Natural Attenuation of

MTBE in the

Subsurface under

Methanogenic Conditions

United States

Environmental Protection

Agency

Office of Research and

Development

Washington DC 20460

EPA/600/R-00/006

January 2000
EPA/600/R-00/006

January 2000

Natural Attenuation of MTBE in the

Subsurface under

Methanogenic Conditions

National Risk Management Research Laboratory

Office of Research and Development

U.S. Environmental Protection Agency

Cincinnati, OH 45268

John T. Wilson

Jong Soo Cho

 Barbara H. Wilson

Subsurface Protection and Remediation Division

National Risk Management Research Laboratory

Ada, Oklahoma 74820

James A. Vardy

Civil Engineering Branch

United States Coast Guard

Cleveland, Ohio 41199-2060

Project Officer

John T. Wilson

Subsurface Protection and Remediation Division

National Risk Management Research Laboratory

Ada, Oklahoma 74820
ii

Notice

This work was carried out by staff of the U.S. Environmental Protection Agency (Office of Research and

Development, National Risk Management Research Laboratory) and by staff of the U.S. Coast Guard, in a

collaboration funded in part under Interagency Agreement # RW-69-937352. It has been subjected to the

Agency’s peer and administrative review and has been approved for publication as an EPA document. Mention

of trade names or commercial products does not constitute endorsement or recommendation for use. Certain

samples were collected or analyzed by employees of ManTech Environmental Research Services Corp., an inhouse contractor to the U.S. Environmental Protection Agency.

Representations and interpretations of the behavior of methyl-tertiary-butyl-ether (MTBE) or tertiary-butyl

alcohol (TBA) apply only to the site of the case study presented in this report. The authors, the U.S.

Environmental Protection Agency, and the U.S. Coast Guard make no claim in this report concerning the

behavior of methyl-tertiary-butyl-ether (MTBE) or tertiary-butyl alcohol (TBA) at other sites.

All research projects making conclusions or recommendations based on environmentally related measurements and funded by the Environmental Protection Agency are required to participate in the Agency Quality

Assurance Program. This project was conducted under an approved Quality Assurance Project Plan. The

procedures specified in this plan were used without exception. Information on the plan and documentation of the

quality assurance activities and results are available from John T. Wilson.
iii

Foreword

The U.S. Environmental Protection Agency is charged by Congress to protect the Nation’s land, air, and water

resources. Under a mandate of national environmental laws, the Agency strives to formulate and implement actions

leading to a compatible balance between human activities and the ability of natural systems to support and nurture

life. To meet these mandates, EPA’s research program is providing data and technical support for solving

environmental problems of today and building a science knowledge base necessary to manage our ecological

resources wisely, understand how pollutants affect our health, and prevent or reduce environmental risks in the

future.

The National Risk Management Research Laboratory is the Agency’s center for investigation of technological

and management approaches for reducing risks from threats to human health and the environment. The focus of the

laboratory’s research program is on methods for the prevention and control of pollution to air, land, water, and

subsurface resources; protection of water quality in public water systems; remediation of contaminated sites and

ground water; and prevention and control of indoor air pollution. The goal of this research effort is to catalyze

development and implementation of innovative, cost-effective environmental technologies; develop scientific and

engineering information needed by EPA to support regulatory and policy decisions; and provide technical support

and information transfer to ensure effective implementation of environmental regulations and strategies.

The U.S. Environmental Protection Agency’s Office of Underground Storage Tanks uses a risk management

approach to protect ground water from contamination with the soluble components of fuels that are accidentally

spilled or released from underground storage tanks. Contamination of ground water with MTBE and TBA associated

with spills from underground storage tanks is an emerging problem in the United States. Little is known of the

prospects for biodegradation of MTBE and TBA in ground water. Consistent with the Agency’s goal of sound science

as a basis for risk management, the Subsurface Protection and Remediation Division is developing information on

the rate and extent of natural attenuation of MTBE and TBA in ground water. This research effort emphasizes

natural biodegradation under various geochemical environments. This report describes natural attenuation of MTBE

under methanogenic conditions. It is the first in a series of reports; subsequent reports will examine natural

attenuation of MTBE under aerobic conditions, and under sulfate-reducing and iron-reducing conditions.

Clinton W. Hall, Director

Subsurface Protection and Remediation Division

National Risk Management Research Laboratory
iv

Abstract

At many fuel spill sites, the spread of contamination from benzene, toluene, ethylbenzene, and the xylenes

(BTEX compounds) is limited by natural biodegradation of the petroleum hydrocarbons in the ground water. At

present there is much uncertainty about whether MTBE from fuel spills will follow the same pattern as the petroleumderived hydrocarbons, or whether MTBE is biologically recalcitrant in ground water. If MTBE does not biodegrade

in ground water, then dilution and dispersion are the only mechanisms that are available to attenuate MTBE. As a

consequence, plumes of MTBE could expand farther than plumes of benzene or the BTEX compounds in the

absence of biodegradation.

This case study was conducted at the former Fuel Farm Site at the U.S. Coast Guard Support Center at

Elizabeth City, North Carolina. The geochemistry of the site is typical of sites where natural biodegradation limits the

spread of BTEX compounds. The plume is undergoing extensive anaerobic oxidation of petroleum hydrocarbons, as

well as fermentation of hydrocarbons to methane. The hydrocarbon metabolism through sulfate and iron oxidation

is approximately equivalent to the hydrocarbon metabolism through methanogenesis. The amount of hydrocarbon

metabolized through anaerobic pathways is about ten times the amount degraded with molecular oxygen.

There are two laboratory studies in the literature that report the biotransformation of MTBE in aquifer material

under methanogenic conditions. Neither study included an evaluation of the field-scale performance of natural

attenuation. This case study is intended to answer the following questions: Can MTBE be biodegraded under

methanogenic conditions in ground water that was contaminated by a fuel spill? Will biodegradation produce

concentrations of MTBE that are less than regulatory standards? Is the rate of degradation in the laboratory

adequate to explain the distribution of MTBE in the ground water at the field site? What is the relationship between

the degradation of MTBE and degradation of the BTEX compounds? What is the rate of natural attenuation of the

source area?

The apparent first order rate of removal of MTBE in the field was a sensitive function of ground-water seepage

velocity. The rate of removal was calculated for an upper boundary on velocity, an average velocity, and a lower

boundary on velocity. The rate was 5.0 per year at the upper boundary; 2.7 per year at the average velocity, and

2.2 per year at the lower boundary. Methane was considered to be a conservative tracer of ground-water flow at the

site. The apparent rate of removal of methane was taken as an estimate of attenuation along the flow path due to

dilution and dispersion. The apparent first order rate of removal of methane at the average estimate of seepage

velocity was 0.50 +/- 0.65 per year.

Biodegradation was evaluated in laboratory microcosms that were constructed with material from the contaminated portion of the aquifer. After 490 days of incubation, the average concentration of MTBE remaining in six

replicates of a treatment that was supplemented with BTEX compounds was 81 μg/l, compared to 5680 μg/l at the

beginning of incubation. The average concentration remaining in the control treatment after 490 days was 1470 μg/l,

compared to 3330 μg/l at the beginning of incubation. MTBE was also removed in microcosms that were not

supplemented with alkylbenzenes. After 490 days of incubation, the concentration of MTBE in all six of the replicate

microcosms that were sampled was below 40 μg/l, compared to 3110 μg/l at the beginning of incubation. Removal

of MTBE in the microcosms did not require the presence of BTEX compounds. The removal of MTBE did not begin

until the removal of the BTEX compounds was complete.

The first order rate of removal of MTBE in microcosms supplemented with alkylbenzenes was 3.02 per year +/

- 0.52 per year at 95% confidence. Removal in the corresponding controls was 0.39 +/- 0.19 per year at 95%

confidence. The removal in the microcosms without added alkylbenzenes was 3.5 per year +/- 0.65 per year at 95%

confidence. Removal in the corresponding controls was 0.30 per year +/- 0.14 per year at 95% confidence. The rate

of removal of MTBE in the laboratory studies can explain the apparent attenuation of MTBE at field scale.

The rate of natural attenuation of the source area was evaluated by comparing that flux to the total mass of

MTBE in the source area. The mass transfer of MTBE from the source LNAPL to the ground water moving

underneath was estimated by calculating the flux of MTBE moving away from the source, then dividing the flux into

the quantity of MTBE remaining. The flux of MTBE away from the source area in 1996 was 2.76 kg/year. The lower

boundary on the total quantity of MTBE in the source area was 46 kg. If the rate of transfer of MTBE to ground water

is proportional to the amount of MTBE in the source, the instantaneous rate of transfer is 0.06 per year. The average

concentration at the most contaminated location in the transect is 1200 μg/l. At this rate of attenuation of the source,

it would require at least sixty years for the concentration to reach 30 μg/l.

Tertiary Butyl Alcohol (TBA) has been documented as a transformation product of MTBE in a number of studies.

At the Old Fuel Farm Site, there is no evidence of accumulation of TBA in the ground-water plume as a whole. With

two exceptions, the concentration of TBA in ground water downgradient of the source area was less than 200 μg/l.

Ground water from a location immediately downgradient of the source area had a higher concentration of TBA, near

2000 μg/l. In this sample there was a corresponding reduction in the concentration of MTBE. At this location the TBA

was probably produced from transformation of MTBE.
v

Contents

Notice ........................................................................................................................................................ ii

Foreword .................................................................................................................................................. iii

Abstract .................................................................................................................................................... iv

Figures ..................................................................................................................................................... vi

Tables ..................................................................................................................................................... viii

Acknowledgments .................................................................................................................................... ix

Section 1 Introduction .............................................................................................................................. 1

Evidence for Biodegradation of MTBE under Anaerobic Conditions in Ground Water ..................... 1

Purpose of the Case Study ............................................................................................................... 2

Section 2 Laboratory Studies ................................................................................................................... 5

Construction, Sampling and Analysis of Microcosms ....................................................................... 5

Removal of MTBE ............................................................................................................................. 6

Removal of Benzene, Toluene, and Ethylbenzene ....................................................................... 7

Relationship between removal of BTEX compounds and removal of MTBE ............................. 10

Section 3 Site Characterization .............................................................................................................. 11

Site Description and History ............................................................................................................ 11

Core Sampling the Source Area...................................................................................................... 11

Estimation of Total Quantity of TPH and MTBE and the Area Impacted ......................................... 13

Vertical Distribution of TPH and MTBE in Core Samples ................................................................ 13

Distribution of Total Petroleum Hydrocarbons and Hydraulic Conductivity with Depth .................... 15

Distribution of MTBE and BTEX Compounds with Depth................................................................ 16

Section 4 Transport and Fate of MTBE in the Ground Water ................................................................. 23

Estimated Rate of Attenuation in Ground Water ............................................................................. 23

Transfer from the Entire Source Area to the Plume ......................................................................... 27

Transfer of MTBE to TBE ................................................................................................................. 28

Section 5 Summary and Conclusions .................................................................................................... 31

Extent of Biodegradation of MTBE .................................................................................................. 31

Role of BTEX Compounds .............................................................................................................. 31

Rate of Removal of MTBE ............................................................................................................... 31

Expected Persistence of the Source of Ground-water Contamination ............................................ 31

Production and Depletion of TBA .................................................................................................... 31

Geochemical Context of the Plume that Biodegraded MTBE ......................................................... 32

References ............................................................................................................................................. 33

Appendix A: Temporal Variation in the Hydraulic Gradient and the Direction of Ground Water Flow ..... 34

Appendix B: Geochemical Context of the MTBE Plume ........................................................................ 45
vi

Figures

Figure 1.1 Site selected for the case study of natural attenuation of MTBE under methanogenic conditions. The shaded

concentric circles represent the residual LNAPL from a fuel spill. The arrow represents the distance traveled

by ground water in three years. .......................................................................................................................................................... 4

Figure 2.1 Removal of MTBE in microcosms constructed with MTBE and BTEX compounds compared to removal in

control microcosms that were autoclaved to prevent biotransformation of MTBE............................................................................. 7

Figure 2.2 Removal of MTBE in microcosms constructed with MTBE but without supplemental concentrations of BTEX

compounds compared to removal in control microcosms that were autoclaved to prevent biotransformation of MTBE. ................ 8

Figure 2.3 Removal of Toluene in microcosms constructed with MTBE and BTEX compounds compared to removal in

control microcosms that were autoclaved to prevent biotransformation of toluene. The solid line is fit through the

removal in the controls. ....................................................................................................................................................................... 8

Figure 2.4 Removal of Benzene in microcosms constructed with MTBE and BTEX compounds compared to removal in

control microcosms that were autoclaved to prevent biotransformation of benzene. The solid line is fit through the

removal in the controls. ....................................................................................................................................................................... 9

Figure 2.5 Removal of Ethylbenzene in microcosms constructed with MTBE and BTEX compounds compared to removal in

control microcosms that were autoclaved to prevent biotransformation of ethylbenzene. ................................................................ 9

Figure 2.6 Comparison of the time lags for removal of MTBE, and of benzene, toluene, and ethylbenzene in microcosms

constructed with all the compounds present together. ..................................................................................................................... 10

Figure 3.1 Relationship between the sampling locations for characterization of the LNAPL source area (labeled CPT-1

through CPT-5), and the former location of storage tanks for fuels. ................................................................................................ 12

Figure 3.2 Inferred location of the fuel release, based on vertical core samples and the location of the steel underground

storage tanks. .................................................................................................................................................................................... 14

Figure 3.3 Vertical distribution of MTBE and Total Petroleum Hydrocarbon (TPH) in core samples at location CPT-1

(See Figure 3.1 for map). .................................................................................................................................................................. 15

Figure 3.4 Vertical distribution of MTBE and Total Petroleum Hydrocarbon (TPH) in core samples from location CPT-2

(See Figure 3.1 for map). .................................................................................................................................................................. 15

Figure 3.5 Vertical distribution of MTBE and Total Petroleum Hydrocarbon (TPH) in core samples from location CPT-3

(See Figure 3.1 for map). .................................................................................................................................................................. 15

Figure 3.6 Relationship between the vertical extent of Hydraulic Conductivity and the vertical extent of Total Petroleum

Hydrocarbon at location CPT-1. ........................................................................................................................................................ 16

Figure 3.7 Relationship between two transects of ground-water samples and the fuel release. The arrow represents

the average direction of ground-water flow. ...................................................................................................................................... 17

Figure 3.8 Location of vertical sampling points along the north-south transect, collected in August 1996. Distance

along the transect extends from south to north (bottom to top in Figure 3.7), in the direction of ground-water flow. ..................... 19

Figure 3.9 Location of vertical sampling points along the east-west transect, collected in December 1997. Distance

along the transect extends from west to east (left to right in Figure 3.7), opposite the direction of ground-water flow. ................. 19

Figure 3.10 Distribution of hydraulic conductivity along the north-south transect, collected in August 1996. Distance along

the transect extends from south to north (bottom to top in Figure 3.7), in the direction of ground-water flow. ............................... 20

Figure 3.11 Distribution of hydraulic conductivity along the east-west transect, collected in December 1997. Distance

along the transect extends from west to east (left to right in Figure 3.7), opposite the direction of ground-water flow. ................. 20

Figure 3.12 Distribution of MTBE along the north-south transect, collected in August 1996. Distance along the transect

extends from south to north (bottom to top in Figure 3.7), in the direction of ground-water flow. ................................................... 21

Figure 3.13 Distribution of MTBE along the east-west transect, collected in December 1997. Distance along the transect

extends from west to east (left to right in Figure 3.7), opposite the direction of ground-water flow. ............................................... 21

Figure 3.14 Distribution of BTEX along the north-south transect, collected in August 1996. Distance along the transect

extends from south to north (bottom to top in Figure 3.7), in the direction of ground-water flow. ................................................... 22

Figure 3.15 Distribution of BTEX along the east-west transect, collected in December 1997. Distance along the transect

extends from west to east (left to right in Figure 3.7), opposite the direction of ground-water flow. ............................................... 22

Figure 4.1 Variation in ground-water flow calculated from eighteen rounds of quarterly monitoring. The length of the

arrow is the distance that would be traveled by MTBE in one year at that hydraulic gradient......................................................... 24

Figure 4.2 Variation in ground-water flow calculated from fourteen rounds of monthly monitoring. The length of the

arrow is the distance that would be traveled by MTBE in one year at that hydraulic gradient. ....................................................... 24

Figure 4.3 Attenuation in concentrations of MTBE, methane, and iron (II) with travel time downgradient from the

location with the highest concentration of MTBE. ............................................................................................................................ 26

Figure 4.4 Locations of ground-water samples included in the calculation of the rate of natural attenuation. The arrow

represents the average direction of ground-water flow. The dark shaded area is the area with LNAPL.

