In cooperation with the

UNITED STATES AIR FORCE

Dover Air Force Base

Assessment of Natural Attenuation of Ground-Water

Contamination at Sites FT03, LF13, and WP14/LF15,

Dover Air Force Base, Delaware

Water-Resources Investigations Report 01–4150

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

U.S. Geological Survey

Assessment of Natural Attenuation of Ground-Water

Contamination at Sites FT03, LF13, and WP14/LF15,

Dover Air Force Base, Delaware

By Jeffrey R. Barbaro

Water-Resources Investigations Report 01-4150

In cooperation with the

UNITED STATES AIR FORCE

Dover Air Force Base

The contents of this report have been approved for public release and unlimited distribution by the U.S. Army--

clearance number 3793-A-6

Baltimore, Maryland

2002
U.S. Department of the Interior

GALE A. NORTON, Secretary

U.S. Geological Survey

Charles G. Groat, Director

The use of trade, product, or firm names in this report is for 

descriptive purposes only and does not imply endorsement 

by the U.S. Government.

For additional information contact:

District Chief

U.S. Geological Survey

8987 Yellow Brick Road

Baltimore, MD 21237

Copies may be purchased from:

U.S. Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80215
iii

Contents

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

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

Purpose and scope .........................................................................................................................................................4

Description of study area...............................................................................................................................................5

Previous studies .............................................................................................................................................................5

Acknowledgments .........................................................................................................................................................6

Data-collection methods.........................................................................................................................................................7

Monitoring network.......................................................................................................................................................7

Development of hydrogeologic framework...................................................................................................................7

Hydrologic data collection.............................................................................................................................................7

Sediment organic-carbon collection ..............................................................................................................................7

Ground-water sample collection and analysis ...............................................................................................................7

Assessment of natural attenuation........................................................................................................................................12

Evidence of natural attenuation ...................................................................................................................................12

Hydrogeologic framework...........................................................................................................................................13

Dispersion, adsorption, and abiotic degradation .........................................................................................................14

Intrinsic biodegradation...............................................................................................................................................17

Areal distribution of electron acceptors and metabolic by-products ..................................................................17

Areal distribution of fuel hydrocarbons..............................................................................................................25

Areal distribution of chlorinated solvents...........................................................................................................26

Site FT03 ....................................................................................................................................................31

Site LF13 ....................................................................................................................................................32

Site WP14/LF15 .........................................................................................................................................33

Temporal variability............................................................................................................................................35

Vertical distribution of selected compounds .......................................................................................................35

Revised conceptual model and possible further study........................................................................................39

Summary and conclusions....................................................................................................................................................39

References cited ...................................................................................................................................................................41

Appendix A. Hydraulic-head measurements from wells screened in the surficial aquifer in the

East Management Unit, Dover Air Force Base, Delaware, March 15–16, 2000......................................43

Figures

1-1. Maps showing location of:

(a) Dover Air Force Base, Dover, Delaware.......................................................................................................2

(b) The East Management Unit and the natural attenuation study area at Dover Air Force Base ......................3

2-2. Map showing the location of the contaminant source areas, the monitor wells and piezometers sampled

for water-quality constituents, and the trace of section A-A′ in the natural attenuation study area ....................4

3-3. Diagram showing hydrogeologic section A-A′..........................................................................................................6

4-4. Map showing location of slug tests and sediment organic-carbon samples.............................................................10

5-6. Maps showing hydraulic-head distribution in the:

5. Upper part of the surficial aquifer, March 15–16, 2000................................................................................14

6. Lower part of the surficial aquifer, March 15–16, 2000 ...............................................................................15

7-8. Diagrams showing:

7. Vertical hydraulic-head distribution and ground-water flow direction along section A-A′ .........................16

8. Possible biotic and abiotic reaction pathways for the chlorinated ethenes and ethanes ...............................20
iv

Figures—Continued

19-19. Maps showing the distribution of selected compounds in the upper part of the surficial aquifer,

 January–March 2000:

(a) Dissolved oxgyen .....................................................................................................................................23

(b) Ferrous iron..............................................................................................................................................24

(c) Methane....................................................................................................................................................25

10-10. Map showing the distribution of terminal electron-accepting processes in the upper part of the

 surficial aquifer, January–March 2000............................................................................................................26

11-11. Maps showing the distribution of selected volatile organic compounds in the upper part of the

 surficial aquifer, January–March 2000:

(a) Tetrachloroethene.....................................................................................................................................30

(b) Trichloroethene ........................................................................................................................................31

(c) Cis-1,2,-dichloroethene ............................................................................................................................32

(d) Vinyl chloride ..........................................................................................................................................33

12-12. Graphs showing concentrations of selected volatile organic compounds in ground-water samples

collected from wells MW039S and DM110S, 1994–2000 ..............................................................................34

13-13. Graphs showing concentrations of selected volatile organic compounds, electron acceptors,

and metabolic by-products in ground-water samples collected from well DM108D, 1994–2000 ..................36

14-14. Graphs showing vertical distribution of selected volatile organic compounds, electron acceptors,

and metabolic by-products in ground-water samples collected from multilevel piezometers

GS045ML and GS047ML................................................................................................................................37

15-15. Graphs showing concentrations of tetrachloroethene and methane in ground-water samples collected

from multilevel piezometers GS045ML and GS047ML, and the adjacent long-screened wells

MW011 and MW039S .....................................................................................................................................38

Tables

11. Construction details for wells and piezometers sampled at Dover Air Force Base, Delaware,

January–March 2000..........................................................................................................................................8

22. Hydraulic conductivity values from slug tests on wells screened in the surficial aquifer in the

East Management Unit.....................................................................................................................................11

33. Weight fractions of organic carbon (foc ) in surficial-aquifer sediment samples collected in the

East Management Unit.....................................................................................................................................12

44. Retardation factors and relative velocities for selected volatile organic compounds detected in

the natural attenuation study area.....................................................................................................................17

55. Concentrations of nutrients, sulfate, and chloride in ground water from wells screened in the

surficial aquifer at sites FT03, LF13, and WP14/LF15, January–March 2000................................................18

66. Concentrations of field constituents and methane in ground water from wells screened in the

surficial aquifer at sites FT03, LF13, and WP14/LF15, January–March 2000................................................21

77. Concentrations of selected volatile organic compounds in ground water from wells screened

in the surficial aquifer at sites FT03, LF13, and WP14/LF15, January–March 2000......................................27
v

Acronyms and Abbreviations

BTEX benzene, toluene, ethylbenzene, total xylenes

COC chain of custody

DAFB Dover Air Force Base

cis-1,2-DCE cis-1,2-dichloroethene

cm centimeter

1,2-DCA 1,2-dichloroethane

DIC dissolved inorganic carbon

EMU East Management Unit

foc weight fraction of organic carbon

ft foot

ft/d foot per day

ft/ft foot per foot

FT03 Fire Training Area Three

g gram

g/cm3 gram per cubic centimeter

g/g gram per gram

in inch

in/yr inch per year

Kd distribution coefficient

Koc organic-carbon-based partition coefficient

L3

/M volume per unit mass

LF13 Rubble Area Landfill

LF15 Receiver Station Landfill

MCL maximum contaminant level

MDL method detection limit

mg/L milligram per liter

mL/g milliliter per gram

n porosity

NAPL non-aqueous phase liquid

OD outer diameter

1,1,2,2-TeCA 1,1,2,2-tetrachloroethane

PCE tetrachloroethene

POC point of compliance

PVC polyvinylchloride

QC quality control

R retardation factor

RI remedial investigation

TEAP terminal electron-accepting process

TCE trichloroethene

trans-1,2-DCE trans-1,2-dichloroethene

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

VOC volatile organic compound

WP14 Liquid Waste Disposal Landfill

ρb dry bulk density

μg/L microgram per liter

VC vinyl chloride

yr year
vi

Conversion Factors and Vertical Datum

_____________________________________________________________________________________________________________________________________

Multiply By To obtain

_____________________________________________________________________________________________________________________________________

inch (in.) 2.54 centimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

foot per day (ft/d) 30.48 centimeter per day

inch per year (in/yr) 2.54 centimeter per year

acre 0.4047 hectare

_____________________________________________________________________________________________________________________________________

Temperature in degrees Fahrenheit (°F) can be converted to degrees Celsius (°C) by using the following equation:

°C = 5/9 x (°F - 32) 

Vertical datum: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929—a geodetic datum 

derived from a general adjustment of the first-order level nets of the United States and Canada, formerly called Sea Level 

Datum of 1929.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 25 o

C).
Abstract 1

Assessment of Natural Attenuation of Ground-Water 

Contamination at Sites FT03, LF13, and WP14/LF15, Dover Air 

Force Base, Delaware

By Jeffrey R. Barbaro

Abstract

Water-quality, aquifer-sediment, and hydrologic data were used to assess the effectiveness of 

natural attenuation of ground-water contamination 

at Fire Training Area Three, the Rubble Area 

Landfill, the Liquid Waste Disposal Landfill, and 

the Receiver Station Landfill in the East Management Unit of Dover Air Force Base, Delaware. 

