)

Kaq-11.1 ft/d 

Kbio = 0 d'1 Kad = 0 ft3/mg

[ | ROAD

EXPLANATION

MONITORING WELL AND 

IDENTIFICATION NUMBER

_*j

_ 

LINE OF EQUAL SIMULATED BENZENE

CONCENTRATION- Interval, in mg/L, is variable.

FENCE LINE

(B)

2022

Kad = 0 ft3/mg

_ , _ LINE OF EQUAL SIMULATED BENZENE 

Ul1 CONCENTRATION- Interval, in mg/L, is variable.

(C)

Kaq =11.1 ft/d 

Kbio = 0 d'1 Kad = 0 ft3/mg

EXPLANATION

ROADLINE OF EQUAL SIMULATED BENZENE

Figure 24. Simulated 1994 (A), 2022 (B), and 2050 (C) distributions of benzene assuming Conservative Model transport,

at Laurel Bay Exchange, Marine Corps Air Station Beaufort, S.C.
(A) (B)

1994

Kaq = 8.9 ft/d 

Kbio = -0.00025 d'1 

Kad = 2.0x 10'9 ft3/mg

EXPLANATION

ROAD

LINE OF EQUAL SIMULATED BENZENE 

CONCENTRATION- Interval, in mg/L, 

is variable.

MONITORING WELL AND 

IDENTIFICATION NUMBER

2022

Kaq = 8 9 ft/d 

Kbio =-0.00025 d'1 

Kad = 2.0x10-9 ft3/mg

EXPLANATION

ROAD

LINE OF EQUAL SIMULATED BENZENE 

CONCENTRATION- Interval, in mg/L, 

is variable.

MONITORING WELL AND 

IDENTIFICATION NUMBER

Figure 25. Simulated 1994 (A) and 2022 (B) distributions of benzene using a minimum

value of hydraulic conductivity, at Laurel Bay Exchange,

Marine Corps Air Station Beaufort, S.C.

47
(A)

Kaq =17.3ft/d 

Kbio = -0.00025 d 1 

Kad = 2.0x10-9 ft3/mg

EXPLANATION

ROAD

_ni _ LINE OF EQUAL SIMULATED BENZENE 

CONCENTRATION- Interval, in mg/L, 

is variable.

MONITORING WELL AND 

IDENTIFICATION NUMBER

(B)

2022

Kaq =17.3 ft/d 

Kbio = -0.00025 d'1 

Kad = 2.0x10-9 ft3/mg

EXPLANATION

ROAD

LINE OF EQUAL SIMULATED BENZENE 

CONCENTRATION- Interval, in mg/L, 

is variable.

MONITORING WELL AND 

IDENTIFICATION NUMBER

Figure 26. Simulated 1994 (A) and 2022 (B) distributions of benzene using a maximum

value of hydraulic conductivity, at Laurel Bay Exchange,

Marine Corps Air Station Beaufort, S.C.

48
CONCLUSIONS

Exploratory modeling simulations indicate that there is no combination of reasonable ranges 

of known hydrologic, geochemical, or microbial conditions, as determined at the sites, that could 

result in petroleum-hydrocarbon contaminant transport to a point-of-contact with humans or 

wildlife at the Laurel Bay Exchange. It appears that contaminants are effectively isolated under 

current site conditions, that they are not affecting adjacent surface-water bodies, nor are they 

likely to in the future. This, in turn, suggests that intrinsic bioremediation is a possible remedial 

option for containing contaminated ground water at Laurel Bay. Available evidence indicates that 

removal of free product- or sorption-contaminated sediments can accelerate the intrinsic 

bioremediation of contaminated ground water at the site.

49
REFERENCES CITED

Chapelle, F.H., Landmeyer, J.E., and Bradley, P.M., 1995, Assessment of intrinsic bioremediation 

of jet fuel contamination in a shallow aquifer, Beaufort, South Carolina: U.S. Geological 

Survey Water-Resources Investigations Report 96-4262, 30 p.

Chiang, C.Y., Salanitro, J.R, Chai, E.Y., Colthart, J.D., and Klein, C.L., 1989, Aerobic 

biodegradation of benzene, toluene, and xylene in a sandy aquifer data analysis and 

computer modeling: Ground Water, v. 27, no. 6, p. 823-834.

Hach Company, 1989, Water analysis handbook, Loveland, Colo., 689 p.

Lovley, D.R., Baedecker, M.J., Lonergan, D.J., Cozarelli, I.M., Phillips, E.J.P., and Siegel, D.I., 

1989, Oxidation of aromatic contaminants coupled to microbial iron reduction: Nature, 

v. 339, p. 297-299.

McAllister, P.M., and Chiang, C.Y., 1994, A practical approach to evaluating natural attenuation 

of contaminants in ground water: Ground Water Monitoring and Review, Spring 1994, 

p. 161-173.

Mormile, M.R., Liu, Shi, and Suflita, J.M., 1994, Anaerobic biodegradation of gasoline 

oxygenates: Extrapolation of information to multiple sites and redox conditions: 

Environmental Science & Technology, v. 28, p. 1727-1732.

Schwarzenbach, R.P., and Westall, J., 1981, Transport of nonpolar organic compounds from 

surface water to groundwater. Laboratory sorption studies: Environmental Science & 

Technology, v. 15, no. 11, p. 11-18.

Stookey, L.L., 1970, FerroZine, a new spectrophotometric reagent for iron: Analytical Chemistry, 

v. 42, p. 779-781.

U.S. Navy Department, Southern Division Naval Facilities Engineering Command, 1993,

Contamination assessment report, Laurel Bay Exchange Service Station, Marine Corps Air 

Station Beaufort, South Carolina, Ground-Water Protection Division Site #A-07-AA-13575.

Voss, C.I., 1984, A finite-element simulation model for saturated-unsaturated, fluid-densitydependent ground-water flow with energy transport or chemically-reactive single-species 

solute transport: U.S. Geological Survey Water-Resources Investigations Report 84-4369, 

409 p.

<r U.S. GOVERNMENT PRINTING OFFICE 1996 - 742-068/23002

50