The larger lightly shaded area is the area downgradient where the ground water contains high

concentrations of methane and iron (II). Only wells in the shaded area were included in the calculation of the

rate of natural attenuation. ................................................................................................................................................................ 27
vii

Figure 4.5 Relationship between the direction of ground-water flow and the ground-water sampling locations in the

transect sampled in December 1997. Ground-water flow vectors were calculated from the gradients in

water table elevation in eighteen different rounds of monitoring The length of arrow is the distance that

would be traveled by MTBE in one year of flow at that gradient. ..................................................................................................... 27

Figure 4.6 Concentrations of MTBE in a transect that extends across the plume in a direction that is roughly

perpendicular to ground-water flow. See Figure 4.5 for the positions of the sampling locations identified as

15 through 25 in both Figures. Depicted at each location are the flow-weighted average concentrations of

MTBE in ground-water samples from a vertical profile extending across the aquifer at each location. .......................................... 28

Figure 4.7 Relationship between the concentration of TBA in ground water and the concentration of MTBE, in water

samples collected in a transect across the plume in December 1997. ........................................................................................... 29

Figure 4.8 Depth distribution of MTBE in three locations downgradient of the LNAPL source area. See Figure 4.5 for

position of the locations on a map. Compare location 19 to location 19 in Figure 4.9. ................................................................... 30

Figure 4.9 Depth distribution of TBa in locations downgradient of the source area. See Figure 4.5 for position of the

locations on a map. Compare location 19 to location 19 in Figure 4.8. ........................................................................................... 30

Figure A.1 Variation in elevation of water in the Pasquotank River over a time interval extending from

September 5, 1996 to October 30, 1996. ......................................................................................................................................... 34

Figure A.2 Location of the permanent monitoring wells used to estimate the hydraulic gradient and direction during each

round of monitoring. .......................................................................................................................................................................... 35

Figure A.3 Variation in elevation of the water table at the fuel farm site over time. Consult Figure A.1 for the

location of the monitoring wells. Well ESM-10 is closest to the Pasquotank River, the point of ground-water

discharge. Wells ESM-14, ESM-6, and ESM-7 are farther inland. ................................................................................................... 35

Figure A.4 Direction and gradient of ground-water flow on a sample date in September 1994. ....................................................................... 40

Figure A.5 Direction and gradient of ground-water flow on a sample date in December 1994. ........................................................................ 40

Figure A.6 Direction and gradient of ground-water flow on a sample date in March 1995. ............................................................................... 41

Figure A.7 Direction and gradient of ground-water flow on a sample date in May 1995. .................................................................................. 41

Figure A.8 Direction and gradient of ground-water flow on a sample date in August 1995. .............................................................................. 41

Figure A.9 Direction and gradient of ground-water flow on a sample date in December 1995. ........................................................................ 41

Figure A.10 Direction and gradient of ground-water flow on a sample date in March 1996. ............................................................................... 42

Figure A.11 Direction and gradient of ground-water flow on a sample date in June 1996. ................................................................................. 42

Figure A.12 Direction and gradient of ground-water flow on a sample date in September 1996. ....................................................................... 42

Figure A.13 Direction and gradient of ground-water flow on a sample date in December 1996. ........................................................................ 42

Figure A.14 Direction and gradient of ground-water flow on a sample date in March 1997. ............................................................................... 43

Figure A.15 Direction and gradient of ground-water flow on a sample date in June 1997. ................................................................................. 43

Figure A.16 Direction and gradient of ground-water flow on a sample date in September 1997. ....................................................................... 43

Figure A.17 Direction and gradient of ground-water flow on a sample date in December 1997. ........................................................................ 43

Figure A.18 Direction and gradient of ground-water flow on a sample date in March 1998. ............................................................................... 44

Figure A.19 Direction and gradient of ground-water flow on a sample date in June 1998. ................................................................................. 44

Figure A.20 Direction and gradient of ground-water flow on a sample date in September 1998. ....................................................................... 44

Figure A.21 Direction and gradient of ground-water flow on a sample date in December 1998. ........................................................................ 44

Figure B.1 Distribution of methane along the north-south transect, collected in August 1996. Distance along the transect

extends from south to north (bottom to top in Figure 3.7), in the direction of ground-water flow. ................................................... 46

Figure B.2 Distribution of MTBE along the north-south transect, collected in August 1996. Distance along the transect

extends from south to north (bottom to top in Figure 3.7), in the direction of ground-water flow. ................................................... 46

Figure B.3 Distribution of methane along the east-west transect, collected in December 1997. Distance along the transect

extends from west to east (left to right in Figure 3.7), opposite the direction of ground-water flow. ............................................... 47

Figure B.4 Distribution of MTBE along the east-west transect, collected in December 1997. Distance along the transect

extends from west to east (left to right in Figure 3.7), opposite the direction of ground-water flow. ............................................... 47

Figure B.5 Distribution of oxygen along the north-south transect, collected in August 1996. Distance along the transect

extends from south to north (bottom to top in Figure 3.7), in the direction of ground-water flow.. .................................................. 48

Figure B.6 Distribution of sulfate along the north-south transect, collected in August 1996. Distance along the transect

extends from south to north (bottom to top in Figure 3.7), in the direction of ground-water flow. ................................................... 48

Figure B.7 Distribution of iron (II) along the north-south transect, collected in August 1996. Distance along the transect

extends from south to north (bottom to top in Figure 3.7), in the direction of ground-water flow. ................................................... 49

Figure B.8 Distribution of alkalinity along the north-south transect, collected in August 1996. Distance along the transect

extends from south to north (bottom to top in Figure 3.7), in the direction of ground-water flow. ................................................... 49
viii

Tables

Table 1.1 Temporal variation in the concentrations of MTBE, Benzene, and Methane at the most contaminated permanent

sampling location that is downgradient of the LNAPL area. ............................................................................................................... 3

Table 2.1 The concentration of MTBE and alkylbenzenes in the most contaminated sample of ground water from the LNAPL

source area, in the permanent monitoring well at the location where the sediment used to construct the microcosms

was acquired, and the initial concentrations achieved in the microcosms. ....................................................................................... 6

Table 3.1 Quantity of Total Petroleum Hydrocarbon and MTBE at seven sampling locations in or near the point of release of fuel. ............ 13

Table 3.2 Distribution of Hydraulic Conductivity (K) in the North-South transect sampled in August, 1996 (Figure 3.7). .............................. 18

Table 3.3 Distribution of Hydraulic Conductivity (K) in the East-West transect sampled in December, 1997 (Figure 3.7). ........................... 18

Table 4.1 Sensitivity analysis of the estimates of the seepage velocity of ground water at the site. These estimates were

used to calculate a first order rate of attenuation of MTBE in ground water downgradient of the source area. ............................. 24

Table 4.2 Concentration of MTBE, methane, and iron (II) at monitoring locations used to calculate the rate of attenuation of

MTBE, methane, and iron (II) with time of travel downgradient of the location with the highest concentration. ............................. 25

Table 4.3 The apparent first order rate of attenuation of MTBE, methane, and iron (II) with time of travel downgradient from the

location with the highest concentration of MTBE. ............................................................................................................................ 26

Table A.1. Elevation of the water table in permanent monitoring wells during eighteen rounds of quarterly

monitoring extending from September 1994 through December 1998. The elevations are reported in

feet above mean sea level. Compare Figure A.1 for the location of the monitoring wells. ............................................................. 37

Table A.2. Elevation of the water table in permanent monitoring wells during fourteen rounds of monthly

monitoring extending from September 1994 through December 1998. The elevations are reported in

feet above mean sea level. Compare Figure A.1 for the location of the monitoring wells. ............................................................. 37

Table A.3. Equation of a linear plane that was fit using a least-squares regression through the elevation of the

water table in permanent monitoring wells during each of eighteen rounds of quarterly sampling. The

plane is in an x,y,z coordinate system where x increases toward the east, y increases toward the north,

and z increases with elevation above mean sea level. The equation is in the form Ax+By+C+z where

x and y are the grid location in UTM meters and z is the elevation of the water table in feet. ........................................................ 38

Table A.4. Equation of a linear plane that was fit using a least-squares regression through the elevation of the

water table in permanent monitoring wells during each of fourteen rounds of monthly sampling. The

plane is in an x,y,z coordinate system where x increases toward the east, y increases toward the north,

and z increases with elevation above mean sea level. The equation is in the form Ax+By+C+z where

x and y are the grid location in UTM meters and z is the elevation of the water table in feet. ........................................................ 39
ix

The authors appreciate the excellent technical support extended by Frank Beck and Cherri Adair of the SPRD,

by Kelly Hurt while he was a National Research Council associate, by staff of ManTech Environmental Research

Services Corp., and by staff of Dynamac, Inc.

Acknowledgments
1

SECTION 1

Introduction

MTBE is widely distributed in ground water. The U.S.

Geological Survey sampled shallow ambient ground water

from eight urban areas in 1993 and 1994. MTBE was

detected at concentrations at or above 0.2 μg/l in 27% of

the 210 wells and springs that were sampled (Squillace et

al.,1996). The U.S. Environmental Protection Agency has

tentatively classified MTBE as a possible human carcinogen (U.S. EPA, 1996). There is currently much concern

about the occurrence and behavior of MTBE in ground

water that might be used for a drinking water supply.

Higher concentrations of MTBE in ground water are the

result of releases of gasoline containing oxygenates from

underground storage tanks (Landmeyer et al., 1998).

Fuels also contaminate ground water with benzene and

alkylbenzenes including toluene, ethylbenzene, and the

xylenes (BTEX compounds). Careful and detailed studies

of the transport and fate of the BTEX compounds demonstrated that these compounds were biologically degraded

under natural conditions in ground water (summarized in

Wiedemeier et al. 1999).

The data supports the theory that the spread of BTEX

contamination at many sites was limited by natural biodegradation processes. As a result of our increased understanding of benzene plume behavior, natural attenuation is now being formally recognized as a component of

many risk-based remedies at petroleum fuel spill sites.

There is little recognition in the literature that natural

biodegradation may control the spread of MTBE contamination in ground water. In his review, Chapelle (1999)

noted that “Field studies of MTBE biodegradation relative

to BTEX compounds ... indicate that MTBE is biodegraded

in shallow aquifers, but that biodegradation is less than for

BTEX compounds.” Mormile et al. (1994) conclude “the

common ether oxygenates resist both anaerobic and aerobic decay and must be considered recalcitrant chemicals.”

As will be discussed in the next section, there is evidence

in the literature that anaerobic degradation of MTBE is

possible. There are also many experiments where degradation was not detected.

Evidence for Biodegradation of MTBE

under Anaerobic Conditions in Ground

Water

There are two reports of MTBE biotransformation in

laboratory studies under methanogenic conditions. Yeh

and Novak (1994) constructed static soil and water microcosms with material from three sites; a site at a wooded

area at the Virginia Polytechnic Institute (VPI) at

Blacksburg, Virginia, that is largely unsaturated clay; a

site at VPI in a low area that is mainly sandy loam that

receives runoff from a feedlot, and a site at Newport

News, Virginia, that is mainly silty loam. Ethanol and

starch were added as a source of molecular hydrogen.

Potassium phosphate and ammonium chloride were added

as nutrients. Cysterine and sodium sulfide were added to

encourage anaerobic processes. Sodium molybdate was

added to inhibit sulfate-reducing microorganisms in the

microcosms that were intended to simulate methanogenic

conditions. The initial concentration of MTBE in the

microcosms was 100 mg/l. The microcosms were incubated at 20o

C for times extending from 250 days to 300

days. Removals in microcosms were compared to removals in autoclaved controls prepared for each site.

After 250 days of incubation, there was no removal of

MTBE in excess of removal in autoclaved controls in

microcosms constructed with material from the sandy

loam site downgradient of the feed lot, or the silty loam

site. However, in excess of 99 percent of MTBE was

removed in a microcosm constructed with clay material

collected at a depth of 1.5 meters below land surface, and

80 percent of MTBE was removed in a microcosm constructed from material collected at a depth of 3.0 meters

below land surface. The rate of removal in the microcosm

constructed with material from a depth of 1.5 meters

corresponded to a first-order rate of removal of 3.3 per

year, or a half life of eleven weeks. The removal in the

microcosm constructed with material from a depth of

3.0 meters corresponded to a first-order rate of removal of

2.0 per year or a half life of eighteen weeks.

In their studies, Yeh and Novak (1994) found no evidence of MTBE biodegradation under anaerobic conditions where nutrients and a hydrogen source were not

added, or under denitrifying conditions or sulfate-reducing

conditions when nutrients and a hydrogen source were

added. The removal of MTBE was only associated with

methanogenic conditions.

Mormile et al. (1994) examined material from a sandy

water-table aquifer near Empire, Michigan, that had been

contaminated with gasoline; sediment from the Ohio River

that had been impacted by oil storage and barge loading

facilities; and, sediment from Mill Creek in Cincinnati,

Ohio, that had been impacted with industrial and municipal sewage sludge. Microcosms were constructed with

slurries of sediment and ground water. The slurries were
2

amended with sodium sulfide, but no additional nutrients or

hydrogen sources were added. Some of the microcosms

received sodium sulfate or sodium nitrate to stimulate

sulfate-reducing and nitrate-reducing conditions. The initial concentration of MTBE was 50 mg/l as carbon.

There was removal of MTBE in one of three replicate

microcosms constructed with the sediment from the Ohio

River. After 152 days of incubation, the concentration of

MTBE was reduced to 22 mg/l carbon from an initial

concentration of 48 mg/l carbon. There was no removal of

MTBE in an autoclaved control. In the microcosm, the

removal of MTBE was associated with the production of

tertiary butyl alcohol (TBA).

In contrast to the removal of MTBE in the one microcosm constructed with sediment from the Ohio River, there

was no removal in sediment that was contaminated with

gasoline after 230 days of incubation, or in the sediment

impacted with sewage sludge after 180 days of incubation.

In an earlier study, Suflita and Mormile (1993) had examined the degradtion of a variety of oxygenate in material

from a sandy water table aquifer at Norman, Oklahoma,

that had been contaminated with landfill leachate. After

249 days of incubation, there was no evidence of removal

of MTBE.

Consistent with the work of Yeh and Novak (1994),

Mormile et al. (1994) found no removal of MTBE under

sulfate-reducing conditions or nitrate-reducing conditions

in the three materials they examined.

There are two additional reports of MTBE degradation

under anaerobic conditions. Landmeyer et al. (1998)

examined MTBE degradation at a site on Port Royal

Island, South Carolina, in the Lower Coastal Plain of the

Atlantic Coastal Plain geophysical province. The aquifer

had been contaminated with a gasoline spill. Microcosms

were constructed with material from an area with high

concentrations of BTEX contamination and MTBE contamination, and a second area with high concentrations of

MTBE but much lower concentrations of BTEX. Microcosms were constructed with material from each site with a

high or a low concentration of MTBE. Transformation of

MTBE was assayed by collecting radio-labeled carbon

dioxide produced from the transformation of MTBE that

was uniformly labeled with carbon 14 (radio-labeled impurity less than 0.2% of the total label).