These sites, which are contaminated with chlorinated solvents and fuel hydrocarbons, are undergoing long-term monitoring to determine if natural 

attenuation continues to sufficiently reduce contaminant concentrations to meet regulatory requirements. This report is the first assessment of 

the effectiveness of natural attenuation at these 

sites since long-term monitoring began in 1999, 

and follows a preliminary investigation done in

1995–96. This assessment was done by the U.S. 

Geological Survey in cooperation with the U.S. 

Air Force.

Since 1995–96, additional information has been 

collected and used in the current assessment. The 

conclusions in this report are based primarily on 

ground-water samples collected from January 

through March 2000. Previous analytical results 

from selected wells, available geologic and geophysical well logs, and newly acquired information such as sediment organic-carbon measurements, hydraulic-conductivity measurements 

determined from slug tests on wells in the natural 

attenuation study area, and water-level measurements from surficial-aquifer wells also were used 

in this assessment. This information was used to: 

(1) calculate retardation factors and estimate 

contaminant migration velocities, (2) improve 

estimates of ground-water flow directions and 

inferred contaminant migration pathways, (3) better define the areal extent of contamination and the 

proximity of contaminants to discharge areas and 

the Base boundary, (4) develop a better understanding of the vertical variability of contaminant 

concentrations and redox conditions, (5) evaluate 

the effects of temporal changes on concentrations 

in the plumes and source areas, and (6) determine 

whether intrinsic biodegradation is occurring at 

these sites.

The water-quality data indicate that intrinsic 

biodegradation is occurring at all three sites. The 

strongest indication of intrinsic biodegradation is 

the detection of tetrachloroethene and trichloroethene breakdown products within and downgradient of the source areas. The patterns of 

electron acceptors and metabolic by-products 

indicate that contaminant biodegradation has 

changed the prevailing geochemistry of the 

surficial aquifer, creating the strongly reducing 

conditions necessary for chlorinated solvent biodegradation. Geochemical changes include depleted dissolved oxygen and elevated ferrous iron 

and methane levels relative to concentrations in 

uncontaminated zones of the surficial aquifer. At 

Fire Training Area Three and the Rubble Area 

Landfill sites, natural attenuation appears to be 

adequate for controlling the migration of the contaminant plumes. At the third site, the Liquid 

Waste Disposal and Receiver Station Landfills, the 

plume is larger and the uncertainty about the 

effectiveness of natural attenuation in reducing 

contaminant concentrations and controlling plume 

migration is greater. Ground-water data indicate, 

however, that U.S. Environmental Protection 

Agency maximum contaminant levels were not 

exceeded in any point-of-compliance wells located 

along the Base boundary.

The information presented in this report led to 

the development of improved conceptual models 

for these sites, and to the recognition of four issues 
2 Assessment of Natural Attenuation of Ground-Water Contamination at Dover Air Force Base, Delaware

that are currently unclear and may need further 

study. These issues include delineating the areal 

and vertical extent of the contaminant plumes in 

greater detail, determining the extent of intrinsic 

biodegradation downgradient of the Liquid Waste 

Disposal and Receiver Station Landfills, determining the fate of contaminants in the groundwater discharge areas, and determining the effect 

of temporal variability in source concentrations 

and ground-water flow patterns on the plume 

migrating from the Liquid Waste Disposal and 

Receiver Station Landfills. Some of these issues 

have been addressed with additional data collection since the January–March 2000 sampling 

round.

Introduction

Historical activities such as fire training and waste 

disposal in the East Management Unit (EMU) of Dover Air 

Force Base (DAFB), Delaware (figs. 1a and 1b) have 

resulted in subsurface contamination with organic chemicals. 

Previous investigations have shown that ground water 

contains detectable concentrations of a variety of volatile 

organic compounds (VOCs), including fuel hydrocarbons 

such as benzene, toluene, and xylenes, chlorinated solvents 

such as chlorinated ethenes and ethanes, and chlorinated 

benzenes (U.S. Army Corps of Engineers and Dames and 

Moore, Inc., 1997a; Bachman and others, 1998; Beman and 

others, 1999). Because some of these compounds were 

detected in ground water at concentrations exceeding 

maximum contaminant levels (MCLs) established by the 

U.S. Environmental Protection Agency (USEPA), considerable efforts have been made over the past decade to 

investigate the area, and to develop remedial alternatives 

(U.S. Army Corps of Engineers and Dames and Moore, Inc., 

1997a, b). The selected remediation for Fire Training Area 

Three (FT03), the Rubble Area Landfill (LF13), the Liquid 

Waste Disposal Landfill (WP14), and the Receiver Station 

Landfill (LF15) in the EMU (fig. 2) is natural attenuation 

(U.S. Army Corps of Engineers and Dames and Moore, Inc., 

1997b). As part of the selected remediation, the U.S. Geological Survey (USGS), in cooperation with the U.S. Air 

Force, is conducting a long-term monitoring program and an 

assessment of the effectiveness of natural attenuation in the 

surficial aquifer. 

Whereas the steps used to document and quantify natural 

attenuation are the same for all sites, the level of effort 

needed to carry out each step varies substantially according 

to the complexity of the site (National Research Council, 

2000, p. 173). The three basic steps, as defined by the 

National Research Council (2000, p. 204), are:

(1) Conceptual model development (hydrogeologic 

framework, plume and source-area delineation, and 

identification of attenuation processes).

(2) Analysis of ground-water samples to identify the 

characteristic geochemical changes or “footprints” 

associated with the relevant intrinsic biodegradation 

reactions.

(3) On-going site monitoring to determine if the 

documented processes continue to adequately control 

contaminant migration.

A historical data base that shows plume stabilization and/or 

contaminant mass loss over time provides the strongest 

evidence that natural attenuation is effective (Wiedemeier 

and others, 1999, p. 313).
Introduction 3

The natural attenuation sites in the EMU are relatively 

complex. Complicating factors include a complex groundwater flow system, the presence of multiple source areas, 

and the presence of contaminant mixtures, predominantly 

fuel hydrocarbons and chlorinated solvents, in the source 

areas. The main compounds of concern at these sites, the 

chlorinated solvents, require specific geochemical conditions within the plumes that form downgradient of the source 

areas for efficient and sustainable intrinsic biodegradation. 

Accordingly, these sites require a relatively detailed level of 

site characterization and data analysis. A preliminary study 

by Bachman and others (1998) concluded that natural 

attenuation was reducing the concentrations of organic 

compounds as they migrated away from the source areas, 

and that off-site migration was unlikely. The presence of 

appropriate redox conditions and the formation of chlorinated-solvent breakdown products indicated that intrinsic 

biodegradation was the main attenuation mechanism. 

Whereas the preliminary results showed that natural 

attenuation was occurring at all three sites, uncertainties 

about the hydrogeologic framework and contaminant 

distributions also were identified.

Since 1995–96, when the data for the preliminary study 

were collected (Bachman and others, 1998), additional 

information has been obtained at these sites. The new 

results were used in the current assessment and include:

(1) Improved estimates of ground-water velocities in the 

EMU from hydraulic-conductivity values calculated 

from slug tests.

(2) Improved estimates of contaminant migration 

velocities from retardation factors calculated from 

sediment organic-carbon analyses.

(3) Improved understanding of ground-water flow 

directions and inferred contaminant migration 
4 Assessment of Natural Attenuation of Ground-Water Contamination at Dover Air Force Base, Delaware

pathways from 2000 ground-water-level measurements.

(4) Better definition of the areal extent of contamination 

and the proximity of contaminants to discharge areas 

and the DAFB boundary from the results of comprehensive ground-water sampling performed in 2000.

(5) A better understanding of the vertical variability of 

contaminant concentrations and redox conditions from 

samples collected from multilevel piezometers.

(6) A better understanding of temporal changes in plume 

and source-area concentrations based on time-series 

plots using results from both the January–March 2000 

and earlier sampling rounds.

Purpose and Scope

The purpose of this report is to present the results of an 

assessment of the effectiveness of natural attenuation in the 

EMU of DAFB. Monitor wells and newly installed 

multilevel piezometers completed in the surficial aquifer 

were sampled to determine the distributions of VOCs, 

electron acceptors, and metabolic by-products in the natural 

attenuation study area. Ground-water samples collected 

from January through March 2000 from 53 monitor wells 

and piezometers in the natural attenuation study area were 

analyzed. Analytical data from previous sample rounds 

also were used to determine temporal changes in VOC concentrations at selected wells in the plumes. Ground-water 

samples were analyzed for VOCs, total metals, dissolved 

metals, nitrate, ammonia, sulfate, sulfide, chloride, methane, 

dissolved oxygen (DO), total iron, and ferrous iron. The 

MCLs for metals were not exceeded in any sample collected during the January–March 2000 sampling round

(K.C. Alexander and others, U.S. Geological Survey, written 
Introduction 5

commun., 2000); therefore, the natural attenuation of metals 

will not be discussed in this report.

Hydraulic conductivities, hydraulic heads, and organiccarbon concentrations in aquifer sediment were measured to 

develop a better understanding of the hydrogeology of the 

surficial aquifer, and contaminant movement within this 

aquifer. Ten organic-carbon measurements, 15 hydraulicconductivity measurements from slug tests on existing wells, 

ground-water-level measurements from surficial-aquifer 

wells in the EMU, and available geologic and geophysical 

well logs were used in this assessment.

Description of Study Area

DAFB is located in the Delmarva Peninsula, which is in 

the Atlantic Coastal Plain Physiographic Province. The 

surrounding area is primarily cropland and wetlands, with 

some residential development along the southwestern 

boundary of DAFB. The Base is underlain by a thick 

sequence of unconsolidated clastic deposits that form a 

regional aquifer system consisting of layered aquifers and 

confining units. Hinaman and Tenbus (2000, p. 7) provide a 

detailed description of the regional geologic framework and 

hydrogeology. The topographic relief on DAFB is relatively 

small. Land-surface elevations range from about 5 to 30 ft 

(feet) above sea level. Precipitation averages about 46 in/yr 

(inches per year), and is distributed uniformly throughout the 

seasons (Hinaman and Tenbus, 2000, p. 10).

The EMU is bounded on the south and west by runways, 

and on the north and east by cropland and light residential 

development. Four contaminant source areas have been 

identified in the area (fig. 2). Site FT03 is a 2- to 3-acre 

grassy area that was used for fire training until 1989. Waste 

oils and fuels were stored and periodically burned at the site. 

Site LF13 is an 8-acre landfill that has received construction 

rubble and small amounts of other refuse since the 1960s. 

Site WP14 is a former liquid-waste disposal area. It received 

liquid waste during the 1950s and early 1960s. Site LF15 is 

a former landfill site 200 ft east of WP14. During the 1960s, 

LF15 received general refuse and small quantities of shop 

wastes. For purposes of discussion, WP14 and LF15 are 

combined, and referred to as WP14/LF15. Some of the 

source material (contaminated soil) at FT03 was excavated 

during the early 1990s (U.S. Army Corps of Engineers 

and Dames and Moore, Inc., 1997b). Both FT03 and WP14/

LF15 have received clean fill, and a part of FT03 is covered 

with a clay cap. Source materials, such as hydrocarbons and 

chlorinated solvents, probably are present in all four source 

areas in the EMU.

The surficial aquifer underlying DAFB consists of 

interbedded layers of sand and gravel with discontinuous 

layers of silt and clay (fig. 3). The sand consists of quartz 

with minor amounts of feldspar and mica (Bachman and 

others, 1998, p. 14). The orange color of the aquifer 

sediment indicates that ferric oxyhydroxide minerals are 

abundant. The aquifer is bounded on the top by the water 

table and on the bottom by the upper confining unit of the 

Calvert Formation. The saturated thickness of the surficial 

aquifer ranges from about 30 to 60 ft in the natural attenuation study area. The thickness is affected by seasonal 

fluctuations in the altitude of the water table. The surficial 

aquifer is thinnest in the northwest part of this area 

(fig. 3).

Ground water in the natural attenuation study area flows 

to the northwest. Flow is predominantly horizontal but 

vertical flow can occur near the fine-grained deposits and 

surface drainages (Hinaman and Tenbus, 2000, p. 16). 

Ground-water flow directions vary both spatially and 

temporally. Synoptic measurements taken during previous 

studies have shown a seasonal effect on the flow field, with a 

shallower water table and steeper hydraulic gradients during 

wetter conditions (Bachman and others, 1998, p. 14; 

Hinaman and Tenbus, 2000, p. 16). The natural attenuation 

study area is drained by Pipe Elm Branch and approximately 

10 shallow drainage ditches. Drainage ditches that flow into 

Pipe Elm Branch drain surface water from source areas FT03 

and WP14/LF15 (fig. 2). Drainage ditches that flow to the 

south, out of the natural attenuation study area, drain surface 

water from LF13. Strong upward hydraulic gradients in the 

nested streambed piezometers indicate that Pipe Elm Branch 

and the deep drainage ditch adjacent to FT03 (fig. 2) function as ground-water discharge boundaries.

Shallow ground water on the Delmarva Peninsula 

typically contains low total dissolved solids, pH in the range 

of 5 to 6.5, and detectable DO (Bachman and others, 1998, 

p. 23). Anaerobic conditions may be associated locally with 

clay lenses and organic-rich deposits in the aquifers, and 

with the organic-rich sediments commonly found in the discharge areas. Uncontaminated ground water in the surficial 

aquifer underlying DAFB typically contains low concentrations of nitrate (less than 1 mg/L, or milligram per liter, 

nitrate as N), variable concentrations of sulfate, and undetectable concentrations of iron and methane. Methane 

may be produced locally from microbial degradation of 

naturally occurring carbon in the organic-rich stream sediments, however.

Previous Studies

Investigations were performed in the EMU in 1984–85 

(Science Applications International Corporation, 1986), the 

late 1980s (Science Applications International Corporation, 

1989; Hazardous Waste Remedial Actions Program, 1990), 

and the early 1990s (Hazardous Waste Remedial Actions 

Program, 1993). In general, these studies included soil, 

sediment, ground-water, and surface-water sampling, as well 

as geophysical (magnetic, conductivity, and ground-penetrating radar) and soil-gas surveys. Additional sampling in 

support of the comprehensive Basewide Remedial Investigation (RI) was performed in 1993–94 (U.S. Army Corps of 

Engineers and Dames and Moore, Inc., 1997a). The purpose of the RI was to identify potential source areas and 

delineate contaminant distributions in soil and the surficial 

aquifer. In addition to geologic and hydrologic (groundwater level and slug test) data collection, this investigation 

included ground-water samples from available and new 

monitor wells, additional soil samples from borings and test 

pits, surface-water and sediment samples from drainage
6 Assessment of Natural Attenuation of Ground-Water Contamination at Dover Air Force Base, Delaware

ditches, and geophysical and soil-gas surveys. Samples were 

analyzed for a comprehensive list of inorganic and organic 

constituents. The RI also contains summaries of the investigations performed prior to 1997.

The sizes and locations of the source areas discussed in 

this report (fig. 2) are taken from the RI report. The composition and amount of waste materials deposited in these 

areas is not known.

The preliminary USGS investigation to determine 

whether natural attenuation was occurring in the EMU took 

place in 1995–96 (Bachman and others, 1998; Beman and 

others, 1999). In 1997, the USEPA and the Delaware 

Department of Natural Resources and Environmental 

Control selected natural attenuation as the remedial measure 

for sites LF13, WP14/LF15, and FT03. In 1999, the USGS 

began semi-annual, long-term monitoring at these sites.

Acknowledgments

The author thanks the 436th Support Group, Civil 

Engineer Squadron, Environmental Flight (436 SPTG/CEV) 

at DAFB for technical and logistical support. The author 

also thanks the following U.S. Geological Survey personnel 

and contractors: Kristen Alexander, Frederick Gebhardt, 

Kurt Hinaman, George Zynjuk, and Dana Erickson (contractor) for assisting in the collection of the water-quality, 

aquifer-sediment, and hydraulic-head data used in this 

report; Charles Brush for collecting and analyzing the slugtest data; Lisa Donohoe for contributing to the development 

of the geologic framework; Lisa Olsen and Tracey Spencer 

for the methane analysis; Barbara Bekins and William Fleck 

for peer reviews; Valerie Gaine for the editorial review; 

Donna Knight for layout; and Timothy Auer for graphics.
Data-Collection Methods 7

Data-Collection Methods

The ground-water monitoring network, the methods used 

to characterize the hydrogeologic framework, and the 

methods used to collect hydrologic, aquifer sediment, and 

water-quality data are described in this section. The hydrogeologic framework is based on available lithologic and 

geophysical logs, hydraulic conductivity measurements from 

slug tests, and hydraulic-head measurements. Contaminant 

fate-and-transport interpretations are based on organiccarbon measurements in aquifer sediments and laboratory 

and field analyses on ground-water samples.

Monitoring Network

The ground-water monitoring network used in this 

study consists of 44 polyvinylchloride (PVC) monitor wells, 

7 short-screened (3 ft or less), stainless-steel piezometers in 

the drainage ditches and Pipe Elm Branch, and 2 multilevel 

piezometers that were installed adjacent to long-screened 

wells. At many locations, well clusters installed during 

previous investigations were used to obtain data from 

varying depths in the surficial aquifer. Well locations and 

construction details are shown in figure 2 and table 1, 

respectively.

The monitoring network consists of a core group of 

34 wells that are part of the current long-term monitoring 

program, plus a group of additional wells and piezometers 

that were sampled in an attempt to achieve better plume 

characterization. A list of the core group of wells was 

submitted to the USEPA in the external draft of the longterm monitoring plan (J. Williams and L.J. Bachman, 

U.S. Geological Survey, written commun., 1999).