The headspace of the microcosms was helium, which

resulted in iron-reducing conditions in the microcosms.

After 28 weeks of incubation, between 2.0% and 3.0% of

the label was transformed to carbon dioxide (mean of

triplicate microcosms for all four experimental treatments).

Attenuation under iron-reducing conditions was real, but

the rate of transformation was slow, corresponding to a

first-order rate of attenuation of 0.06 per year. Anaerobic

biodegradation did not attenuate the plume of MTBE in

ground water before it approached the receptor.

Church et al. (1997) examined the effluent from a column microcosm constructed with core material from a site

in Trenton, New Jersey. The influent concentration of

MTBE was near 100 μg/l. After 35 days of operation, the

effluent concentration of MTBE was 160 μg/l, and there

was no detectable concentration of TBA (interpretation of

their Figure 2). After 44 days of operation, the effluent

concentration of MTBE was 160 μg/l and the concentration

of TBA was 20 μg/liter. After 52 days of operation, the

effluent concentration of MTBE was reduced to 40 μg/l,

and the concentration of TBA increased to 60 mg/l.

Purpose of the Case Study

This case study is intended to answer the following

questions. Can MTBE be biodegraded under methanogenic

conditions in ground water that was contaminated by a

fuel spill? Will biodegradation reach concentrations of

MTBE that are less than regulatory standards? Is the rate

of degradation in the laboratory adequate to explain the

distribution of MTBE in the ground water at the field site?

What is the relationship between the degradation of MTBE

and degradation of the BTEX compounds? How long can

the fuel release continue to contaminate ground water at

the site?

The case study was conducted at a former fuel farm that

had been operated by the United States Coast Guard at

their Support Center at Elizabeth City, North Carolina. Fuel

for aircraft was stored at the site until December 31, 1991.

The fuel farm had been in use since 1942, and originally

consisted of a 50,000-gallon concrete underground storage tank, and two steel underground storage tanks with a

volume of 12,000-gallons and 15,000-gallons, respectively

(adjacent to location CPT-1 in Figure 1.1). The steel tanks

were apparently removed in the mid-1980s.

The United States Coast Guard has conducted extensive free product recovery efforts at the site. Figure 1.1

depicts the location of residual LNAPL, the direction and

speed of ground-water flow, and the ground-water sampling locations at the site.

A GeoProbe push point sampler was used to acquire

water samples at the location in Figure 1.1. At each

location the aquifer was sampled in a vertical profile that

extended from the water table, through a shallow silty clay

layer, into a fine sand unit, and then into a silty clay unit

beneath the sand. The GeoProbe push points were

screened over a vertical interval of 1.5 feet. The

GeoProbe samples extended from 5 feet below land

surface to 30 feet below land surface. The aquifer was

confined to an interval between 10 and 25 feet below land

surface. At a minimum, every other 1.5 foot vertical

interval was sampled.

The hydraulic conductivity of the material sampled was

estimated for each GeoProbe sample. The measured

concentration of MTBE, benzene, and methane were

weighted by the hydraulic conductivity of the interval being

sampled before the vertical samples were averaged. Because the GeoProbe samples are flow-weighted averages, there is no danger that the data could give a false

impression of natural attenuation due to hydraulic averaging along the flow path in the aquifer.

The concentration of MTBE in ground water under the

spill was high, 1740 μg/L at location CPT-1 in Figure 1.1.

However, the only monitoring locations downgradient of
3

Table 1.1 Temporal variation in the concentrations of MTBE, Benzene, and Methane at the most contaminated

permanent sampling location that is downgradient of the LNAPL area.

Vertical

Profile

 Permanent Well ESM-14

8/1996 8/1996 10/1997 10/1998 12/1998 7/1999 9/1999

 ---------(mg/l)--------

MTBE 383 353 194 154 65 259 609

Benzene 139 631 389 1280 1300 2185 1070

Methane 7,780 11,500 15,400 16,200 10,900 8,400 962

the source area that had concentrations of MTBE that

exceeded regulatory standards were monitoring locations

ESM-14 and ESM-3, and the concentrations were approximately 20% of the maximum concentration in the source

area.

The concentration of MTBE in a permanent monitoring

well at location ESM-14 (see Figure 1.1) showed good

agreement with the weighted-average concentration from

the vertical profile sampling (see Table 1.1). In order to

estimate the temporal variability of the plume, data from

the permanent monitoring well at location ESM-14 are

reported for sampling events extending from August 1996

through September 1999. The August 1996 sampling

event was almost five years after the site was no longer

used for fuel storage. In the interval from August 1996 to

September 1999 there are no recognizable trends in the

concentration of MTBE, benzene, or methane. There is no

evidence that the concentrations of MTBE leaving the

source are decreasing over time.

The site was selected for study for two reasons. The

source of the MTBE plume was relatively stable, based on

trends in MTBE concentrations in the most contaminated

permanent monitoring well at the site (Table 1.1). This

made it possible to differentiate attenuation in concentration as water moved downgradient from attenuation of the

source area itself. The site was also selected because the

concentrations of MTBE were greatly attenuated at monitoring locations that were relatively close to the source

area in terms of time of travel of ground water. The time

required for ground water to travel from the source area to

the most distant sampling location is less than five years.

This suggested that the kinetics of natural attenuation at

this site should be rapid.
4

Figure 1.1 Site selected for the case study of natural attenuation of MTBE under methanogenic conditions. The shaded concentric circles

represent the residual LNAPL from a fuel spill. The concentration of MTBE at location CPT-1 was 1740 μg/l. The arrow represents

the distance traveled by the ground water in three years. The only sampling location with concentrations of MTBE above 20 μg/l

was ESM-14.

PASQUOTANK RIVER

GP-1

ESM-10 ESM-9

ESM-14

Approximate Scale in Feet

200 200 100 0

N

ESM-3

ESM-11

CPT-3

CPT-1

CPT-2

Approximate Scale in Meters

0 50 100 150

Source
5

SECTION 2

Laboratory Studies

Construction, Sampling and Analysis of

Microcosms

Microcosms were constructed with aquifer material from

location ESM-14. This location had the highest concentration of MTBE in the permanent monitoring wells that

were available at the time the samples for the microcosm

study were collected. At this location, approximately 10

feet (3 meters) of silty clay overlies 15 feet of silty sand

and fine sand. The water table is near 10 feet below land

surface. A hollow stem auger was advanced into the

earth to a depth of approximately 15 feet (4.6 meters).

The auger was maintained at this depth, and was rotated

to elevate material on the auger flights. The initial material

that was elevated was silty clay; this material was discarded. Approximately 0.3 cubic meters of fine sand was

elevated on the auger flights and discarded, then 8 liters of

sediment was collected for construction of microcosms.

The sediment was collected and stored in 1-quart glass

jars. To protect the anaerobic microorganisms that might

be present in the samples from oxygen in the atmosphere,

the head space above the sediment was replaced with

ground water from the borehole immediately after collection. The samples were cooled and shipped to the Robert

S. Kerr Environmental Research Center with water ice,

and stored at 4 oC until used to construct microcosms.

To protect anaerobic microorganisms from oxygen in

the atmosphere, all manipulations to prepare the microcosms were carried out in a glove box with a concentration of oxygen in the atmosphere that was less than

1 ppm (v/v). This corresponds to a concentration of oxygen in water (at equilibrium) of 0.00004 mg/l. Microcosms

were prepared in glass serum bottles with a volume of 25

ml. Ground water from the bore hole was added to the

sediment to make a thick slurry. This slurry was transferred to the serum bottles with a scoop. Each microcosm

received 40 gm wet weight of slurry and 1.0 ml of a dosing

solution containing MTBE, or MTBE and alkybenzenes.

The remaining volume (3 to 4 ml) was filled with autoclaved ground water from the bore hole. The microcosms

were sealed with a grey butyl rubber septum and a crip

cap. The microcosms were stored in the same glove box,

under an atmosphere that was 2% to 5% hydrogen and

contained less than 1 ppm oxygen. Ground-water temperatures from permanent wells at the site varied from 19o

to 24oC between sampling dates in December and September. The microcosms were incubated at room temperature (20o

 to 22oC).

To prepare abiotic controls, a portion of the sediment

was autoclaved overnight. Four treatments were prepared; sediment amended with MTBE alone, autoclaved

sediment amended with MTBE alone, sediment amended

with MTBE and alkylbenzenes, and autoclaved sediment

amended with MTBE and alkylbenzenes. The initial concentrations of MTBE and alkylbenzenes that were achieved

in the microcosms are listed in Table 2.1.

The microcosms were sampled and analyzed as follows. The contents of the microcosms were vigorously

stirred with a vortex mixer. The microcosms were centrifuged to settle the solids. Then the crimp cap and septa

were removed, and 1.0 ml of water was transferred to

39 ml of dilution water and sealed in 40 ml VOA bottle with

a Teflon-faced silicone septum and a screw cap. The

dilution water was distilled water that had been boiled,

each sample received one drop of sulfuric acid to preserve the sample. The concentration of MTBE and

alkylbenzenes were determined by purge and trap analysis using gas chromatography with a PID detector. The

limit of quantification for MTBE and the alkylbenzenes was

1 μg/l, corresponding to a limit of quantification of 40 μg/l

in the pore water of the microcosms.

The sediment selected to construct the microcosms

was intended to represent the region in the aquifer where

natural attenuation of MTBE was in progress. The microcosms were constructed with material from a location

where the apparent natural attenuation of MTBE was

extensive, but was not complete. Table 2.1 compares the

concentration of MTBE in the source area of the plume to

the concentration in a permanent monitoring well at the

location that was used to acquire the sediment for the

microcosms. At the sample location, the concentration of

MTBE was reduced approximately tenfold from the concentration in the LNAPL source area. Another tenfold

reduction would approach concentrations that would meet

regulatory standards for MTBE.

The microcosms incubated over time were intended to

represent the travel of a representative volume of water

along the entire flow path, starting with the source and

extending to a potential receptor. The sediments in the

microcosms were amended with MTBE to simulate the

highest concentration in the source area. The travel time

of MTBE from the source to a potential receptor is on the

order of four years. The microcosm study was designed to

last for eighteen months to two years.
6

To determine whether there was any interaction between

the presence of alkylbenzenes and the removal of MTBE,

one set of treatments was amended with benzene, toluene, ethylbenzene, the three xylenes, and the three

trimethylbenzenes, and one set of treatments was not

amended. The initial concentrations of individual

alkylbenzenes in the microcosms were higher than their

concentration in the source area, with the exception of

benzene, where the concentration in the microcosms was

approximately one-fourth the maximum concentration in

the source area (Table 2.1). Background concentrations

of alkylbenzenes were not detected in microcosms that

were not amended with alkylbenzenes.

Removal of MTBE

Removal of MTBE in material that was supplemented

with alkylbenzenes was extensive (Figure 2.1). There was

no evidence of MTBE removal over removal in the controls

in the first 175 days of incubation. The concentrations of

MTBE in replicate microcosms from both the living and

control treatments show relatively little scatter. After

385 days of incubation, there is evidence of removal in the

living treatment. The data after 385 days of incubation

show a great deal of scatter. The range in concentrations

in six replicates is over an order of magnitude wide. After

490 days of incubation, there is consistent removal of

MTBE compared to the controls.

Table 2.1 The concentration of MTBE and alkylbenzenes in the most contaminated sample of ground water from

the LNAPL source area, in the permanent monitoring well at the location where the sediment used to

construct the microcosms was acquired, and the initial concentrations achieved in the microcosms.

Most Sample

 Impacted Location Microcosms

Ground Water Ground Water

CPT-1 Well MTBE MTBE MTBE MTBE

Compound 3.1 m to ESM-14 alone alone plus BTEX plus BTEX

3.6 m bls control control

Single Analysis Mean (Sample Standard Deviation, n = 3)

μg/l

MTBE 3640 353 3112 2908 5680 3330

(188) (538) (138) (360)

Benzene 7830 631 <40 <40 2079 1953

(218) (174)

Toluene 383 1.8 <40 <40 2183 1996

(233) (176)

Ethylbenzene 396 1.9 <40 <40 1256 1109

(130) (108)

o-xylene 23 <1 <40 <40 1911 1697

(170) (146)

m-Xylene <40 <40 1592 1404

(158) (134)

p-Xylene <40 <40 1556 1361

(151) (139)

m+p-Xylene 1250 2.9

1,2,3-trimethylbenzene 286 1.2 <40 <40 887 747

(54) (78)

1,2,4-trimethylbenzene 430 <1 <40 <40 625 521

(40) (62)

1,3,5-trimethylbenzene 107 <1 <40 <40 664 551

(45) (69)
7

The average concentration remaining in six replicates of

the living treatment was 81 μg/l, compared to 5680 μg/l at

the beginning of incubation. The average concentration

remaining in the control treatment after 490 days was 1466

μg/l, compared to 3330 μg/l at the beginning of incubation.

The removal in the controls was a little more than twofold,

while removal in the living microcosms was 70-fold.

Removal of MTBE in material that was not supplemented

with alkybenzenes was also extensive (Figure 2.2). There

was little evidence of removal in the first 175 days of

incubation. After 385 days, the removal of MTBE in the

living microcosms was extensive. After 490 days of incubation, the concentration of MTBE in six replicate microcosms was below 40 μg/l, compared to 3112 μg/l at the

beginning of incubation. After 490 days, the average

concentration of MTBE in the control microcosms was

1571 μg/l, compared to an initial concentration in the

controls of 2908 μg/l.

A first-order rate of removal was fitted to the data by a

linear regression of the natural logarithm of the concentration of MTBE on the time of incubation. The rate of

removal of MTBE in microcosms supplemented with

alkylbenzenes was 3.02 per year ±0.52 per year at 95%

confidence. Removal in the corresponding controls was

0.39 +/- 0.19 per year at 95% confidence. The removal in

the microcosms without added alkylbenzenes was 3.5 per

year +/- 0.65 per year at 95% confidence. Removal in the

corresponding controls was 0.30 per year, ±0.14 per year

at 95% confidence. The rate constants were fit to the

entire data; no correction was made for the apparent lag

period.

A container control was not done. There is no way to

determine if the removals in the controls are due to kinetically slow sorption to the aquifer solids, or to diffusion out

of the microcosm through the septa.

Removal of Benzene, Toluene, and

Ethylbenzene

Toluene was removed rapidly and extensively in the

microcosms. After only 40 days or 47 days of incubation,

toluene removal was extensive in most of the microcosms

sampled (Figure 2.3). After 110 days of incubation, the

concentration of toluene was less than 40 μg/l in all of the

microcosms sampled. Benzene was also rapidly removed

from the microcosms. There was no evidence of benzene

removal after 40 days or 47 days of incubation (Figure 2.4)

although removal of toluene was evident. After 110 days

of incubation, the concentration of benzene in all the

microcosms sampled was less than 40 μg/l. There is no

evidence of a lag for removal of toluene. There may have

been a slight lag in the removal of benzene.

The behavior of ethylbenzene (Figure 2.5) is representative of the behavior of the xylenes and trimethylbenzenes

as well. Removal was extensive in both living microcosms

and controls. There was no difference in removal in living

and control microcosms through 110 days of incubation.

After 175 days of incubation, the concentration of

ethylbenzene in the living microcosms is below the limit of

quantification (40 μg/l) but the average concentration in the

controls is only 113 μg/l compared to an initial concentration of 1109 μg/l. After 490 days of incubation, the concentration in both living and control microcosms was less than

the quantification limit.

Figure 2.1 Removal of MTBE in microcosms constructed with MTBE and BTEX compounds compared to removal in control microcosms that

were autoclaved to prevent biotransformation of MTBE.