The two multilevel piezometers, GS045ML and 

GS047ML, were installed by the USGS in 1999 to obtain 

better vertical resolution of contaminant distributions 

adjacent to two contaminated wells, MW011 and MW039S, 

that are located near the source areas. These piezometers 

are constructed with a series of 1/8-in. (inch) outer diameter 

(OD) Teflon sample tubes strapped to a 3/4-in. PVC center 

tube. The ends of the sample tubes are covered with nylon 

screen. Each piezometer contains multiple sample tubes 

spaced evenly over a given interval to obtain a vertical 

water-quality profile. GS045ML contains eight ports spaced 

at 5-ft intervals between 19.3 and 54.3 ft bgs (below ground 

surface). GS047ML contains five ports spaced at 2-ft 

intervals between 13.0 and 21.0 feet bgs.

Development of Hydrogeologic Framework

The hydrogeologic framework of the natural attenuation 

study area was developed primarily from geologic logs 

obtained by the USGS and other contractors during monitor 

well and piezometer installation, geophysical logs from 

selected wells, and interpretations from available reports 

(Hinaman and Tenbus, 2000, p. 7). The geophysical logs are 

included in the data report by Beman and others (1999, 

p. 82). The lithologic and geophysical logs (primarily 

gamma logs) were used to prepare a hydrogeologic section 

(fig. 3) parallel to the general direction of ground-water flow.

Hydrologic Data Collection

Fifteen wells in the area were slug-tested to obtain 

estimates of hydraulic conductivity in the surficial aquifer 

(fig. 4). Rising-head tests were done with an air-pressurized 

slug testing system (Greene and Shapiro, 1995). To obtain 

hydraulic conductivity values, the empirical Bouwer and 

Rice solution (Bouwer and Rice, 1976) for aquifers under 

unconfined conditions was applied to the rising-head data. 

Values range from 3 ft/d (feet per day) (GS003M) to 313 ft/d 

(MW037D), with a geometric mean of 43 ft/d (table 2). 

Most of the high values were obtained from tests on the 

wells that screen the base of the surficial aquifer. This result 

is consistent with the available geologic information, which 

indicates that the deposit that forms the surficial aquifer 

coarsens downward.

A synoptic round of water-level measurements was conducted on March 15–16, 2000 at most of the available wells 

in the EMU to determine ground-water flow directions and 

hydraulic-head gradients (Appendix A). These measurements were used to develop plan-view potentiometricsurface maps for the shallow and deep parts of the surficial 

aquifer, and a hydrogeologic section parallel to the general 

ground-water flow direction. Data from continuous recorders on wells in the natural attenuation study area indicated 

that water levels rose sharply during the 2-month period 

preceding this synoptic round of measurements, indicating 

that a spring recharge event was affecting the ground-water 

flow system when these measurements were taken.

Sediment Organic-Carbon Collection

Samples were collected from three boreholes to 

determine concentrations of organic carbon in the aquifer 

sediment (fig. 4). These data were used to estimate retardation factors for the organic contaminants in ground water. 

Ten samples were obtained from shallow, intermediate, and 

deep depth intervals within the surficial aquifer. Samples of 

aquifer sediment were obtained from cores collected with a 

Geoprobe direct-push drill rig. To obtain a sample, aquifer 

sediment was removed from a 1-ft section of core with a 

stainless-steel spatula. Sediment adjacent to the core-tube 

wall was discarded. After collection, samples were refrigerated until shipment to the University of Waterloo in 

Waterloo, Ontario, for analysis using the method described 

in Churcher and Dickhout (1987). Concentrations, expressed as the weight fraction of organic carbon (foc) on a dry 

weight basis (grams of naturally occurring carbon per gram 

of dry aquifer sediment), ranged from 0.00007 to 0.00087, 

with an arithmetic mean of 0.00029 (table 3). The method 

detection limit (MDL) for the analysis is 0.00004.

Ground-Water Sample Collection and Analysis

Ground-water and quality-control (QC) sample collection and analysis procedures for the January–March 2000 

sampling round are described in detail in the data report 

prepared by Alexander and others (K.C. Alexander and 

others, U.S. Geological Survey, written commun., 2000).

To obtain representative ground-water samples, wells 

were purged of approximately three well volumes of water 
8 Assessment of Natural Attenuation of Ground-Water Contamination at Dover Air Force Base, Delaware

Table 1. Construction details for wells and piezometers sampled at 

Dover Air Force Base, Delaware, January–March 2000

[ft, feet; ft bgs, feet below ground surface; ft s.l., feet above or below (negative altitude) sea level; 

in., inch; the last letter of a well or piezometer identification number indicates the following sampling 

location depths—S, H, or B, shallow; M, intermediate; D, deep; ML, multilevel piezometer]

Well and

piezometer

identification

number

Screen

length

 (ft)

Total 

depth

(ft bgs)

Altitude of 

ground surface

 (ft s.l.)

Altitude 

of top 

of screen

(ft s.l.)

Altitude

of bottom

of screen

(ft s.l.)

DM101S 10 16.2 23.1 16.92 6.92

DM101D 10 53.0 22.9 -20.11 -30.11

DM107S 10 16.4 13.8 7.47 -2.53

DM107D 10 38.8 13.6 -15.15 -25.15

DM108S 10 16.9 11.7 4.75 -5.25

DM108D 10 32.8 11.5 -11.33 -21.33

DM109S 10 16.7 21.6 14.88 4.88

DM109D 10 43.9 21.4 -12.54 -22.54

DM110S 10 20.3 25.7 15.33 5.33

DM110D 10 73.4 25.4 -38.00 -48.00

DM113S 10 18.8 20.9 12.06 2.06

DM113D 10 63.6 20.9 -32.68 -42.68

GS002M 1 12.6 7.1 -4.49 -5.49

GS002D 3 23.0 7.4 -12.60 -15.60

GS003S 3 9.0 4.8 -1.28 -4.28

GS003M 3 14.9 4.4 -7.53 -10.53

GS004S 0.5 5.8 3.3 -2.03 -2.53

GS004M 1 9.8 3.8 -5.07 -6.07

GS004D 3 21.2 4.5 -13.73 -16.73

GS010S 5 25.2 16.9 -3.29 -8.29

GS010D 5 48.8 16.9 -26.89 -31.89

GS011S 5 17.4 20.1 7.73 2.73

GS011D 5 43.5 20.2 -18.34 -23.34

GS012S 10 15.4 21.0 15.53 5.53

GS012D 5 52.0 20.9 -26.09 -31.09

GS013D 10 34.8 6.8 -17.96 -27.96

GS014D 10 64.8 23.3 -31.51 -41.51

GSCP3M 3 22.8 16.8 -3.00 -6.00

GSCP5M 3 29.5 18.2 -8.34 -11.34

GSCP6D 3 40.5 18.8 -18.70 -21.70

MW011 44 55.0 19.8 8.74 -35.26

MW019 35 44.2 14.4 5.26 -29.74

MW037S 10 20.3 14.0 3.69 -6.31

MW037D 5 48.0 14.4 -28.61 -33.61

MW038S 10 29.9 17.2 -2.71 -12.71

MW038D 5 57.2 17.2 -34.96 -39.96

MW039S 10 19.6 17.0 7.41 -2.59

MW039D 5 58.0 18.3 -34.70 -39.70

MW040S 10 22.1 18.7 6.57 -3.43

MW040D 5 47.0 18.8 -23.27 -28.27

MW061S 10 27.5 24.7 7.21 -2.79

MW062S 10 25.0 19.5 4.49 -5.51

MW062D 5 67.7 19.8 -42.94 -47.94
Data-Collection Methods 9

Table 1. Construction details for wells and piezometers sampled at 

Dover Air Force Base, Delaware, January–March 2000—Continued

[ft, feet; ft bgs, feet below ground surface; ft s.l., feet above or below (negative altitude) sea level; 

in., inch; The last letter of a well or piezometer identifideepowing sampling 

location depths—S, H, B, shallow; M, intermediate; D, deeep; ML, multilevel piezometer]

Well and

piezometer

identification

number

Screen

length

 (ft)

Total 

depth

(ft bgs)

Altitude of 

ground surface

 (ft s.l.)

Altitude

of top

of screen

(ft s.l.)

Altitude

of bottom

of screen

(ft s.l.)

MW064S 10 34.8 21.7 -3.09 -13.09

MWD4H 3 9.5 19.0 12.47 9.47

MWD4B 10 12.9 14.2 11.25 1.25

MW206M 10 28.3 14.1 -4.22 -14.22

MW206D 10 44.3 14.3 -20.05 -30.05

MW227S 10 22.9 19.4 6.41 -3.59

MW227M 10 33.3 19.4 -3.98 -13.98

MW227D 10 46.2 19.4 -16.85 -26.85

Multilevel

piezometer 

Diameter of

multilevel port

 (in.)

Depth

(ft bgs)

Altitude of

ground surface

 (ft s.l.)

Altitude of multilevel 

piezometer port

 (ft s.l.)

GS045ML 57.0 19.8

Port H 0.125 19.3 19.8 0.45

Port G 0.125 24.3 19.8 - 4.5

Port F 0.125 29.3 19.8 - 9.5

Port E 0.125 34.3 19.8 -14.5

Port D 0.125 39.3 19.8 -19.5

Port C 0.125 44.3 19.8 -24.5

Port B 0.125 49.3 19.8 -29.5

Port A 0.125 54.3 19.8 -34.5

GS047ML 22.0 17.0

Port E 0.125 13.0 17.0 0 4.0

Port D 0.125 15.0 17.0 0 2.0

Port C 0.125 17.0 17.0 0 0.0

Port B 0.125 19.0 17.0 - 2.0

Port A 0.125 21.0 17.0 - 4.0
10 Assessment of Natural Attenuation of Ground-Water Contamination at Dover Air Force Base, Delaware

 
Data-Collection Methods 11

 

prior to sample collection. Purging was stopped when 

temperature, pH, DO, and specific conductance stabilized. 

Measurements were made with field meters connected to a 

flow-through cell. A peristaltic pump was used for purging 

and sampling. Teflon tubing was used to collect groundwater samples. Each day, tubing was decontaminated in the 

laboratory with hot, soapy water and a triple rinse with 

deionized water. To minimize cross-contamination from 

sampling equipment and procedures, wells were sampled 

approximately in the order of least to most contaminated (as 

best known). The water-level measuring tape, electrodes, 

flow-through cell, and glassware also were rinsed with 

deionized water between wells.

Samples were collected from the discharge line of the

peristaltic pump after the wells were purged. Severn Trent 

Laboratory in Arvada, Colorado provided pre-preserved 

sample bottles. Samples requiring laboratory analysis were 

placed on ice in coolers prior to shipment. Field analyses 

were done in a mobile laboratory immediately after sample 

collection.

The ground-water samples and equipment blanks 

discussed in this report were analyzed for VOCs (SW846 

Method 8260), dissolved nutrients (sum of nitrate plus 

nitrite) (MCAWW Method 353.2) and ammonia as 

nitrogen (MCAWW Method 350.1), chloride (MCAWW 

Method 300.0A), and sulfate (MCAWW Method 

300.0A) (U.S. Environmental Protection Agency, 1983, 

1986). Trip and ambient blanks were analyzed for

Table 2. Hydraulic conductivity values from slug tests on wells screened in the

 surficial aquifer in the East Management Unit, Dover Air Force Base, Delaware

[See figure 4 for location of slug-tested wells; ft, feet, ft bgs, feet below ground surface;

 ft/d, feet per day; *, arithmetic mean of values from two slug tests]

Well

Screen

length

(ft)

Total 

depth

(ft bgs)

Hydraulic 

conductivity

(ft/d)

DM101D 10 53.0 85 *

GS001M 1 6.5 4

GS003M 3 14.9 3

GS013D 10 34.8 30

GS303D 10 59.7 22

GSCP3M 3 22.5 24

GSCP4M 3 24.3 46 *

GSCP8 3 33.2 8 *

MW037D 5 48.0 313

MW040D 5 47.0 119

MW206D 10 44.3 110

MW206M 10 28.3 146

MW207D 10 47.5 138 *

MW207M 10 34.7 55 *

MW226M 10 46.9 130

Geometric Mean 1 43

 1 Geometric Mean: 

where

 

 

K K1K2...Kn = n

K = geometric mean of the hydraulic conductivity,

K1K2...Kn = individual values of the hydraulic conductivity, and

n = number of values.
12 Assessment of Natural Attenuation of Ground-Water Contamination at Dover Air Force Base, Delaware

VOCs only. Duplicate ground-water samples from all wells 

also were submitted to the USGS field laboratory at 

Aberdeen Proving Ground, Maryland for methane analysis 

using the procedure described in Baedecker and Cozzarelli 

(1992). Because of the potential for methane loss during and 

after sample collection, the highest concentration from each 

pair of replicate samples that contained methane was 

reported.

Temperature, pH, specific conductance, DO, ferrous and 

total iron, and sulfide were analyzed in the field. Temperature, pH, specific conductance, and DO were measured with 

meters (YSI Model 3000, Orion Model 260, or Hydrolab 

Model H20 multiprobe) connected to a flow-through cell on 

the discharge line of the peristaltic pump. When the DO 

concentration was 2 mg/L or less with the Hydrolab, a 

modified Winkler titration was used to determine the DO 

concentration. This method is more accurate and reproducible at low DO concentrations (Baedecker and 

Cozzarelli, 1992). The MDL for the modified Winkler 

titration is 0.2 mg/L DO.

Ferrous and total iron and sulfide were measured 

colorimetrically immediately after collection in the mobile 

field laboratory. Iron was analyzed with a Hach DR2000 

spectrophotometer. Iron check standards were run daily 

along with ground-water samples. Sulfide was analyzed 

with a Chemetrics spectrophotometer. The spectrophotometers were zeroed with raw ground water before each 

sample was read in the instrument. The MDL for ferrous 

and total iron is 0.02 mg/L.

The QC samples presented in this report include nine 

laboratory duplicates, seven field blanks, three ambient 

blanks, six trip blanks, one equipment blank, and one sourcewater blank, denoted respectively by the extensions “DUP,” 

“FDB,” “AMB,” “TPB,” “EQB,” and “SWB” (K.C. 

Alexander and others, U.S. Geological Survey, written 

commun., 2000). Ground-water sample identifiers consist of 

the well name followed at most locations by an “S,” “M,” 

“D,” or “ML,” which represent shallow wells, intermediate 

wells, deep wells, and multilevel piezometers, respectively. 

Actual well depths are shown in table 1.

Assessment of Natural Attenuation

The hydrogeologic framework and the spatial and temporal concentration trends used to assess natural attenuation 

are presented in this section. The general approach used in 

this report to determine the effectiveness of natural attenuation is consistent with established protocols (Wiedemeier 

and others, 1999, p. 309; National Research Council, 2000, 

p. 150).

Evidence of Natural Attenuation

Natural attenuation refers to the reduction in concentration or mass of a dissolved constituent over time and distance from the source of contamination in response to 

naturally occurring physical, chemical, and biological processes. The main processes in a ground-water environment 

are intrinsic biodegradation, abiotic degradation, adsorption, 

and dispersion. Intrinsic biodegradation typically is the 

dominant natural attenuation process for organic compounds 

(Wiedemeier and others, 1999, p. 164).

Following initial conceptual model development, three 

broadly defined lines of evidence commonly are used at sites 

contaminated with chlorinated solvents to determine the 

extent of natural attenuation (Wiedemeier and others, 1999, 

p. 13):

(1) A historical data base of ground-water samples that 

shows whether the plume is stable or shrinking, and/or 

whether there has been a loss of contaminant mass 

over time. Demonstrating contaminant loss at the 

field scale is the strongest evidence of natural 

attenuation.

(2) Measurements of compounds in ground water to 

identify the characteristic geochemical changes, or 

footprints, of the intrinsic biodegradation reactions 

occurring at the site. These changes include the 

depletion of electron acceptors and production of 

metabolic by-products relative to background conditions, and the depletion of parent compounds and 

production of chlorinated-solvent breakdown products 

along ground-water flow paths. This line of evidence 

Table 3. Weight fractions of organic carbon (foc) 

in surficial-aquifer sediment samples 

collected in the East Management 

Unit, Dover Air Force Base, Delaware

 [See figure 4 for sample-collection locations; ft bgs, feet below 

ground surface; foc , weight fraction of organic carbon; g/g, grams of 

naturally occurring carbon per gram of dry aquifer sediment]

Location

Sample

identification

 Depth

 (ft bgs)

 foc 

 (g/g)

GP553 GSFOC-1 12–16 0.00083

GP553 GSFOC-2 32–35 .00020

GP553 GSFOC-3 42–44 .00008

GS037D GSFOC-4 10–12 .00015

GS037D GSFOC-5 12–16 .00014

GS037D GSFOC-6 24–28 .00011

GS037D GSFOC-7 40–44 .00007

GP554 GSFOC-8 12–16 .00087

GP554 GSFOC-9 24–28 .00039

GP554 GSFOC-10 28–30 .00008

Arithmetic mean: 0.00029
Assessment of Natural Attenuation 13

is strongest when contaminant reductions and biodegradation reactions are linked quantitatively to show 

a cause-and-effect relation (National Research 

Council, 2000, p. 172).

(3) Laboratory microcosm data that demonstrate mass 

loss from biodegradation under the prevailing geochemical conditions in the aquifer.

In this report, concentration trends that show (1) the 

depletion of parent organic compounds, (2) the production of 

chlorinated-solvent breakdown products, and (3) the consumption of electron acceptors and production of metabolic 

by-products along generalized ground-water flow paths are 

used as the main lines of evidence of natural attenuation. 