10

100

1000

10000

-100 0 100 200 300 400 500 600

Time (Days)

MTBE (ug/L)

MTBE plus BTEX

MTBE plus BTEX

Control

MTBE (

μg/l)
8

Figure 2.2 Removal of MTBE in microcosms constructed with MTBE but without supplemental concentrations of BTEX compounds compared

to removal in control microcosms that were autoclaved to prevent biotransformation of MTBE.

10

100

1000

10000

-100 0 100 200 300 400 500 600

Time (Days)

MTBE (ug/L)

MTBE Alone

MTBE Alone Control

Figure 2.3 Removal of Toluene in microcosms constructed with MTBE and BTEX compounds compared to removal in control microcosms

that were autoclaved to prevent biotransformation of toluene. The solid line is fit through the removal in the controls.

10

100

1000

10000

-100 0 100 200 300 400 500 600

Time (Days)

Toluene (ug/L)

Toluene

Toluene Control

MTBE (

μg/l) Toluene (

μg/l)
9

Figure 2.4 Removal of Benzene in microcosms constructed with MTBE and BTEX compounds compared to removal in control microcosms

that were autoclaved to prevent biotransformation of benzene. The solid line is fit through the removal in the controls.

10

100

1000

10000

-100 0 100 200 300 400 500 600

Time (Days)

Benzene (ug/L)

Benzene

Benzene Control

Figure 2.5 Removal of Ethylbenzene in microcosms constructed with MTBE and BTEX compounds compared to removal in control microcosms that were autoclaved to prevent biotransformation of ethylbenzene.

10

100

1000

10000

-100 0 100 200 300 400 500 600

Time (Days)

Ethylbenzene (ug/L)

Ethylbenzene

Ethylbenzene Control

Benzene (

μg/l)

Ethylbenzene (

μg/l)
10

Figure 2.6 Comparison of the time lags for removal of MTBE, and of benzene, toluene, and ethylbenzene in microcosms constructed with all

the compounds present together.

Relationship between removal of BTEX

compounds and removal of MTBE

 Removal of MTBE did not require the presence of BTEX

compounds. Figure 2.6 plots the removal of MTBE, benzene, toluene, and ethylbenzene. Toluene was entirely

depleted within 40 to 47 days, benzene was entirely depleted within 110 days, and ethylbenzene was entirely

depleted within 175 days. During this time period there

was no evidence of removal of MTBE. After 385 days

there was evidence of extensive removal of MTBE in one

microcosm, limited removal in three microcosms, and no

evidence of removal in two microcosms. The removal of

10

100

1000

10000

-100 0 100 200 300 400 500 600

Time (Days)

Compound (ug/L)

MTBE 

Benzene

Toluene

Ethylbenzene

MTBE did not begin until the removal of the BTEX compounds was complete.

There is a possibility that removal of MTBE may have

been inhibited by the presence of BTEX compounds. However, the lag for removal of MTBE in microcosms without

BTEX compounds was also long, at least 175 days (Figure 2.6). Removal of MTBE in microcosms that did not

contain detectable concentrations of alkylbenzenes was

not detected until 385 days of incubation. There is not

enough resolution in the sampling schedule to determine if

the presence of BTEX compounds inhibited MTBE removal.

Compound (

μg/l)
11

SECTION 3

Site Characterization

Site Description and History

The case study was conducted at a former fuel farm

located at the U.S. Coast Guard Support Center at Elizabeth City, North Carolina. The following description is

excerpted from the Former Fuel Farm Work Plan, a part of

the Remediation Feasibility Assessment Work Plan prepared for the U.S. Coast Guard Support Center, Elizabeth

City (SCES), North Carolina, by Parsons Engineering Science, 1996.

The Support Center is located on the southern bank of

the Pasquotank River. The former fuel farm was located

south of a concrete ramp used to recover seaplanes from

the Pasquotank River (Figure 3.1). Currently a plume of

MTBE and fuel hydrocarbons in ground water emanates

from a source area in the location of the former fuel farm,

and flows under the concrete ramp toward the Pasquotank

River to the north, and toward a drainage canal along the

western side of the seaplane ramp. This source area

corresponds to the former location of fuel storage tanks on

the site (Figure 3.1).

Fuel was stored at the site until December 31, 1991. The

fuel farm had been in use since 1942, and originally consisted of a 50,000-gallon concrete underground storage

tank (TANK 23 in Figure 3.1), and two steel underground

storage tanks with a volume of 12,000-gallons and

15,000-gallons, respectively (adjacent to location CPT-1 in

Figure 3.1). The steel tanks were apparently removed in

the mid-1980s. In addition to the underground storage

tanks, two steel, above-ground storage tanks with a capacity of 50,000 gallons were installed in the mid-1980s. There

was evidence of corrosion in the transfer lines from these

tanks. They were taken out of service and removed from

the site. No evidence of a release from the pipes was

discovered.

The U.S. Coast Guard began a free product recovery

effort at the site in September 1990. Eight recovery wells

were arranged around the source area in a circle. By March

1992, a total of 79,000 gallons of fuel was recovered.

Core Sampling the Source Area

In September 1996, a GeoProbe was used to acquire

core samples in continuous vertical profiles at seven locations in or near the source area (locations CPT-1 through

CPT-7 in Figure 3.1). The water table was 7.0 to 8.0 feet

below land surface (2.1 to 2.4 meters below land surface).

The cores extended from the surface to a depth 12 to 16

feet (3.7 to 4.9 meters).

The cores were cut into subcores that were 4 inches long

(10 cm long). A plug of material approximately 1.0 cm in

diameter and 5 cm long was acquired from each subcore

with a paste sampler. The plugs were immediately transferred in the field into 40-ml glass vials with 5 ml of

methylene chloride and 10 ml of distilled water. The vials

were sealed with Teflon-faced septa and screw caps,

then they were shaken to extract organic components into

the methylene chloride. The contents were allowed to

settle, then the methylene chloride was taken for analysis

by gas chromotography using a mass spectrometer as a

detector. The limit of quantitation for MTBE and for BTEX

compounds was 0.01 mg/kg; the limit of quantitation for

Total Petroleum Hydrocarbons was 50 mg/kg.

The cores at each sampling location extended from clean

soil above the release, through the release to clean aquifer

material below the release. The quantity of Total Petroleum Hydrocarbon (TPH) or MTBE in individual cores was

summed to determine the total amount of TPH and MTBE

present at each location. The subcores were 10 cm long.

The concentration reported in mg/kg was considered representative of a block of soil that was 1.0 meter square and

0.1 meter deep. The dry bulk density of the soil or

sediment was assumed to be 1,820 kg/ m3

. Each block of

soil would have a weight of 182 kg/m2

. The concentration

reported in mg/kg was multiplied by 182 kg/m2

 to determine

the quantity in each block. The quantity in each block was

summed to determine the total quantity at each location.

Results are presented in Table 3.1.

The greatest quantity of TPH was found at locations

CPT-2 and CPT-1 (Table 3.1). These locations were near

the location of the original steel underground storage tanks

(Figure 3.1). These two locations also had the greatest

mass of MTBE, and the highest concentration of MTBE in

the residual fuel. The quantity of TPH at location CPT-3

was high, but the concentration of MTBE in the TPH was

lower than the concentration in locations CPT-1 and CPT-2.

MTBE was detected at location CPT-7, but the quantity

and concentration in the TPH was much lower than at

locations CPT-1 and CPT-2. The fuel release that contains

MTBE is centered around locations CPT-1 and CPT-2, and
12

Figure 3.1 Relationship between the sampling locations for characterization of the LNAPL source area (labeled CPT-1 through CPT-5), and

the former location of storage tanks for fuels.

PASQUOTANK RIVER

CPT-5

N

CPT-3

CPT-1

CPT-4

CPT-2

Approximate Scale in Feet

200 0 100 200

STEEL AST’S

TANK 23

FUEL TANK

FUEL TANK

STEEL UST’S

Former Seaplane Ramp

Drainage Canal

Earthen Dike

CPT-7

CPT-6

Approximate Scale in Meters

0 50 100 150
13

is roughly bound by CPT-7 to the west, by CPT-5 and CPT-3

to the north, and by CPT-6 and by CPT-4 to the east.

Estimation of Total Quantity of TPH and

MTBE and the Area Impacted

The former location of the steel underground storage

tanks was selected as the location of CPT-1. This location

had the highest quantity of MTBE (Table 3.1), and will be

taken as the center of the release. Location CPT-1 is

60 meters from CPT-7, 67 meters from CPT-5, 37 meters

from CPT-3, 88 meters from CPT-6, and 60 meters from

CPT-4 (Figure 3.2). If the source is a circle that fits within

the space bounded by CPT-7, CPT-5, and CPT-4, then its

radius is 60 meters, and its area is 11,000 m2. If the

concentration of TPH in the source is 46 kg/m2 (the average

of locations CPT-1, CPT-2, CPT-3, and CPT-4 in Table 3.1),

the total quantity of fuel hydrocarbons remaining in the

source is 500,000 kg. If the density of the fuel is 0.82, this

corresponds to 620,000 liters or 180,000 gallons of fuel.

Addendum 2 to the Corrective Action Plan, Former Fuel

Farm (SWMU No. 32), U.S. Coast Guard Support Center

Elizabeth City, Elizabeth City, North Carolina, Parsons

Engineering Science, 1997, provides an independent assessment of the mass of fuel remaining. They estimate that

approximately 100,000 gallons (380,000 liters) remain in the

soil, and that the impacted area is approximately 150,000

square feet (14,000 m2). The agreement between their

estimate and our estimate is acceptable.

The average quantity of MTBE at locations CPT-1 and

CPT-2 was 12.8 g/m2 (Table 3.1). If the area of the source is

11,000 m2

, this corresponds to a total quantity of 140 kg of

MTBE. This would be an upper boundary on the quantity of

MTBE in the source.

If the source “hot-spot” is restricted to the interval between CPT-1 and CPT-2 (see Figure 3.2), then the radius of

the “hot-spot” is 29 meters, and the area of the hot spot is

2,600 m2. The average concentration of MTBE at CPT-1

and CPT-2 is 12.8 g/m2, for a total in the “hot spot” of

33.2 kg of MTBE. If the remaining area of the source

(8,400 m2) has a quantity of MTBE equal to that of location

CPT-3 (1.5 g/m2), the addition quantity is 12.6 kg for a grand

total 46 kg in the source area. This would be a lower

boundary on the quantity of MTBE in the source.

Vertical Distribution of TPH and MTBE in

Core Samples

Figure 3.3 presents the vertical distribution of MTBE and

TPH in the continuous core samples from location CPT-1.

The majority of TPH was confined to a depth interval

between 1.5 and 3.0 meters. The relative proportions of

MTBE in the TPH were very consistent over this interval.

Below 3 meters TPH disappears, while the concentration of

MTBE declines gradually with increase in depth. The

MTBE in core samples below a depth of 3 meters can only

be dissolved in the ground water. In this interval there is no

TPH to partition into, and sorption to aquifer solids should

be negligible.

The vertical distribution of TPH and MTBE at location

CPT-2 (Figure 3.4) was very similar to the distribution at

location CPT-1 (Figure 3.3). The majority of the TPH was

confined to an interval between 1.5 and 3 meters below land

surface. The relative proportions of MTBE and TPH were

consistent across the vertical profile, and it was the same

proportion seen at location CPT-1. At the time the cores

were collected, the depth to the water table was 7.0 feet

(2.13 m). One core sample, located right at the water table,

had a TPH concentration of 139,000 mg/kg. This concentration is high enough to represent free product floating on

the water table. The concentration of MTBE in this sample

was 11.5 mg/kg. At location CPT-2, both TPH and MTBE

disappeared below a depth of 3 meters.

At location CPT-3, the TPH was also confined to an

interval between 1.5 and 3.0 meters (Figure 3.5). The

quantity of MTBE relative to TPH was less than the proportion at locations CPT-1 and CPT-2. As was the case at the

other locations, the relative proportion of MTBE to TPH did

not change across the vertical profile.

Table 3.1. Quantity of Total Petroleum Hydrocarbon and MTBE at seven sampling locations in or near the point of

release of fuel.

Location Total Petroleum Hydrocarbons MTBE Mass Fraction MTBE

(kg/m2) (kg/m2) ppm MTBE per TPH

CPT-7 9.7 0.00017 18

CPT-5 0.220 <0.0001

CPT-3 26 0.0015 57.7

CPT-1 54 0.0152 282

CPT-2 75 0.0104 139

CPT-6 1.3 <0.0001

CPT-4 30 <0.0001
14

PASQUOTANK RIVER

CPT-5

N

CPT-3

CPT-4

Approximate Scale in Feet

200 0 100 200

CPT-6

CPT-7

Approximate Scale in Meters

0 50 100 150

CPT-1

CPT-2

Figure 3.2 Inferred location of the fuel release, based on vertical core samples and the location of the steel underground storage tanks.
15

Figure 3.3 Vertical distribution of MTBE and Total Petroleum

Hydrocarbon (TPH) in core samples at location CPT-1

(see Figure 3.1 for map).

Figure 3.4 Vertical distribution of MTBE and Total Petroleum

Hydrocarbon (TPH) in core samples from location CPT2 (see Figure 3.1 for map).

Figure 3.5 Vertical distribution of MTBE and Total Petroleum

Hydrocarbon (TPH) in core samples from location CPT3 (see Figure 3.1 for map).

Distribution of Total Petroleum

Hydrocarbons and Hydraulic Conductivity

with Depth

Water samples were acquired using GeoProbe rods

with an outer diameter of 1.0 inch (2.54 cm). The leading

rod had 1.5 vertical feet (0.46 meter) of vertical mill slot

screens. In addition to collecting samples for analysis of

chemical parameters, the hydraulic conductivity was determined at each depth interval using an inverse specific

capacity test following the procedure of Wilson et al. (1997).

Figure 3.6 depicts the vertical relationship of Total

Petroleum Hydrocarbon (TPH) and Hydraulic Conductivity

at location CPT-1 (Figure 3.6). The TPH was confined to

an interval extending from 5 to 10 feet below land surface.

This may represent a “smear” zone around the water table,

which was located 7 feet below land surface when the core

and water samples were collected in August 1996. The

core material containing significant concentrations of TPH

was silty clay to clay with very low hydraulic conductivity.

The depth interval with significant hydraulic conductivity

extended from 10 to 27 feet (3 to 8 meters) below land

surface (Figure 3.6). The interval containing TPH was just

above the interval allowing significant flow of ground water.

Figure 3.7 plots the locations of ground-water samples at

the site. At each location, sampling was attempted at

depths that extended across the first aquifer, starting in the

0 20000 40000 60000

2

4

6

8

1 0

1 2

1 4

1 6

1 8

D epth

(f ee t)

T o ta l Pe tro leum H y dr o c ar bo ns ( m g /kg )

0 2 4 6 8 10

M T BE ( m g/ kg )

M TBE

TPH

D epth

(m eters)

2

3

4

1

5

0 20000 40000 60000

0

2

4

6

8

10

12

14

16

18

Depth 

(feet)

Total Petroleum Hydrocarbon (mg/kg)

0 2 4 6 8 10

MTBE (mg/kg)

Depth

(meters)

2

3

1

MTBE

TPH

MTBE

0 20000 40000 60000

0

2

4

6

8

1 0

1 2

1 4

1 6

1 8

Depth 

(f eet)

T o tal Pe tr o leum H y d ro c arb o n (m g/ k g )

0 2 4 6 8 10

M T BE ( m g/k g )

D e p th

( meter s) 

1

2

3

4

5

MTBE TPH 
16

low conductivity material near the surface and extending

across the aquifer to the confining zone below the aquifer.

The vertical distribution of hydraulic conductivity in Figure 3.6 is representative of the distribution of hydraulic

conductivity across the transects depicted in Figure 3.7. At

a minimum, the hydraulic conductivity was measured in

every other 1.5 foot interval at each location. At many

locations, the hydraulic conductivity was measured in every successive 1.5 foot interval. For each location in

Figure 3.7, Tables 3.2 and 3.3 compare the average hydraulic conductivity across the aquifer, the highest hydraulic conductivity measured, and the conductivity in the

confining layers above and below the aquifer.

In the area that was sampled, hydraulic conductivity in

this aquifer was remarkably uniform. The aquifer abruptly

pinched out to the southeast (compare GP-21 and GP-22

in Table 3.3), and pinched out to a more limited extent to

the northwest. There was no systematic change in hydraulic conductivity moving north toward the Pasquotank

River. In general, the highest hydraulic conductivity was

twice the average conductivity, as would be expected if the

shape of the distribution of hydraulic conductivity with

depth was triangular.

Figure 3.6 Relationship between the vertical extent of Hydraulic

Conductivity and the vertical extent of Total Petroleum

Hydrocarbon at location CPT-1.

Figures 3.8 and 3.9 depict the vertical distribution of the

actual sampling locations on transects that are depicted in

Figure 3.7, and summarized in Tables 3.2 and 3.3. These

figures are offered to provide the reader an indication of the

density of the data that are contoured in Figures 3.10

through 3.15.

The depth to the water table was approximately 7 feet in

the source area and 6 feet below land surface farther

toward the Pasquotank River. Figures 3.10 through 3.15

depict contours as depth below the water table.

Figures 3.10 and 3.11 contour the distribution of hydraulic conductivity. In both transects, the conductive interval

starts approximately 5 to 6 feet below the water table and

extends to 18 to 20 feet below the water table. In the

horizontal plane, there is little indication that flow of ground

water is confined to preferential flow channels with markedly higher hydraulic conductivity than surrounding aquifer

material.

Distribution of MTBE and BTEX Compounds

with Depth

The distribution of MTBE in the north-south transect is

depicted in Figure 3.12. The highest concentrations of

MTBE are in the shallow ground water underneath the

LNAPL at the south end of the transect. As the ground

water moves north (to the right in Figure 3.12), the highest

concentrations of MTBE are found in the depth intervals

with the highest hydraulic conductivity. There is a threefold

reduction in the concentration of MTBE at the most contaminated depth interval with each 200 feet north of the

source area. All the ground water from the location that

was closest to the Pasquotank River had less than 1 μg/l

MTBE.

The transect of samples collected in December 1997,

runs approximately northwest to southeast. The transect

is oriented 50 degrees west of north. As discussed in

Appendix A, the average direction of ground-water flow is

8 degrees west of north. The angle between the transect

and the direction of flow is 42 degrees. The distribution of

MTBE in the northwest to southeast transect reveals a

plume that is approximately 350 feet wide. The distance

containing MTBE that is perpendicular to ground-water

flow is calculated by multiplying the sine of 42 degrees by

the contaminated length along the transect. The width of

the plume perpendicular to ground-water flow (230 feet, or

90 meters) is slightly less than the diameter of the source

area (compare Figure 3.7) . The MTBE plume attenuates

abruptly on its northwest side, and attenuates more gradually on its southeastern side.

The distribution of BTEX compounds along the northsouth transect is depicted in Figure 3.14. As was the case

with MTBE, the highest concentrations are in the shallow

ground water underneath the LNAPL. As ground water

moves away from the source area, the highest concentrations are found in the most conductive depth intervals.

Unlike the pattern seen at other sites, there is no evidence

that BTEX compounds are attenuating while MTBE is

persistent. There is little practical difference in the pattern

of attenuation for MTBE and BTEX compounds.

0 100 00 200 00 300 00 400 00

0

5

10

15

20

25

30

T ota l Pe tro leu m H y dro carb on (m g / kg)

0 0.01 0.02 0.03 0.04

H yd ra ul ic C o n d u ct iv ity ( cm / sec)

D e pth

( fe et) D e pth

(m eters)

0

2

3

4

1

5

6

7

8

9

H y d rau lic Co n du ctiv ity

TPH

W a ter T ab le
17

Figure 3.7 Relationship between two transects of ground-water samples and the fuel release. The arrow represents the average direction of

ground-water flow.

PASQUOTANK RIVER

N

Approximate Scale in Feet

200 0 100 200

December 1997

August 1996

Source Area

Approximate Scale in Meters

0 50 100 150

Average Direction of

Ground Water Flow
18

Table 3.2 Distribution of Hydraulic Conductivity (K) in the north-south transect sampled in August 1996

(Figure 3.7).

South to Number of Average K Highest K Lowest K in Lowest K in

North measurements 10 to 26.5 Higher Interval Lower Interval

feet

cm/sec cm/sec cm/sec cm/sec

CPT-2 11 0.015 0.029 0.0004 0.00035

CPT-1 9 0.020 0.033 0.0036 0.016

CPT-3 10 0.026 0.041 0.0014 0.0001

CPT-5 10 0.022 0.042 0.00001 0.012

ESM-14 9 0.025 0.046 0.0003 0.0005

ESM-10 10 0.031 0.060 0.0077 0.0033

GP-1 7 0.024 0.041 0.0015 0.0015

Table 3.3 Distribution of Hydraulic Conductivity (K) in the east-west transect sampled in December 1997

(Figure 3.7).

West to Number of Average K Highest K Lowest K in Lowest K in

East measurements 10 to 26.5 Higher Interval Lower Interval

feet

cm/sec cm/sec cm/sec cm/sec

GP-25 4 0.013 0.038 0.0064

GP-24 6 0.019 0.047 0.0007 0.013

GP-23 8 0.025 0.057 0.00015 0.00021

GP-19 7 0.032 0.084 0.00004 0.012

GP-18 8 0.027 0.063 0.00001 0.00004

GP-17 8 0.024 0.052 0.0016 0.00004

GP-16 7 0.017 0.031 0.0003

GP-15 8 0.024 0.050 0.00024 0.00023

GP-20 6 0.024 0.051 0.0034 0.010

GP-21 7 0.022 0.052 0.00013 0.00004

GP-22 6 0.0001 0.020 0.00003 0.00002

It is important not to interpret the apparent attenuation

along this transect as sufficient evidence for natural attenuation along the flow path. The transect may be askew of

the flow path, and may sample water at the distal locations

that was never in the plume. The north-south transect is

oriented 16 degrees east of north (Figure 3.2). This is only

24 degrees from the average direction of ground-water

flow, but this difference is large enough to move out of a

plume that is 230 feet wide at a distance of 600 feet from

the edge of the source area.

To determine whether a particular location along the

transects sampled the plume or missed the plume, the

concentration of MTBE at each location was compared to a

number of geochemical indicators that correlate with biological activity in ground water. The concentrations of

methane, iron (II), oxygen, sulfate, and alkalinity at the

transect locations were compared to their concentrations

in ground water that was not impacted by the plume (see

Figures B.1 through B.6 in Appendix B). The geochemistry

of the site is discussed in detail in Appendix B. All of the

sampling locations on the north-south transect that were

downgradient of the source area were depleted of oxygen

and sulfate, and had elevated concentrations of methane,

iron (II), and alkalinity, indicating that these locations in the

north-south transect sampled the plume.

Figure 3.15 reveals that the plume, as sampled by the

east-west transect, is heterogeneous with respect to the

distribution of MTBE and BTEX compounds. The higher

concentrations of MTBE extend from 50 to 350 feet along

the transect (Figure 3.13). The higher concentrations of

BTEX extend from 200 feet along the transect to 400 feet

along the transect (Figure 3.15). The northwestern reach

of the transect has MTBE but no BTEX, the central reach

has both MTBE and BTEX, and the southeastern reach

has BTEX but no MTBE.
19

Figure 3.8 Location of vertical sampling points along the north-south transect, collected in August 1996. Distance along the transect extends

from south to north (bottom to top in Figure 3.7), in the direction of ground-water flow.

Figure 3.9 Location of vertical sampling points along the east-west transect, collected in December 1997. Distance along the transect

extends from west to east (left to right in Figure 3.7), opposite the direction of ground-water flow.

Location of Sampling Points on North-South Transect

Depth Below Water Table (ft)

Location on Transect (ft)
20

Figure 3.10 Distribution of hydraulic conductivity along the north-south transect, collected in August 1996. Distance along the transect extends

from south to north (bottom to top in Figure 3.7), in the direction of ground-water flow.

Figure 3.11 Distribution of hydraulic conductivity along the east-west transect, collected in December 1997. Distance along the transect

extends from west to east (left to right in Figure 3.7), opposite the direction of ground-water flow.

Location on Transect (ft)

H ydraulic C onductivity (cm /sec) at East-West Transect

D epth B elow W ater Table (ft)
21

Figure 3.12 Distribution of MTBE along the north-south transect, collected in August 1996. Distance along the transect extends from south to

north (bottom to top in Figure 3.7), in the direction of ground-water flow.

Figure 3.13 Distribution of MTBE along the east-west transect, collected in December 1997. Distance along the transect extends from west to

east (left to right in Figure 3.7), opposite the direction of ground-water flow.

0.0

3.0

6.0

9.0

12.0

15.0

18.0

21.0

24.0

27.0

30.0

0 100 200 300 400 500 600 700 800 900

Location on Transect (ft)

M T BE ( g/l) at N orth-South Transect μ

D epth B elow Water Table (ft)

3000

300

1000

3 0
22

Figure 3.14 Distribution of BTEX along the north-south transect, collected in August 1996. Distance along the transect extends from south to

north (bottom to top in Figure 3.7), in the direction of ground-water flow.

Figure 3.15 Distribution of BTEX along the east-west transect, collected in December 1997. Distance along the transect extends from west to

east (left to right in Figure 3.7), opposite the direction of ground-water flow.

L ocation on Transect (ft)

D epth B elow W ater Table (ft)
23

SECTION 4

Transport and Fate of MTBE in the Ground Water

Estimated Rate of Attenuation in Ground

Water

Ground-water flow carrying the plume of contamination

is contained within a semi-confined aquifer. Most of the

plume occurs under the concrete of an operational apron

at the U.S. Coast Guard Support Center. The bottom of

the drainage ditch on the northwest side of the site does

not penetrate the upper layer of silty clay, and does not

communicate with the sandy layer that carries the plume

of contamination. As a result, there is little opportunity for

the ground-water flow field to be influenced by local recharge or local discharge. As a simplification, we assume

that the shape of the water table is a plane during any

particular round of sampling.

The direction of ground-water flow was determined by

using a least squares regression technique to fit a plane

through the elevation of the water table in eight permanent

monitoring wells at the site. A separate regression was

performed for each of eighteen rounds of quarterly monitoring starting in September 1994 and extending through

December 1998, and each of fourteen rounds of monthly

sampling starting in February 1998 and extending to

March 1999. Appendix A contains a map showing the

location of the permanent monitoring wells used to estimate flow direction, tables showing the water table elevation in the monitoring wells at each round of sampling, and

tables showing results of the regression analyses, with

estimates of plume direction, hydraulic gradient, and goodness of fit. Appendix A also contains maps that compare

the contours of the water table as estimated by the

regressions to the measured elevations of the water for

the eighteen rounds of quarterly monitoring.

The regional ground-water flow direction is north, directly toward the Pasquotank River; however, the hydraulic gradient at any one time in the study area is strongly

influenced by the stage of the Pasquotank River. The

direction and magnitude in the flow of ground water as

predicted in each round of quarterly sampling is depicted

in Figure 4.1. For each round of sampling, an arrow in the

figure represents the distance ground water would move

in one year under the conditions of gradient and direction

that were encountered in that round of sampling. The

variation in direction and magnitude in the fourteen rounds

of monthly sampling is depicted in Figure 4.2.

The average direction of ground-water flow was calculated by weighting the direction of flow on any particular

round of sampling by the hydraulic gradient at that particular round, then taking an average of the weighted flow

directions.

For the eighteen rounds of quarterly sampling, the average flow direction was 8.6 degrees west of north, with a

standard deviation of 9.1 degrees. For the fourteen rounds

of monthly sampling, the average flow direction was 8.7

degrees west of north, with a standard deviation of 23

degrees.

The seepage velocity of the plume was estimated from

Darcy’s Law. To calculate seepage velocity, the hydraulic

conductivity was multiplied by the hydraulic gradient, and

divided by the porosity. Table 4.1 summarizes the statistical properties of the parameters used to calculate seepage velocity. There is relatively little variation in hydraulic

conductivity; the range of eight samples is only 56% of the

mean. There is greater variation in hydraulic gradient

from one round of sampling to another. For eighteen

rounds of quarterly sampling, the hydraulic gradient varied

over an order of magnitude. Nothing is known directly

about the range of porosity at the site. The total pore

space in core samples was calculated by comparing the

wet and dry weight of core samples from five locations.

The total porosity was very close to 0.34 to 0.36. A value

of 0.35 was taken as an upper boundary on effective

porosity. A survey of the literature and professional

judgment was used to assign an average effective porosity of 0.3, and lower boundary on effective porosity of 0.25.

The average seepage velocity was calculated by multiplying the average hydraulic conductivity by the average

hydraulic gradient, and dividing by an assumed effective

porosity of 0.3. The upper boundary on seepage velocity

was calculated by multiplying the upper 95% confidence

intervals for hydraulic conductivity and hydraulic gradient,

then dividing by an assumed effective porosity of 0.25.

The lower boundary was calculated by multiplying the

lower 95% confidence intervals for hydraulic conductivity

and hydraulic gradient, then dividing by an assumed

porosity of 0.35. The average calculated seepage velocity

at the site was 82 meters per year. The upper boundary

was 150 meters per year, and the lower boundary was

67 meters per year. These estimates of seepage velocity
24

PASQUOTANK RIVER

GP-1

ESM-9

ESM-10

ESM-14

CPT-5 CPT-3

CPT-1

Approximate Scale in Feet

200 0 100 200

ESM-3

ESM-11 N

50 100

Approximate Scale in Meters

0 150

CPT-2

Figure 4.1. Variation in ground-water flow calculated from eighteen

rounds of quarterly monitoring. The length of the arrow

is the distance that would be traveled by MTBE in one

year at that hydraulic gradient.

PASQUOTANK RIVER

GP-1

ESM-10 ESM-9

ESM-14

CPT-5 CPT-3

CPT-1

Approximate Scale in Feet

200 0 100 200

ESM-3

ESM-11 N

50 100

Approximate Scale in Meters

0 150

CPT-2

Figue 4.2. Variation in ground-water flow calculated from fourteen

rounds of monthly monitoring. The length of the arrow

is the distance that would be traveled by MTBE in one

year at that hydraulic gradient.

Parameter Hydraulic

Conductivity

Hydraulic

Gradient

Porosity Seepage Velocity

Unit cm per second meter per meter Fraction pore

space

meter per year

Basis of

Boundary

95% Confidence

Interval

95% Confidence

Interval

Range of

Literature

Calculated

Number of

Samples

8 18 Calculated

Mean 0.027 0.0029 0.30 82

Maximum 0.036 0.0067

Minimum 0.020 0.00059

Upper Boundary 0.031 0.0037 0.25 150

Lower Boundary 0.023 0.0028 0.35 67

Table 4.1 Sensitivity analysis of the estimates of the seepage velocity of ground water at the site. These estimates were used to calculate a

first-order rate of attenuation of MTBE in ground water downgradient of the source area.
25

were used to calculate time of travel of ground water from

the most contaminated location (CPT-1) to the downgradient

locations.

For various sampling locations presented in Figure 4.1,

Table 4.2 compares the distance downgradient to the

concentration of MTBE, and the concentrations of methane and iron (II). Methane is expected to be a conservative tracer in ground water once it forms, and iron (II)

appears to be conservative at this site as well. Methane

and iron (II) will be used as tracers for the plume of

contaminated ground water. Monitoring locations CPT-1,

CPT-3, CPT-5, ESM-3, ESM-10, ESM-14, and GP-1 have

high concentrations of methane and iron (II) indicating that

these locations sample the plume. These locations were

included in the calculation of the natural biodegradation

rate constant.