Time-series graphs of VOC concentrations at selected wells 

within the plumes also are used in this assessment. The 

historical data base is currently inadequate for mapping the 

plumes, and laboratory microcosm data are not available.

Documentation of the disappearance of a contaminant is 

an important indication of natural attenuation, but this 

information alone is generally not sufficient (National 

Research Council, 2000, p. 11). Cause and effect must be 

established by linking the observed contaminant losses to 

natural-attenuation processes occurring in the aquifer. For 

natural attenuation to be acceptable as a remedial measure, it 

also must be shown that the remedial objective established 

for the site is met. For the sites in the EMU, the remedial 

objective is to attain cleanup levels (USEPA MCLs or sitespecific background concentrations) for the compounds of 

concern in the area of attainment, defined as the zone inside 

the boundary of the contaminant plume (Hazardous Waste 

Remedial Actions Program, written commun., 1997). Pointof-compliance (POC) wells are used to determine 

concentrations at the DAFB boundary. The POC wells 

sampled in January–March 2000, GS004S, GS004M, 

GS004D, GS010S, GS010D, GS012S, GS012D, GS014D, 

GSCP3M, GSCP6D, MW227M, and MWD4H, are shown 

on figure 2.

Hydrogeologic Framework

The surficial aquifer may be conceptualized as a single, 

heterogeneous hydrostratigraphic unit that is bounded on the 

top by the water table and on the bottom by the regional 

Calvert confining unit (fig. 3). Ground water flows generally from southeast to northwest, toward Pipe Elm Branch, 

which is the major discharge boundary in the natural 

attenuation study area (figs. 5, 6, and 7). As noted by 

Bachman and others (1998, p. 14), flow systems in the 

surficial aquifer can be categorized with two scales. In the 

shallow flow system, ground water recharged at local 

topographically high areas flows under the influence of 

gravity along shallow flow paths to the nearest discharge 

area. The ground-water traveltimes in these shallow flow 

cells probably range from weeks to years. The deeper flow 

system apparently recharges near a local ground-water 

divide beneath LF13, and possibly further upgradient, and 

discharges to Pipe Elm Branch. The ground-water traveltimes in the deep flow system probably range from years to 

decades. Although hydraulic heads fluctuate seasonally, 

synoptic rounds of hydraulic-head measurements collected 

in 1993–94 (Hinaman and Tenbus, 2000, p. 25–26), in 

1995–96 (Bachman and others, 1998, p. 14) and in 

March 2000 indicate that the general configuration of 

hydraulic heads in the surficial aquifer, and, therefore, 

ground-water flow paths, does not change substantially.

Near LF13, radial flow associated with a local groundwater divide occurs in the upper part of the surficial aquifer 

(fig. 5). This divide appears to be the main recharge boundary for the flow systems in the natural attenuation study area. 

Local water-table “highs” generally form above laterally 

extensive, fine-grained deposits in the surficial aquifer 

(Hinaman and Tenbus, 2000, p. 16). Hydrographs from well 

DM110 demonstrate that the high water table beneath LF13 

persists throughout the year, although the magnitude of the 

downward vertical gradient decreases during the summer 

and fall (Hinaman and Tenbus, 2000, p. 16). 

On March 15–16, 2000, hydraulic gradients varied substantially across the natural attenuation study area (figs. 5 

and 6). Vertical gradients were measured in ground water 

beneath LF13 and Pipe Elm Branch (fig. 7). Near WP14/

LF15, vertical gradients were relatively small and flow was 

predominantly horizontal; the horizontal gradient between 

wells DM107S and DM108S was 0.003 ft/ft (feet per feet) 

(fig. 7). Large horizontal gradients were observed, however, 

in the upper part of the surficial aquifer between the LF13 

and WP14/LF15 source areas. These patterns are consistent 

with previous hydraulic-head distributions in this area 

(Bachman and others, 1998, p. 15; Hinaman and Tenbus, 

2000, p. 26). 

As shown on section A-A′, the surficial aquifer consists 

predominantly of sand and gravel with discontinuous lenses 

of silt and clay (fig. 3). The fine-grained deposits are more 

prevalent in the upper part of the aquifer. With the simplifying assumption of aquifer homogeneity, Darcy’s Law 

(Freeze and Cherry, 1979, p. 15) can be used to estimate a 

representative ground-water flow rate in the natural attenuation study area. Using the geometric mean hydraulic 

conductivity of 43 ft/d calculated from the 15 slug tests 

conducted in the area (table 2), and a horizontal gradient of 

0.003, which is representative of the area between WP14/

LF15 and Pipe Elm Branch, the specific discharge is 0.1 ft/d. 

Using an assumed effective porosity of 0.25 for sand and 

gravel (Wiedemeier and others, 1999, p. 129), the average 

ground-water velocity in the natural attenuation study area is 

0.4 ft/d. At this velocity, the traveltime from source areas 

WP14/LF15 to Pipe Elm Branch at the Base boundary,

a distance of 2,200 ft, is approximately 15 yrs (years). Using 

the highest and lowest measured hydraulic conductivities in 

this area (313 ft/d and 2.9 ft/d), and the representative horizontal hydraulic gradient of 0.003 ft/ft, traveltimes range 

from 2 yrs to 170 yrs. This is consistent with other estimates 

of traveltime in the surficial aquifer beneath DAFB (U.S. 

Army Corps of Engineers and Dames and Moore, Inc., 

1997b; Hinaman and Tenbus, 2000, p. 63), and in other areas 

of the Delmarva Peninsula (Dunkel and others, 1992).
14 Assessment of Natural Attenuation of Ground-Water Contamination at Dover Air Force Base, Delaware

Dispersion, Adsorption, and Abiotic Degradation

Natural gradient experiments have shown that transverse 

dispersion is a weak mixing process in unconsolidated 

geologic deposits (Sudicky, 1986). At FT03, where the 

source area is close to adjacent surface-water discharge 

areas, dispersion probably has a negligible effect on the 

attenuation of the plume prior to discharge. At WP14/LF15 

and LF13, flow paths are longer and the plume migrates 

through more aquifer heterogeneities, so dispersion may 

have a more substantial effect on contaminant distributions 

and on increased mass losses from aerobic biodegradation at 

the plume boundaries. The effects of dispersion cannot be 

determined with the information that is currently (2001) 

available, however.

To obtain a rough estimate of the effect of adsorption on 

contaminant migration velocities in the surficial aquifer, 

retardation factors were calculated for a selected group of 

organic compounds using a simple estimation procedure that 

is based on the organic-carbon content of the aquifer sediment. This procedure assumes that the association between 

the nonpolar organics and the natural organic matter in the 

sediment is the dominant adsorption mechanism. It 

should be noted that in sediments with low organic carbon 

contents (foc less than about 0.002), adsorption to mineral
Assessment of Natural Attenuation 15

surfaces, particularly clays, might become significant 

(Schwarzenbach and others, 1993, p. 284). The importance 

of this sorption mechanism in the surficial aquifer cannot be 

determined with the information that is currently available.

The retardation factor is defined as R = 1+ (ρb /n) Kd ,

where

ρb = the dry bulk density of the aquifer sediment in 

g/cm3 (grams per cubic centimeter),

n = the total porosity, and

Kd = the distribution coefficient in mL/g (milliliters 

per gram) (Freeze and Cherry, 1979, p. 404).
16 Assessment of Natural Attenuation of Ground-Water Contamination at Dover Air Force Base, Delaware

This equation is based on the assumption that adsorption is 

fast, reversible, and independent of the concentration of the 

organic constituent. Distribution coefficients were estimated 

from the relation Kd = foc Koc ,

where

foc = the mean weight fraction of organic carbon in 

the aquifer in g/g (grams of naturally 

occurring organic carbon per gram of dry 

aquifer sediment), and

Koc = the organic-carbon distribution coefficient in 

mL/g (milliliters per gram).

Organic-carbon distribution coefficients were obtained from 

the published literature (table 4). Using assumed values for 

dry bulk density and porosity of 1.7 g/cm3

 and 0.35, respectively (Freeze and Cherry, 1979, p. 405), the measured 

arithmetic mean foc of the aquifer sediment of 0.00029, and 

the literature Koc values, retardation factors ranging from 

1.35 (PCE) to 1.00 vinyl chloride (VC) were calculated 

(table 4). Corresponding dimensionless relative velocities, 

defined as the migration velocity of the contaminant divided 

by the ground-water velocity, ranged from 0.74 (PCE) to 

1.00 (VC). The low values for the distribution coefficients 

also indicate that the mass adsorbed to the aquifer solids is 

small relative to the mass in solution.

On the basis of the calculations presented above, adsorption to aquifer solids is not a strong natural attenuation 

mechanism in the surficial aquifer. The amount of adsorp-
Assessment of Natural Attenuation 17

 

tion may be substantially greater in the finer-grained sediments around LF13 and the drainage ditches, which likely 

contain greater amounts of both organic carbon and clay. 

For the relatively coarse layers through which most of the 

contaminant migration occurs, however, both the clay and 

organic contents are low, and the effect of adsorption on 

migration velocities and bioavailable contaminant mass are 

expected to be small.