Although location ESM-11 is directly downgradient of the

source area (Figure 4.1), it had low concentrations of

methane (Table 4.2). The low concentration of MTBE at

location ESM-11 may have resulted from simple dilution;

as a result, location ESM-11 was not included in the

calculation of the biodegradation rate constant. In contrast, location ESM-9 is not directly downgradient from the

source area. However, the geochemical parameters indicate that ground water sampled in this location was in the

plume of contamination. As depicted in Figure 4.1, the

plume of methane and iron (II) moves more to the east

than would be expected from the hydraulic gradient alone.

This may reflect anisotrophic flow in the aquifer, rather

than error in estimating the direction of the hydraulic gradient. Location ESM-9 was included in the calculation of the

biodegradation rate constant.

Figure 4.3 plots the logarithm of concentration of iron (II),

methane, and MTBE against the calculated travel time of

ground water downgradient of the source. In three years'

travel time, there is no attenuation of iron (II), the concentration of methane is attenuated by an order of magnitude,

and the concentration of MTBE is attenuated by three

orders of magnitude. The attenuation of methane is the

best estimate of the effects of attenuation due to dilution

and dispersion. The attenuation of MTBE in excess of the

attenuation of methane must be due to natural biodegradation.

The first-order rate of attenuation was calculated by a

linear regression of the natural logarithm of concentration

on time of travel along the flow path. Table 4.3 compares

the apparent rate of attenuation, and the 95% confidence

interval on that rate, for MTBE, methane, and iron (II). The

average total rate of attenuation of MTBE was near 2.7 per

year. The attenuation of methane, which may be taken as

Location Distance from

Source (CPT-1)

Date

Sampled

MTBE Methane Iron (II)

meters Month/Year mg/l mg/l mg/l

Source Area

CPT-1 0 8/1996 1,740 3.5 34

CPT-3 40 8/1996 823 13.5 96

Downgradient of

Source

CPT-5 70 8/1996 672 5.9 56

ESM-14 104 8/1996 383 7.8 84

ESM-3 134 6/1999 319 1.3 86

ESM-9 180 8/1996 <1 3.24 42

ESM-10 195 8/1996 9.7 4.6 20

ESM-11 238 6/1999 13.5 0.12 33

GP-1 250 8/1996 <1 1.0 59

Table 4.2. Concentration of MTBE, methane, and iron (II) at monitoring locations used to calculate the rate of attenuation of MTBE, methane,

and iron (II) with time of travel downgradient of the location with the highest concentration.
26

0.1

1

10

100

1000

10000

100000

-0.5 0.5 1.5 2.5 3.5

Travel Time (years)

Iron

Methane

MTBE

Figure 4.3 Attenuation in concentrations of MTBE, methane, and iron (II) with travel time downgradient from the location with the highest

concentration of MTBE.

Table 4.3 The apparent first-order rate of attenuation of MTBE, methane, and iron (II) with time of travel downgradient from the location with

the highest concentration of MTBE.

Analyte Estimate of 

Plume Velocity

Apparent Rate of

Attenuation

Upper 95%

Confidence

Interval

Lower 95%

Confidence

Interval

 per year

MTBE Upper Boundary 5.0 7.21 2.69

MTBE Average 2.7 3.89 1.45

MTBE Lower Boundary 2.2 3.26 1.22

Methane Average 0.5 1.15 -0.160

Iron (II) Average 0.12 0.619 -0.384

Concentration (

μg/l)
27

a surrogate for attenuation of MTBE due to dilution and

dispersion, was 0.5 per year. The lower 95% confidence

interval for attenuation of MTBE does not overlap the

upper 95% confidence interval for methane attenuation.

The rates are different at 95% confidence. There was no

appreciable attenuation in concentrations of iron (II).

The rate of attenuation in the field compares well with the

rate of attenuation in the laboratory microcosm study. The

average rate of attenuation in the field was 2.7 per year.

The average rate of attenuation in the laboratory was

3.02 ± 0.52 per year at 95% confidence.

Transfer from the Entire Source Area to the

Plume

The mass transfer of MTBE from the source LNAPL to

the ground water moving underneath was estimated by

calculating the flux of MTBE moving away from the source

across the east-west transect, then comparing that flux to

the total mass of MTBE in the source area.

Figure 4.5 depicts the relationship between the locations

on the east-west transect, the direction of ground-water

flow, and the source area.

The flow-weighted average concentration of MTBE in

ground water at each location along the east-west transect

was calculated by multiplying the concentration of each

vertical sample by the hydraulic conductivity at that point,

dividing each product by the average hydraulic conductivity to produce a weighted concentration, then taking the

simple arithmetic average of the weighted concentrations.

The results are presented in Figure 4.6. The highest

concentrations of MTBE were directly downgradient of the

“hot-spot” at CPT-1.

Each location was considered to represent a length

along the transect equal to the distance to the mid points

between the neighboring locations. The linear distance

between the transect locations was 50 feet. For locations

18, 17, 16, 15, 20, 21, and 22, the length that was perpendicular to ground-water flow was 34 feet. For locations 25,

50 100

PASQUOTANK RIVER

GP-1

ESM-10 ESM-9

ESM-14

Approximate Scale in Feet

200 200 100 0

ESM-3

ESM-11

CPT-3

CPT-1

CPT-2

Approximate Scale in Meters

0 150

Source

CPT-5

Figure 4.4 Locations of ground-water samples included in the

calculation of the rate of natural attenuation. The arrow

represents the average direction of ground-water flow.

The dark shaded area is the area with LNAPL. The

larger lightly shaded area is the area downgradient

where the ground water contains high concentrations of

methane and iron (II). Only wells in the shaded area

were included in the calculation of the rate of natural

attenuation.

Figure 4.5 Relationship between the direction of ground-water flow

and the ground-water sampling locations in the transect

sampled in December 1997. Ground-water flow vectors

were calculated from the gradients in water table

elevation in eighteen different rounds of monitoring The

length of arrow is the distance that would be traveled by

MTBE in one year of flow at that gradient.

PASQUOTANK RIVER

N

Approximate Scale in Feet

200 0 100 200

Source Area

Approximate Scale in Meters

0 50 100 150

Direction of Ground

Water Flow

25 24 17 1615 20 21 22 23 19 18
28

24, and 23 the distance perpendicular to ground-water flow

was 17.1 feet. For location 19, the length was 26 feet.

For each location, the vertical interval that was averaged

to get the average concentration (25.5 feet) was multiplied

by the length perpendicular to flow to get the cross section,

then by the Darcy velocity to get the flux of water. The flux

at all the locations was summed. Approximately 5,300

cubic meters of water crosses the transect per year. The

flux of water at each location was multiplied by the average

concentration at each location, then summed. The flux of

MTBE from the source area across the transect was

2.8 kg/year.

The lower boundary on the total quantity of MTBE in the

source area was 46 kg (Section 3). If this flux did not

change over time, it would take seventeen years to remove

the MTBE from the source.

When the MTBE diffuses out of the LNAPL to be swept

away by flow in the aquifer, the less soluble petroleum

hydrocarbons are left behind. As a result, the concentration of MTBE in the fuel decreases over time. It is not likely

that the mass transfer rate of MTBE from the LNAPL

remains the same, regardless of the concentration of

MTBE remaining in the LNAPL. Diffusion is the mechanism that drives the mass transfer process. The diffusive

flux is proportional to concentration gradients, and the

gradients of MTBE are proportional to the remaining concentration of MTBE in the LNAPL. The mass transfer of

MTBE would be directly proportional to the concentration

of MTBE remaining in the LNAPL. If the rate of transfer of

MTBE to ground water is proportional to the amount of

MTBE in the source, the instantaneous rate of transfer is

0.06 per year. The average concentration at the most

contaminated location in the transect is 1200 μg/year. At

this rate of attenuation of the source, it would require

approximately sixty years for the concentration to reach

30 μg/liter.

Transformation of MTBE to TBA

Tertiary Butyl Alcohol (TBA) has been documented as a

transformation product of MTBE in a number of studies

(Mormile et al., 1994; Squillace et al., 1996; Church et al.,

1997). In the water samples collected in the transect

across the plume in December 1997, the concentration of

TBA was measured using solid phase micro-extraction.

The limit of quantitation was 1 μg/l. Figure 4.7 compares

the concentration of TBA to the concentration of MTBE in

all the water samples from the transect. With two exceptions, the concentration of TBA was less than 200 μg/l.

There is no evidence of accumulation of TBA in the transect

as a whole.

In general, it is difficult to determine whether TBA in

ground water was a component of the original spill, or if it

was produced from biological transformation of MTBE

(Church et al., 1997; Landmeyer et al., 1998). Two groundwater samples had higher concentrations of TBA. In one

0

200

400

600

800

1000

1200

1400

0 100 200 300 400 500

Distance Along Transect (feet)

25

24 23

19

18

17

16

15

20 21 22

Figure 4.6 Concentrations of MTBE in a transect that extends across the plume in a direction that is roughly perpendicular to ground-water

flow. See Figure 4.5 for the positions of the sampling locations identified as 15 through 25 in both Figures. Depicted at each

location are the flow-weighted average concentrations of MTBE in ground-water samples from a vertical profile extending across

the aquifer at each location. Average Concentration of MTBE (

μg/L)
29

of these samples there was a corresponding reduction in

the concentration of MTBE. At these locations the TBA

was probably produced from transformation of MTBE.

Figure 4.8 depicts the distribution of MTBE with depth at

three locations immediately downgradient of the source

area. Location 19 is between locations 18 and 23. Locations 18 and 23 have a peak in MTBE concentration near

2000 μg/l at a depth of 6 meters. At location 19, at a depth

of 6 meters there is a decline in the concentration of MTBE,

with higher concentrations at depths of 5 and 7 meters.

Figure 4.9 depicts the distribution of TBA with depth at the

same locations. There is little accumulation of TBA at

locations 18 and 23. However, there is a large accumulation of TBA at a depth of 6 meters at location 19, at the

same location where MTBE was depleted.

Figure 4.7 Relationship between the concentration of TBA in ground water and the concentration of MTBE in water samples collected in a

transect across the plume in December 1997.

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 500 1000 1500 2000 2500

MTBE (μg/l)

TBA (

μg/l)
30

Figure 4.8 Depth distribution of MTBE in three locations

downgradient of the LNAPL source area. See Figure 4.5

for position of the locations on a map. Compare location

19 to location 19 in Figure 4.9.

0 500 1000 1500 2000 2500

0

1

2

3

4

5

6

7

8

9

Depth 

(meters) 18

19

23

Figure 4.9 Depth distribution of TBA in locations downgradient of

the source area. See Figure 4.5 for position of the

locations on a map. Compare location 19 to location 19

in Figure 4.8.

0 500 1000 1500 2000 2500

0

1

2

3

4

5

6

7

8

9

Depth 

(meters)

18

19

23

MTBE (μg/l) TBA (μg/l)
31

SECTION 5

Summary and Conclusions

Extent of Biodegradation of MTBE

Removal of MTBE in microcosms that were supplemented with alkylbenzenes was extensive. There was no

evidence of MTBE removal over removal in the controls in

the first 175 days of incubation. After 385 days of incubation, there is strong evidence of removal in the living

treatment. After 490 days of incubation, there was very

extensive removal of MTBE compared to the controls.

The average concentration remaining in six replicates of

the living treatment was 81 μg/l, compared to 5680 μg/l at

the beginning of incubation. The average concentration

remaining in the control treatment after 490 days was

1470 μg/l, compared to 3330 μg/l at the beginning of

incubation. The removal in the controls was a little more

than twofold, while removal in the living microcosms was

70-fold.

Removal of MTBE in microcosms that were not supplemented with alkybenzenes was also extensive. There

was little evidence of removal in the first 175 days of

incubation. After 385 days, the removal of MTBE in the

living microcosms was extensive. After 490 days of

incubation, the concentration of MTBE in all six of the

replicate microcosms that were sampled was below 40 μg/l,

compared to 3110 μg/l at the beginning of incubation.

Role of BTEX Compounds

Removal of MTBE in the microcosms did not require the

presence of BTEX compounds. Toluene was entirely

depleted within 40 to 47 days, benzene was entirely

depleted within 110 days, and ethylbenzene was entirely

depleted within 175 days. During this time period there

was no evidence of removal of MTBE. The removal of

MTBE did not begin until the removal of the BTEX compounds was complete.

Rate of Removal of MTBE

The first order rate of removal of MTBE in microcosms

supplemented with alkylbenzenes was 3.02 per year ±0.52

per year at 95% confidence. Removal in the corresponding controls was 0.39 per year ±0.19 per year at 95%

confidence. The removal in the microcosms without

added alkylbenzenes was 3.5 per year ±0.65 per year at

95% confidence. Removal in the corresponding controls

was 0.30 per year ±0.14 per year at 95% confidence.

The apparent first order rate of removal of MTBE in the

field was a sensitive function of ground-water seepage

velocity. The rate of removal was calculated for an upper

boundary on velocity, an average velocity, and a lower

boundary on velocity. The rate was 5.0 per year at the

upper boundary, 2.7 per year at the average velocity, and

2.2 per year at the lower boundary. Methane was considered to be a conservative tracer of ground-water flow at

the site. The apparent rate of removal of methane was

taken as an estimate of attenuation along the flow path

due to dilution and dispersion. The apparent first order

rate of removal of methane at the average estimate of

seepage velocity was 0.50 ±0.65 per year.

 The rate of removal of MTBE in the laboratory studies

can explain the apparent attenuation of MTBE at field

scale.

Expected Persistence of the Source of

Ground-water Contamination

The mass transfer of MTBE from the source LNAPL to

the ground water moving underneath was estimated by

calculating the flux of MTBE moving away from the source,

then comparing that flux to the total mass of MTBE in the

source area. The flux of MTBE away from the source area

in 1996 was 2.76 kg/year. The lower boundary on the total

quantity of MTBE in the source area was 46 kg. If this flux

did not change over time, it would take seventeen years to

remove the MTBE from the source. If the rate of transfer

of MTBE to ground water is proportional to the amount of

MTBE in the source, the instantaneous rate of transfer is

0.06 per year. The average concentration at the most

contaminated location in the transect is 1200 μg/l. At this

rate of attenuation of the source, it would require approximately sixty years for the concentration to reach 30 μg/l.

Production and Depletion of TBA

Tertiary Butyl Alcohol (TBA) has been documented as a

transformation product of MTBE in a number of studies.

At the Old Fuel Farm Site, there is no evidence of accumulation of TBA in the ground-water plume as a whole.

With two exceptions, the concentration of TBA in ground

water downgradient of the source area was less than 200

μg/l. Ground water from a location immediately

downgradient of the source area had a higher concentration of TBA, near 2000 μg/l. In this sample there was a

corresponding reduction in the concentration of MTBE. At

this location the TBA was probably produced from transformation of MTBE.
32

Geochemical Context of the Plume that

Biodegraded MTBE

The entire MTBE plume is contained within a plume of

methane. Methane concentrations generally exceed

3.0 mg/l, and often exceed 10 mg/l . Concentrations of

methane average 7 mg/l, which corresponds to 9 mg/l of

hydrocarbon originally metabolized.

Ground water in the region of the aquifer that contains

MTBE and BTEX compounds is also depleted of sulfate.

Sulfate concentrations are reduced from a background of

near 28 mg/l to less than 4 mg/l; many regions have less

than 1 mg/l. A depletion of 24 mg/l of sulfate would oxidize

5 mg/l of fuel hydrocarbons. The same regions that are

depleted in molecular oxygen and sulfate have significant

accumulations of iron (II) . Background concentrations of

iron (II) are less than 0.1 mg/l. Many regions of the aquifer

with MTBE and BTEX compounds have iron (II) concentrations greater than 50 mg/l. This accumulation of iron (II)

would be capable of oxidizing 3 mg/l of hydrocarbons.