The potential for abiotic degradation in the natural 

attenuation study area is discussed in detail in Bachman 

and others (1998, p. 31). In short, abiotic degradation of 

the chlorinated ethenes is not important because half-lives 

are very long (for example, in the range of 106 to 1010 yr) 

relative to the decadal residence times in the natural attenuation study area. Chlorinated ethanes, on the other hand, 

may undergo relatively rapid hydrolysis. Half-lives range 

from 0.01 to 72.0 yr (Bachman and others, 1998, p. 13). 

1,1,2,2-TeCA (1,1,2,2-tetrachloroethane) degrades to trichloroethene under alkaline conditions (Jeffers and others, 

1989). Because chlorinated ethanes have been detected 

periodically in the WP14/LF15 source area, TCE could be 

produced from abiotic degradation reactions. Based on 

information that is currently available, however, these compounds are not widespread in the natural attenuation study 

area, so the production of TCE by this mechanism is not 

expected to be important.

Intrinsic Biodegradation

Intrinsic biodegradation refers to contaminant transformation mediated by the indigenous microbial community 

in the aquifer. The anaerobic degradation pathways for the 

chlorinated ethenes and ethanes are shown in figure 8. The 

fuel hydrocarbons biodegrade relatively easily in ground 

water under both aerobic and anaerobic conditions. By 

depleting electron acceptors, they create the reducing conditions necessary for the efficient biodegradation of the 

chlorinated solvents. Converging lines of evidence from the 

spatial and temporal distributions of VOCs, electron 

acceptors, and metabolic by-products were used to assess the 

occurrence and extent of intrinsic biodegradation in the 

natural attenuation study area.

Areal Distribution of Electron Acceptors and 

Metabolic By-Products In response to the presence of 

biodegradable carbon, aquifer microorganisms will consume 

electron acceptors and produce metabolic by-products, 

causing changes in the prevailing geochemistry of the 

ground water. A well-defined sequence of microbially 

mediated oxidation-reduction reactions, based on freeenergy yield, is observed in most contaminated aquifers. 

Dissolved-oxygen reduction occurs first, followed by nitrate 

reduction, manganese (IV) reduction, iron (III) reduction, 

sulfate reduction, and finally methane production 

(Weidemeier and others, 1999, p. 165; Chapelle and others, 

1995). The consumption of electron acceptors is 

accompanied by the production of characteristic byproducts. These by-products include reduced nitrogen, 

manganese and iron species, sulfide, methane, and carbon 

dioxide, leading to increased levels of dissolved inorganic 

carbon (DIC).

When chlorinated solvents are biodegraded, chloride ion 

also is produced as a metabolic by-product. Thus, under 

ideal conditions when background chloride concentrations 

are low, for example, elevated chloride concentrations can be 

used as evidence of chlorinated solvent degradation. The 

chloride results from the January–March 2000 sampling 

round (table 5) were inconclusive, however, and will not be

Table 4. Retardation factors and relative velocities for selected volatile organic compounds

detected in the natural attenuation study area, Dover Air Force Base, Delaware

[Koc, organic carbon-based partition coefficient; Kd, distribution coefficient; R, retardation factor; vc, velocity of contaminant front; 

v, average linear ground-water velocity; mL/g, milliliters per gram; --, dimensionless quantity]

Compound

log Koc

(mL/g) Reference

Kd

(mL/g)

R

(--)

vc /v

(--)

Tetrachloroethene 2.4 Abdul and others, 1987 0.073 1.35 0.74

Trichloroethene 1.8 Abdul and others, 1987 0.018 1.09 0.92

cis-1,2-Dichloroethene 1.4 Wiedemeier and others, 1999, p. 145 0.0073 1.03 0.97

Vinyl chloride 0.39 Montgomery, 1996, p. 1,050 0.0007 1.00 1.00

1,1,2,2-Tetrachloroethane 2.07 Schwille, 1988, p. 130 0.034 1.16 0.86

Chlorobenzene 2.2 Walton and others, 1992 0.046 1.22 0.82

Benzene 1.7 Abdul and others, 1987 0.014 1.07 0.93
18 Assessment of Natural Attenuation of Ground-Water Contamination at Dover Air Force Base, Delaware

 

Table 5. Concentrations of nutrients, sulfate, and chloride in ground water from wells 

screened in the surficial aquifer at sites FT03, LF13, and WP14/LF15,

Dover Air Force Base, Delaware, January–March 2000—Continued

Sample

identification

number

Collection

date

Nitrate plus nitrite

 (mg/L as N)

 

 Ammonia

 (mg/L as N)

 Sulfate

 (mg/L)

 Chloride

 (mg/L)

RL: 0.10 0.10 5.0 3.0

DM101D 01/12/2000 0.30 ND 3.2 B 11.9

DM101S 01/12/2000 .63 ND 220 Q 6.3

DM107D 02/24/2000 .75 0.029 B 3.6 B 9.9

DM107S 02/24/2000 .16 .033 B 15.0 5.6

DM108D 02/24/2000 .20 .040 B 4.4 B 11.4

DM108D.DUP 02/24/2000 .53 .034 B 4.5 B 11.4

DM108S 02/24/2000 .55 .034 B 12.9 9.8

DM109D 01/19/2000 .066 B .088 B 18.5 9.8

DM109S 01/19/2000 ND ND 176.0 Q 16.6

DM110D 02/25/2000 1.1 .038 B 46.7 6.2

DM110D.DUP 02/25/2000 1.1 .091 B 49.0 6.1

DM110S 02/29/2000 .17 .29 94.5 Q 95.3 Q

DM110S.DUP 02/29/2000 .28 .37 97.1 Q 99.1 Q

DM113D 03/09/2000 2.1 .034 B 1.2 B 10.8

DM113S 03/09/2000 .33 .065 B 22.0 3.6

GS002D 02/16/2000 ND .38 B, G 3.4 11.6

GS002M 02/10/2000 .41 B, G .23 B, G 4.5 B 10.8

GS003M 02/23/2000 .46 .38 19.9 4.6

GS003S 02/23/2000 ND .11 14.6 16.7

GS004D 02/25/2000 .013 B .28 18.2 5.2

GS004D.DUP 02/25/2000 .021 B .30 17.7 5.0

GS004M 02/15/2000 ND G .17 16.7 10

GS004S 02/15/2000 ND G .24 16.6 9.6

GS010D 02/17/2000 .20 B, G .32 B, G 10 11.4

GS010S 02/17/2000 .26 B, G 1.2 G 36.2 18.9

GS010S.DUP 02/17/2000 .14 B, G .68 G 35.8 19.1

GS011D 01/13/2000 .53 .65 16.2 12.2

GS011S 01/13/2000 ND .44 416. Q 8.1

GS012D 02/17/2000 .38 B, G .98 B, G 5.6 12.1

GS012S 02/17/2000 ND .34 B, G 170 Q 78.0

GS013D 02/29/2000 .087 B .42 14.4 5.0

GS014D 02/16/2000 1.0 .31 B, G 22.5 11.1

GS045MLA 03/13/2000 ND .18 1.6 B 10.8

GS045MLB 03/13/2000 1.1 .035 B 3.2 B 12.5

GS045MLC 03/13/2000 1.7 .14 1.0 B 13.6

GS045MLD 03/13/2000 1.0 .030 B 2.8 B 5.6

GS045MLE 03/13/2000 ND .047 B 9.1 11.6

GS045MLF 03/13/2000 ND .085 B 10.9 4.0

GS045MLG 03/13/2000 ND .066 B 13.4 4.8

GS045MLH 03/13/2000 ND .060 B 12.9 5.6

GS045MLH.DUP 03/13/2000 ND .064 B 13.0 5.8

Table 5. Concentrations of nutrients, sulfate, and chloride in ground water from wells 

screened in the surficial aquifer at sites FT03, LF13, and WP14/LF15,

Dover Air Force Base, Delaware, January–March 2000

[mg/L, milligrams per liter; N, nitrogen; DUP, duplicate sample; EQB, equipment blank; SWB, source-water blank; FDB, field blank; B, estimated 

result; result is less than the reporting limit; RL, reporting limit; ND, not detected; Q, elevated reporting limit; the reporting limit is elevated due to 

high analyte levels; G, elevated reporting limit; the reporting limit is elevated because of matrix interference]
Assessment of Natural Attenuation 19

Table 5. Concentrations of nutrients, sulfate, and chloride in ground water from wells 

screened in the surficial aquifer at sites FT03, LF13, and WP14/LF15,

Dover Air Force Base, Delaware, January–March 2000—Continued

[no., number; mg/L, milligrams per liter; N, nitrogen; DUP, duplicate sample; EQB, equipment blank; SWB, source-water blank; FDB, field 

blank; B, estimated result. Result is less than the laboratory reporting limit; RL, reporting limit; ND, not detected; Q, elevated reporting limit. The 

reporting limit is elevated due to high analyte levels; G, elevated reporting limit. The reporting limit is elevated because of matrix interference]