The plume is also undergoing extensive anaerobic oxidation of petroleum hydrocarbons, as well as fermentation

of hydrocarbons to methane. The hydrocarbon metabolism through sulfate and iron oxidation is approximately

equivalent to the hydrocarbon metabolism through

methanogenesis. The amount of hydrocarbon metabolized through anaerobic pathways is about ten times the

amount degraded with molecular oxygen.
33

Chapelle, F.H., 1999, Bioremediation of petroleum

hyrocarbon-contaminated ground water: the

perspectives of history and hydrology, Ground Water

37(1):122-132.

Church, C.D., Isabelle, L.M., Pankow, J.M., Rose, D.L.,

and Tratnyek, P.G., 1997, Method for determination of

methyl tert-butyl ether and its degradation products in

water, Environ. Sci. Technol. 31(12):3723-3726.

Landmeyer, J.E., Chapelle, F.H., Bradley, P.M.., Pankow,

J.F., Church, C.D., and Tratnyek, P.G., 1998, Fate of

MTBE relative to benzene in a gasoline-contaminated

aquifer (1993-98), Ground Water Monitoring and

Remediation, Fall, 1998, pp. 93-102.

Mormile, M.R., Liu, S, and Suflita, J.M., 1994, Anaerobic

biodegradation of gasoline oxygenates:extrapolation

of information to multiple sites and redox conditions,

Environ. Sci. Technol. 28(9):1727-1732.

Squillace, P.J., Zogorski,J.S., Wilber,W.G., and Price, C.V.,

1996, Preliminary assessment of the occurrence and

possible sources of MTBE in ground water in the

United States, 1993-1994, Environ. Sci. Technol.

30:1721-1730.

Suflita, J.M., and Mormile, M.R., 1993, Anaerobic

biodegradation of known and potential gasoline

oxygenates in the terrestrial subsurface, Environ. Sci.

Technol. 27(5):976-978.

U.S. Environmental Protection Agency. 1996. Drinking

Water Regulations and Health Advisories. Washington,

D.C.

Wiedemeier, T.H., Rifai, H.S., Newell, C.J., and Wilson,

J.T., 1999, Natural Attenuation of Fuels and Chlorinated

Solvents in the Subsurface, John Wiley & Sons, New

York, ISBN 0-471-19749-1.

Wilson, J.T., Cho,J.S., Beck, F.P., and Vardy,J.A., 1997,

Field Estimation of Hydraulic Conductivity for

Assessments of Natural Attenuation. In Proceedings

of the Fourth International In-Situ and On-site

Bioremediation Symposium, New Orleans, LA, pp.

309-314.

Yeh, C.K., and Novak, J.T., 1994, Anaerobic biodegradation

of gasoline oxygenates in soils, Water Environ. Res.,

66(5):744-752.

References
34

Ground-water flow at the site is strongly influenced by

the elevation of the Pasquotank River. At Elizabeth City,

North Carolina, the Pasquotank River makes a transition

from a conventional river to a large estuary. At Elizabeth

City, the Pasquotank River is a few hundred feet wide. At

the U.S. Coast Guard Air Station, only a few miles farther

down river, the Pasquotank is more than two miles wide.

The average elevation of the Pasquotank River at the fuel

farm site is near sea level. The elevation of the river is not

controlled by recent precipitation and runoff, as is usually

expected for a river. The strongest influence on the

elevation of the river is the recent direction of the wind,

producing a phenomenon known as a wind seiche. The

trend of the valley is from northwest to southeast. Friction

from strong winds coming from the north will drive water

out of the river valley, and lower the elevation of the river.

Winds from the south drive water into the valley, and raise

the water table.

This effect is illustrated in the data in Figure A.1. A

pressure transducer was used to record the elevation of

the Pasquotank River every fifteen minutes over a time

interval extending from September 5, 1996 to October 30,

1996. To minimize the confounding effects of wave action

on the measurement of water table elevation, the transducer was located in a drainage ditch in close communication with the river. Over this two-month interval, the elevation of the Pasquotank River varied from 1.5 feet above

sea level to 1.5 feet below sea level. The most rapid

changes in elevation were drops in elevation on the order

of 1.5 feet within one or two days that were associated with

cold fronts that came through in the first week of September, and the first week of October. There was also a

diurnal cycle that varied from 0.1 to 0.5 feet.

These changes in elevation of the receptor of the

ground-water plume are large, compared to the change in

elevation of ground water across the fuel farm site. As an

illustration, examine Figure A.2 and identify Well ESM-14

located in the center of the study area. The average

elevation of ground water in Well ESM-14 over 27 rounds

of quarterly or monthly sampling was 1.25 feet above sea

level. As a consequence of the temporal variation in the

elevation of the Pasquotank River, the hydraulic gradient

and the ground-water flow velocity across the site vary

widely from one sampling event to another.

Figure A.3 plots the elevation of the water table against

time for four representative monitoring wells extending

across the site (see Figure A.2 for the location of the wells).

Over the period from March 1994 to December 1998, the

elevation of the water table in individual wells changed

from 1.5 to 2.0 feet. The elevation of the water table in all

the wells tended to track each other over time. In some

time intervals the elevations of ground water in all the wells

are nearly the same. At other intervals, the hydraulic

gradient is more strongly expressed. There is no obvious

correlation of the hydraulic gradient to seasons of the year,

or to the average elevation of ground water across the site.

Appendix A. Temporal Variation in the Hydraulic Gradient and the Direction

of Ground-water Flow

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

-5 5 15 25 35 45 55 65

Time (days)

Elevation (feet)

Figure A.1 Variation in elevation of water in the Pasquotank River over a time interval extending from September 5, 1996 to October 30, 1996.
35

0

0.5

1

1.5

2

2.5

3

3.5

Dec-93 Dec-94 Dec-95 Dec-96 Dec-97 Dec-98 Dec-99

Date of Sampling

Elevation of Water Table (feet)

ESM-7

ESM-6

ESM-14

ESM-10

Figure A.3 Variation in elevation of the water table at the fuel farm site over time. Consult Figure A.1 for the location of the monitoring wells.

Well ESM-10 is closest to the Pasquotank River, the point of ground-water discharge. Wells ESM-14, ESM-6, and ESM-7 are

farther inland.

ESM-14

ESM-11

ESM 3

ESM 4

ESM 10

ESM 9

CONCRETE

GRASS

GRASS

GRASS

ASP HALT

GRASS

FUEL

PASQUOTANK

ESM-7

ESM-6

RIVER

D R AIN AG E D IT C H

GRASS

N

862700 862750 862800 862850 862900 862950 863000 863050

283000

283050

283100

283150

283200

283250

283300

283350

283400

SCALE IN METERS

0 50 100

Figure A.2 Location of the permanent monitoring wells used to estimate the hydraulic gradient and direction during each round of monitoring.
36

Water table elevations were available from eighteen

rounds of quarterly monitoring and for fourteen rounds of

monthly monitoring. The elevations of ground water in

wells depicted in Figure A.2 are presented in Table A.1 for

eighteen rounds of sequential quarterly sampling, and

Table A.2 for fourteen rounds of sequential monthly sampling. There are four sampling dates common to the

quarterly monitoring data and the monthly monitoring data.

Rather than take one or a few rounds of sampling and use

professional judgment to construct ground-water elevation

contours that would be representative of the site, a simple

statistical approach was used to give equal weight to each

round of sampling.

The aquifer containing the plume of contamination is

semi-confined across the entire site. The superficial layer

of silty clay, and the concrete operational apron prevent

recharge of precipitation in the study area. Recharge

occurs far inland from the Pasquotank River. The superficial confining layer extends out into the bed of the river.

Discharge occurs some distance into the river, not at its

bank. As a result, ground-water flow at the site is controlled by regional flow in the aquifer. As an approximation,

the ground-water elevation at any round of sampling will be

considered to be a linear plane in three-dimensional space.

The slope of the plane is the hydraulic gradient and flow

direction of ground water.

Table A.3 provides a summary of the regression on the

eighteen rounds of quarterly monitoring. The table provides the equation of the regression, the number of wells

involved in the regression at each date, the coefficient of

correlation r2

, the variance of the estimate of elevations,

and the fitted hydraulic gradient and direction for each

round of sampling. The seepage velocity of the ground

water was calculated from Darcy’s Law using an average

estimate of hydraulic conductivity of 0.27 cm/sec and an

assumed effective porosity of 0.3. In general, the assumption that the water table was a plane was reasonable. The

coefficient of correlation exceeded 0.8 for fifteen of the

eighteen rounds of sampling. Table A.4 provides the summary of the regression of the fourteen rounds of monthly

monitoring. The coefficient of correlation exceeded 0.8 for

nine of the fourteen rounds of sampling.

The results of the individual regressions are presented in

graphical form in Figures 4.1 and 4.2.

The average direction of ground-water flow was calculated by weighting the direction of flow on any particular

round of sampling by the hydraulic gradient at that particular round, then taking an average of the weighted flow

directions. For the eighteen rounds of quarterly sampling,

the average flow direction was 8.6 degrees west of north,

with a standard deviation of 9.1 degrees. The mean of the

hydraulic gradient was 0.00252 with a 95% confidence

interval of 0.0010. For the fourteen rounds of monthly

sampling, the average flow direction was 8.7 degrees west

of north, with a standard deviation of 23 degrees. The

mean of the hydraulic gradient was 0.00252 with a 95%

confidence interval of 0.00070. The results of the regressions for each of the eighteen rounds of quarterly monitoring are presented in Figures A.4 through A.18. The

measured elevations submitted to the regressions are

listed in the figures to allow a well-by-well evaluation of the

fit of the regression.
37

Table A.1 Elevation of the water table in permanent monitoring wells during eighteen rounds of quarterly

monitoring extending from September 1994 through December 1998. The elevations are reported in

feet above mean sea level. Compare Figure A.1 for the location of the monitoring wells.

Well 9/94 12/94 3/95 5/95 8/95 12/95 3/96 6/96 9/96 12/96 3/97 6/97 9//97 12/97 3/98 6/98 9/98 12/98

ESM-3 0.74 0.86 1.08 1.23 1.35 0.49 0.97 1.32 1.5 1.4 1.16 1.31 1.29 0.5 1.56 1.27 1.03 0.58

ESM-4 0.44 0.89 0.89 1.11 1.36 0.17 0.65 1.05 1.45 1.18 0.84 1.53 1.37 0.29 1.18 1.19 1.21 0.83

ESM-6 1.08 1.03 2.18 1.38 1.2 0.97 1.71 1.85 1.89 1.81 1.73 1.28 1.14 0.71 2.35 1.56 1.06 0.74

ESM-7 1.63 1.55 3.16 1.65 1.17 1.67 2.56 2.45 2.3 1.66 1 1.05 2.82 1.95 1.76 1.38

ESM-9 0.73 0.92 0.95 1.23 1.33 0.42 0.89 1.25 1.45 1.34 1.09 1.23 1.31 0.52 1.47 1.25 1.08 0.67

ESM-10 0.68 0.87 0.83 1.19 1.3 0.37 0.8 1.16 1.39 1.32 1.11 1.25 1.32 0.44 1.4 1.34 1.13 0.6

ESM-11 0.4 0.9 0.8 1.09 1.38 0.09 0.61 1.02 1.45 1.13 0.78 1.34 1.34 0.26 1.06 1.15 1.17 0.59

ESM-14 0.92 1.03 1.34 1.33 1.27 0.68 1.24 1.5 1.64 1.53 1.39 1.24 1.23 0.59 1.79 1.37 1.07 0.59

Table A.2. Elevation of the water table in permanent monitoring wells during fourteen rounds of monthly

monitoring extending from September 1994 through December 1998. The elevations are reported in

feet above mean sea level. Compare Figure A.1 for the location of the monitoring wells.

Well 2/11/98 3/10/98 4/7/98 5/13/98 6/16/98 7/9/98 8/6/98 9/2/98 10/1/98 11/14/98 12/7/98 1/6/99 2/1/99 3/5/99

ESM-3 2.34 2.22 1.53 1.06 1.41 1.16 0.52 1.36 1.03 0.85 0.58 0.94 1.05

ESM-4 2.15 1.98 1.29 0.45 1.43 1.4 0.32 1.37 1.21 1.07 0.83 0.78 0.69 0.73

ESM-6 3.14 2.75 1.98 1.59 1.49 1.17 0.82 1.78 1.06 0.88 0.74 1.15 1.42 1.36

ESM-7 3.6 3.13 1.54 2.16 1.73 1.38 2.26 1.76 1.63 1.38 1.62 1.79 1.64

ESM-9 2.17 2.11 1.37 0.91 1.39 1.06 0.47 1.46 1.08 0.91 0.67 0.86 0.66 0.94A

ESM-10 2.12 2.09 1.32 0.85 1.4 0.95 0.47 1.33 1.13 0.99 0.6 0.77 0.68 0.9

ESM-11 2.1 1.94 2.07 0.35 1.44 0.91 0.29 1.36 1.17 1.02 0.59 0.79 0.62 0.69

ESM-14 2.5 2.37 1.61 1.21 1.45 1.19 0.65 1.39 1.07 0.9 0.59 0.98 1.06
38

Table A.3 Equation of a linear plane that was fit using a least-squares regression through the elevation of the wat

monitoring wells during each of eighteen rounds of quarterly sampling. The plane is in an x,y,z coordinate systoward the east, y increases toward the north, and z increases with elevation above mean sea level. The eAx+By+C+z where x and y are the grid location in UTM meters and z is the elevation of the water table in fe

Date n A B C r2 z Hydraulic Direction V

number x coefficient y coefficient constant Variance Gradient (degrees 

of wells east from north) 

Sept. 1994 8 1.03334E-03 -3.25785E-03 31.950 0.96785 0.00700 0.0034 -18 Dec. 1994 8 7.04719E-04 -1.76193E-03 -108.013 0.83375 0.01227 0.0019 -22 Mar. 1995 8 -3.91751E-04 -6.66909E-03 2228.290 0.95519 0.04445 0.0067 3 May 1995 8 4.80194E-04 -1.49026E-03 9.037 0.96893 0.00141 0.0016 -18 Aug. 1995 8 -1.66687E-05 5.84579E-04 -149.892 0.86551 0.00110 0.0006 -2 Dec. 1995 8 8.39634E-04 -4.21060E-03 468.710 0.97192 0.01028 0.0043 -11 Mar 1996 8 5.81895E-04 -5.39052E-03 1025.850 0.96560 0.02110 0.0054 -6 June 1996 7 1.50708E-04 -3.49088E-03 860.107 0.91667 0.01053 0.0035 -2 Sept. 1996 8 1.07050E-04 -2.82168E-03 708.465 0.90858 0.01660 0.0028 -2 Dec. 1996 8 4.21094E-04 -3.16140E-03 533.568 0.97066 0.00613 0.0032 -8 Mar. 1997 7 5.70420E-04 -4.02255E-03 648.421 0.89316 0.01688 0.0041 -8 June 1997 8 -2.55409E-04 -6.23482E-04 398.315 0.27942 0.02470 0.0007 22 Sept. 1997 8 -9.29013E-05 9.90993E-04 -199.269 0.93309 0.00144 0.0010 -5 Dec 1997 8 7.32342E-04 -2.09248E-03 -38.689 0.97696 0.00205 0.0022 -19 Mar. 1998 8 1.01025E-04 -4.85332E-03 1289.130 0.96975 0.01529 0.0049 -1 June 1998 8 3.93228E-04 -2.09999E-03 256.873 0.92957 0.00672 0.0021 -11 Sept. 1998 8 7.92146E-04 -1.41290E-03 -282.118 0.50305 0.03959 0.0016 -29 Dec. 1998 8 4.00185E-04 -1.85079E-03 179.655 0.66928 0.03389 0.0019 -12 
39

Table A.4 Equation of a linear plane that was fit using a least-squares regression through the elevation of the wat

monitoring wells during each of fourteen rounds of monthly sampling. The plane is in an x,y,z coordinate systtoward the east, y increases toward the north, and z increases with elevation above mean sea level. The eAx+By+C+z where x and y are the grid location in UTM meters and z is the elevation of the water table in fe

Date n A B C r2 z Hydraulic Direction V

number x coefficient y coefficient constant Variance Gradient (degrees 

of wells east from north) 

11-Feb-98 8 -8.05822E-04 -4.33641E-03 1925.990 0.96171 0.01669 0.00441 11 10-Mar-98 8 -7.31494E-05 -3.33136E-03 1008.970 0.96560 0.00833 0.00333 1 7-Apr-98 8 -1.75445E-03 9.29881E-05 1489.030 0.19250 0.09695 0.00176 -87 13-May-98 8 9.33425E-04 -4.79741E-03 554.451 0.94123 0.02886 0.00489 -11 16-Jun-98 7 -2.64965E-04 -1.24188E-04 265.225 0.65632 0.00006 0.00029 65 9-Jul-98 8 -1.28825E-04 -1.67917E-03 587.936 0.61699 0.03750 0.00168 4 6-Aug-98 8 7.07976E-04 -2.89156E-03 208.726 0.95649 0.00759 0.00298 -14 2-Sep-98 8 2.28660E-04 -2.54803E-03 525.917 0.89449 0.01567 0.00256 -5 1-Oct-98 8 7.92146E-04 -1.41290E-03 -282.118 0.50305 0.03959 0.00162 -29 14-Nov-98 8 8.51723E-04 -1.43532E-03 -327.332 0.46232 0.04834 0.00167 -31 7-Dec-98 8 4.00185E-04 -1.85079E-03 179.655 0.89316 0.03389 0.00189 -12 6-Jan-99 6 3.50042E-04 -2.31329E-03 354.144 0.93028 0.01330 0.00234 -9 1-Feb-99 8 -7.16128E-04 -3.26365E-03 1543.220 0.95728 0.01074 0.00334 12 5-Mar-99 8 2.01839E-04 -2.56719E-03 553.994 0.95110 0.00695 0.00258 -4 
40

0.74

0.44

1.08

1.63

0.73

0.68

0.4

0.92

ESM-14

ESM-11

ESM 3

ESM 4

ESM 10

ESM 9

CONCRETE

GRASS

GRASS

GRASS

ASPH ALT

GRASS

FUEL

PASQUOTANK

ESM-7

ESM-6

RIVER

D R AIN AG E DITC H

GRASS

N

SCALE IN METERS

0 50 100

SCALE IN METERS

0.86

0.89

1.03

1.55

0.92

0.87

0.9

1.03

ESM-14

ESM-11

ESM 3

ESM 4

ESM 10

ESM 9

CONCRETE

GRASS

GRASS

GRASS

ASPHALT

GRASS

FUEL

PASQUOTANK

ESM-7

ESM-6

RIVER

D R AINAG E D ITC H

GRASS

N

SCALE IN METERS

0 50 100

Figure A.4 Direction and gradient of ground-water flow on a sample

date in September 1994.

Figure A.5 Direction and gradient of ground-water flow on a sample

date in December 1994.
41

1.08

0.89

2.18

3.16

0.95

0.83

0.8

1.34

ESM-14

ESM-11

ESM 3

ESM 4

ESM 10

ESM 9

CONCRETE

GRASS

GRASS

GRASS

ASP HALT

GRASS

FUEL

PASQUOTANK

ESM-7

ESM-6

RIVER

DR AIN AGE D ITCH

GRASS

N

SCALE IN METERS

0 50 100

1.23

1.11

1.38

1.65

1.23

1.19

1.09

1.33

ESM-14

ESM-11

ES M 3

ES M 4

ES M 10

ES M 9

CONCRETE

GRASS

GRASS

GRASS

ASPH ALT

GRASS

FUEL

PASQUOTANK

ESM-7

ESM-6

RIVER

DR AIN AGE D IT CH

GRASS

N

SCALE IN METERS

0 50 100

1.35

1.36

1.2

1.17

1.33

1.3

1.38

1.27

ESM-14

ESM-11

ES M 3

ES M 4

ES M 10

ES M 9

CONCRETE

GRASS

GRASS

GRASS

ASPH ALT

GRASS

FUEL

PASQUOTANK

ESM-7

ESM-6

RIVER

D R AIN AG E DITC H

GRASS

N

SCALE IN METERS

0 50 100

0.49

0.17

0.97

1.67

0.42

0.37

0.09

0.68

ESM-14

ESM-11

ES M 3

ES M 4

ES M 10

ES M 9

CONCRETE

GRASS

GRASS

GRASS

ASPHALT

GRASS

FUEL

PASQUOTANK

ESM-7

ESM-6

RIVER

D R AIN AG E DITC H

GRASS

N

SCALE IN METERS

0 50 100

Figure A.6 Direction and gradient of ground-water flow on a sample

date in March 1995. Figure A.7 Direction and gradient of ground-water flow on a sample

date in May 1995.

Figure A.8 Direction and gradient of ground-water flow on a

sample date in August 1995.

Figure A.9 Direction and gradient of ground-water flow on a

sample date in December 1995.
42

0.97

0.65

1.71

2.56

0.89

0.8

0.61

1.24

ESM-14

ESM-11

ES M 3

ES M 4

ES M 10

ES M 9

CONCRETE

GRASS

GRASS

GRASS

ASPH ALT

GRASS

FUEL

PASQUOTANK

ESM-7

ESM-6

RIVER

D RAIN AGE D ITC H

GRASS

N

SCALE IN METERS

0 50 100

1.32

1.05

1.85

1.25

1.16

1.02

1.5

ESM-14

ESM-11

ES M 3

ES M 4

ES M 10

ES M 9

CONCRETE

GRASS

GRASS

GRASS

ASPHALT

GRASS

FUEL

PASQUOTANK

ESM-6

RIVER

D RAINAGE D ITC H

GRASS

N

SCALE IN METERS

0 50 100

1.5

1.45

1.89

2.45

1.45

1.39

1.45

1.64

ESM-14

ESM-11

ES M 3

ES M 4

ES M 10

ES M 9

CONCRETE

GRASS

GRASS

GRASS

ASP HALT

GRASS

FUEL

PASQUOTANK

ESM-7

ESM-6

RIVER

DR AIN AGE D IT CH

GRASS

N

SCALE IN METERS

0 50 100

1.4

1.18

1.81

2.3

1.34

1.32

1.13

1.53

ESM-14

ESM-11

ESM 3

ESM 4

ESM 10

ESM 9

CONCRETE

GRASS

GRASS

GRASS

ASPHALT

GRASS

FUEL

PASQUOTANK

ESM-7

ESM-6

RIVER

D RAINAGE D ITC H

GRASS

N

SCALE IN METERS

0 50 100

Figure A.10 Direction and gradient of ground-water flow on a

sample date in March 1996.

Figure A.13 Direction and gradient of ground-water flow on a

sample date in December 1996.

Figure A.12 Direction and gradient of ground-water flow on a

sample date in September 1996.

Figure A.11 Direction and gradient of ground-water flow on a

sample date in June 1996.
43

1.31

1.53

1.28

1.66

1.23

1.25

1.34

1.24

ESM-14

ESM-11

ES M 3

ES M 4

ES M 10

ES M 9

CONCRETE

GRASS

GRASS

GRASS

ASPH ALT

GRASS

FUEL

PASQUOTANK

ESM-7

ESM-6

RIVER

D R AIN AG E DITC H

GRASS

N

SCALE IN METERS

0 50 100

1.29

1.37

1.14

1

1.31

1.32

1.34

1.23

ESM-14

ESM-11

ES M 3

ES M 4

ES M 10

ES M 9

CONCRETE

GRASS

GRASS

GRASS

ASPH ALT

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FUEL

PASQUOTANK

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RIVER

D RAIN AGE D ITC H

GRASS

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SCALE IN METERS

0 50 100

1.16

0.84

1.73

1.09

1.11

0.78

1.39

ESM-14

ESM-11

ES M 3

ES M 4

ES M 10

ES M 9

CONCRETE

GRASS

GRASS

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GRASS

FUEL

PASQUOTANK

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RIVER

DR AIN AG E D IT CH

GRASS

N

SCALE IN METERS

0 50 100

0.5

0.29

0.71

1.05

0.52

0.44

0.26

0.59

ESM-14

ESM-11

ESM 3

ESM 4

ESM 10

ESM 9

CONCRETE

GRASS

GRASS

GRASS

ASPH ALT

GRASS

FUEL

PASQUOTANK

ESM-7

ESM-6

RIVER

D R AIN AG E DITC H

GRASS

N

SCALE IN METERS

0 50 100

Figure A.16 Direction and gradient of ground-water flow on a

sample date in September 1997.

Figure A.15 Direction and gradient of ground-water flow on a

sample date in June 1997.

Figure A.14 Direction and gradient of ground-water flow on a

sample date in March 1997.

Figure A.17 Direction and gradient of ground-water flow on a

sample date in December 1997.
44

1.27

1.19

1.56

1.95

1.25

1.34

1.15

1.37

ESM-14

ESM-11

ESM 3

ESM 4

ESM 10

ESM 9

CONCRETE

GRASS

GRASS

GRASS

ASP HALT

GRASS

FUEL

PASQUOTANK

ESM-7

ESM-6

RIVER

D RAINAGE D ITC H

GRASS

N

SCALE IN METERS

0 50 100

1.03

1.21

1.06

1.76

1.08

1.13

1.17

1.07

ESM-14

ESM-11

ES M 3

ES M 4

ES M 10

ES M 9

CONCRETE

GRASS

GRASS

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ASPHALT

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FUEL

PASQUOTANK

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RIVER

D RAINAGE D ITC H

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N

SCALE IN METERS

0 50 100

1.56

1.18

2.35

2.82

1.47

1.4

1.06

1.79

ESM-14

ESM-11

ES M 3

ES M 4

ES M 10

ES M 9

CONCRETE

GRASS

GRASS

GRASS

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GRASS

FUEL

PASQUOTANK

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RIVER

D R AIN AG E DITC H

GRASS

N

SCALE IN METERS

0 50 100

0.58

0.83

0.74

1.38

0.67

0.6

0.59

0.59

ESM-14

ESM-11

ES M 3

ES M 4

ES M 10

ES M 9

CONCRETE

GRASS

GRASS

GRASS

ASPH ALT

GRASS

FUEL

PASQUOTANK

ESM-7

ESM-6

RIVER

D R AIN AG E DITC H

GRASS

N

SCALE IN METERS

0 50 100

Figure A.21 Direction and gradient of ground-water flow on a

sample date in December 1998.

Figure A.20 Direction and gradient of ground-water flow on a

sample date in September 1998.

Figure A.19 Direction and gradient of ground-water flow on a

sample date in June 1998.

Figure A.18 Direction and gradient of ground-water flow on a

sample date in March 1998.
45

Figures B.1 and B.2 compare the distribution of methane

and MTBE in the north-south transect (see Figure 3.7).

Figures B.3 and B.4 compare the distribution of methane

and MTBE in the east-west transect. The entire MTBE

plume is contained within a plume of methane. Methane

concentrations generally exceeded 3.0 mg/l, and often

exceeded 10 mg/l. In general, the distribution of MTBE

was contained within the distribution of methane. However, along the east-west transect, at the sampling locations 50, 100, and 150 feet along the transect, the highest

concentrations of MTBE extended about three feet deeper

into the aquifer than the higher concentrations of methane.

In general, this aquifer is strongly methanogenic. Concentrations of methane averaged 7 mg/l, which corresponds to

9 mg/l of hydrocarbon originally metabolized. (Wiedemeier

et al., 1999, table 5.3, page 214).

Figures B.5 through B.8 compare the other geochemical

parameters in the ground water along the north-south

transect. Figure B.5 shows that the MTBE plume is

contained within a region of the aquifer that is depleted of

molecular oxygen. Many regions of the aquifer have less

than 0.1 mg/l oxygen. Background concentrations of

oxygen in regions of the aquifer that are not impacted by

the fuel spill are near 3.6 mg/l. The depletion in oxygen

would account for 1 mg/l of petroleum hydrocarbon.

Ground water in the region of the aquifer that contains

MTBE and BTEX compounds is also depleted of sulfate

(Figure B.6). Sulfate concentrations are reduced from a

background of near 28 mg/l to less than 4 mg/l. Many

regions have less than 1 mg/l. A depletion of 24 mg/l of

sulfate would oxidize 5 mg/l of fuel hydrocarbons.

The same regions that are depleted in molecular oxygen

and sulfate have significant accumulations of iron (II)

(Figure B.7). Background concentrations of iron (II) are

less than 0.1 mg/l. Many regions of the aquifer with MTBE

and BTEX compounds have iron (II) concentrations greater

than 50 mg/l. This accumulation of iron (II) would be

capable of oxidizing 3 mg/l of hydocarbons.

The plume is undergoing extensive anaerobic oxidation

of petroleum hydrocarbons, as well as fermentation of

hydrocarbons to methane. The hydrocarbon metabolized

through sulfate and iron reduction is approximately equivalent to the hydrocarbon metabolized through

methanogenesis. The amount of hydrocarbon metabolized through anaerobic pathways is about seventeen times

the amount degraded with molecular oxygen.

The pH of the plume is generally near 6.5 and is below

6.0 only in the ground water that is in direct contact with the

Appendix B: Geochemical Context of the MTBE Plume

LNAPL. Under these conditions, carbon dioxide produced

through oxidation of petroleum hydrocarbons will react with

carbonate minerals in the aquifer matrix to produce bicarbonate alkalinity in the ground water. Figure B.8 shows

that as much as 200 mg/l of alkalinity was produced by

oxidation of petroleum hydrocarbons. This corresponds to

88 mg/l of carbon dioxide produced or 28 mg/l of TPH

consumed. There is more than enough carbon dioxide

production to account for the depletion of oxygen and

sulfate, and production of iron (II) and methane.
46

Figure B.1 Distribution of methane along the north-south transect, collected in August 1996. Distance along the transect extends from south

to north (bottom to top in Figure 3.7), in the direction of ground-water flow.

Figure B.2 Distribution of MTBE along the north-south transect, collected in August 1996. Distance along the transect extends from south to

north (bottom to top in Figure 3.7), in the direction of ground-water flow.
47

Figure B.3 Distribution of methane along the east-west transect, collected in December 1997. Distance along the transect extends from west

to east (left to right in Figure 3.7), opposite the direction of ground-water flow.

Figure B.4 Distribution of MTBE along the east-west transect, collected in December 1997. Distance along the transect extends from west to

east (left to right in Figure 3.7), opposite the direction of ground-water flow.

10

1

1

1

1

1 1

10 10

10

10

10

2 7

2 4

2 1

1 8

1 5

1 2

9

6

3

0

0 50 100 150 200 250 300 350 400 450

L ocation on Transect (ft)

D epth B elow Water Table (ft)

M ethane (m g/L ) at E ast - West Transect
48

Figure B.6 Distribution of sulfate along the north-south transect, collected in August 1996. Distance along the transect extends from south to

north (bottom to top in Figure 3.7), in the direction of ground-water flow.

Figure B.5 Distribution of oxygen along the north-south transect, collected in August 1996. Distance along the transect extends from south to

north (bottom to top in Figure 3.7), in the direction of ground-water flow.
49

Figure B.8 Distribution of alkalinity along the north-south transect, collected in August 1996. Distance along the transect extends from south

to north (bottom to top in Figure 3.7), in the direction of ground-water flow.

Figure B.7 Distribution of iron (II) along the north-south transect, collected in August 1996. Distance along the transect extends from south to

north (bottom to top in Figure 3.7), in the direction of ground-water flow.