Sample

identification

number

Collection

date

Nitrate plus nitrite

 (mg/L as N)

 

 Ammonia

 (mg/L as N)

 Sulfate

 (mg/L)

 Chloride

 (mg/L)

RL: 0.10 0.10 5.0 3.0

GS047MLA 03/14/2000 1.6 0.040 B 0.97 B 8.2

GS047MLB 03/14/2000 .90 .060 B 1.0 B 9.2

GS047MLC 03/14/2000 .14 .055 B 1.8 B 10.5

GS047MLD 03/14/2000 .10 .059 B 24.0 12.2

GS047MLE 03/14/2000 .16 .098 B 28.3 9.4

GSCP3M 02/15/2000 ND G .24 3.4 B 14.4

GSCP5M 03/06/2000 ND ND 7.1 14.2

GSCP6D 02/09/2000 ND G .16 11.4 17.9

MW011 03/14/2000 .34 B .078 B 12.1 5.9

MW011.DUP 03/14/2000 .73 .033 B 15.8 11.0

MW019 02/25/2000 .58 .38 9.9 8.0

MW037S 03/10/2000 .27 .035 B 22.4 6.2

MW037D 03/10/2000 .44 .030 B 10.2 10.2

MW038D 03/09/2000 .081 B .038 B 5.5 9.8

MW038S 03/09/2000 ND .055 B 26.7 6.3

MW039D 02/23/2000 1.1 .032 B 5.6 8.4

MW039S 03/14/2000 .73 .033 B 15.8 11.0

MW039S.DUP 03/14/2000 .55 .057 B 15.8 11.0

MW040D 03/07/2000 .88 .14 6.8 7.8

MW040S 03/07/2000 1.7 .030 B 10.7 8.0

MW061S 02/29/2000 .14 .28 11.9 175 Q

MW062D 03/10/2000 2.2 .034 B .79 B 7.8

MW062S 02/11/2000 .25 B, G 1.2 G 5.5 19.5

MW064S 02/16/2000 ND G .71 G 250 Q 32.8

MW206D 03/06/2000 .60 .036 B 2.2 B 10.7

MW206M 03/06/2000 .85 ND 8.9 8.6

MW206M.DUP 03/06/2000 .70 ND 8.7 8.7

MW227D 03/07/2000 .44 .032 B 10.2 8.6

MW227M 02/10/2000 .49 .044 B 15.2 5.9

MW227S 03/07/2000 ND .14 49.7 40.5

MWD4B 03/06/2000 1.6 ND 16.4 4.6

MWD4H 03/15/2000 .029 B .073 B 15.7 5.7

JAN0500.EQB 01/05/2000 ND ND ND ND

JAN0500.SWB 01/05/2000 ND ND ND ND

DM107S.FDB 02/24/2000 ND .05 B ND ND

GS013D.FDB 02/29/2000 ND .33 ND ND

GSCP6D.FDB 02/09/2000 ND 3.7 ND ND

MW011.FDB 03/14/2000 .025 B .14 ND .70 B

MW019.FDB 02/25/2000 ND .032 B ND ND

MW064S.FDB 02/16/2000 .012 B .33 B, G ND .79 B

MW227S.FDB 03/07/2000 ND ND 0.97 B .73 B
20 Assessment of Natural Attenuation of Ground-Water Contamination at Dover Air Force Base, Delaware

 

discussed further in this report. This reaction footprint may 

have been difficult to observe because the concentrations of 

the chlorinated solvents in the plumes are low (less than

1 mg/L generally).

Nitrate and sulfate reduction do not appear to be important reactions in the natural attenuation study area. Because 

of a lack of nitrogen input to the surficial aquifer underlying 

DAFB, nitrate concentrations (nitrate plus nitrite as N in 

tables) in ground water are low (table 5). Sulfate concentrations were variable across the natural attenuation study area, 

and widespread sulfate depletion in contaminated ground 

water was not evident (table 5).

Based on the data collected for this study (table 6), 

oxygen reduction, iron reduction and methanogenesis are the 

dominant terminal electron-accepting processes (TEAPs) in 

the natural attenuation study area. For comparative purposes, the definitions provided by Bachman and others 

(1998, p. 23) of the three main TEAPs in the area are used in 

this discussion: aerobic conditions prevail when DO concentrations are greater than 1 mg/L; iron-reducing when DO 

concentrations are less than 1 mg/L and ferrous iron concentrations are greater than 1 mg/L; and mixed methanogenic and iron-reducing when DO concentrations are less 

than 1 mg/L, ferrous iron concentrations are greater than 

1 mg/L, and dissolved methane concentrations are greater 

than 100 μg/L (micrograms per liter).

The areal distributions of DO, ferrous iron, and methane 

are shown in figures 9a, 9b, and 9c. For illustrative purposes, concentrations in ground water from wells and piezometers screened in the upper part of the surficial aquifer,
Assessment of Natural Attenuation 21

 

Table 6. Concentrations of field constituents and methane in ground water from wells screened 

in the surficial aquifer at sites FT03, LF13, and WP14/LF15, Dover Air Force Base, Delaware, 

January–March 2000

[mg/L, milligrams per liter; μg/L, micrograms per liter; μS/cm, microsiemens per centimeter; °C, degrees Celsius; <, less than method detection 

limit; >, estimated result; concentration exceeded highest calibration standard; DUP, duplicate sample; FDB, field blank; NS, not sampled]

Sample

identification

number 

Collection

date

Ferrous 

iron

(mg/L)

Total

iron

(mg/L)

Sulfide

(mg/L)

Methane

(μg/L)

Dissolved 

oxygen

(mg/L)

pH

(standard

units)

Specific

conductance

(μS/cm)

Temperature

(°C)

DM101D 01/12/2000 0.14 1.2 <0.01 42 0.81 7.6 159 12.6

DM101S 01/12/2000 < .02 .07 < .01 <10 4.5 5.6 625 11.7

DM107D 02/24/2000 .02 .02 < .01 21 3.1 5.4 61 14.6

DM107S 02/24/2000 < .02 .02 < .01 <9.2 4.7 5.2 55 11.3

DM108D 02/24/2000 .04 .19 < .01 58 .59 5.4 64 13.9

DM108D.DUP 02/24/2000 .05 .17 < .01 NS .67 NS NS NS

DM108S 02/24/2000 .15 .20 < .01 <9.4 1.0 5.0 68 12.2

DM109D 01/19/2000 .28 1.2 < .01 <9.9 .76 7.7 122 11.9

DM109S 01/19/2000 < .02 .07 < .01 <8.8 3.7 5.4 430 11.8

DM110D 02/25/2000 .03 .08 < .01 <9.7 6.0 5.0 130 14.2

DM110D.DUP 02/25/2000 NS NS NS <10 NS NS NS NS

DM110S 02/29/2000 36 37 .15 8,700 < .20 5.9 724 11.3

DM113D 03/09/2000 < .02 < .02 < .01 <9.5 4.6 5.5 84 14.2

DM113S 03/09/2000 < .02 < .02 < .01 <9.5 4.6 5.0 88 11.8

GS002D 02/16/2000 1.0 1.4 < .01 47 1.5 5.3 73 14.2

GS002M 02/10/2000 8.0 8.4 < .01 <9.2 < .20 5.5 102 11.7

GS003M 02/23/2000 < .02 < .02 < .01 <9.5 5.9 5.5 72 12.9

GS003S 02/23/2000 10 10 < .01 1,500 < .20 6.1 206 12.8

GS004D 02/25/2000 4.4 4.6 .01 39 < .20 6.0 83 13.2

GS004M 02/25/2000 6.8 7.1 .01 <9.5 1.1 5.4 103 10.2

GS004S 02/15/2000 7.2 8.2 .03 <10 .52 5.2 101 8.1

GS010D 02/17/2000 < .02 < .02 < .01 <9.4 1.1 5.0 60 10.7

GS010S 02/17/2000 4.8 5.5 < .01 14 < .20 5.5 142 12.3

GS010S.DUP 02/17/2000 5.0 5.5 < .01 15 NS NS NS NS

GS011D 01/13/2000 .31 .34 < .01 <10 2.2 4.3 108 13.6

GS011S 01/13/2000 12 14 < .01 30 < .20 5.2 1,270 12.4

GS012D 02/17/2000 3.1 4.0 < .01 15 < .20 5.5 74 11.0

GS012S 02/17/2000 1.9 4.2 < .01 13 .91 5.0 436 10.0

GS013D 02/29/2000 10 10 < .01 <9.5 < .20 6.4 136 14.1

GS014D 02/16/2000 .35 .37 < .01 <10 2.2 4.0 136 14.2

GS045MLA 03/13/2000 19 19 NS 14.2 1.5 NS NS NS

GS045MLB 03/13/2000 < .02 .03 NS <9.4 4.1 NS NS NS

GS045MLC 03/13/2000 < .02 2.9 NS <9.7 6.8 NS NS NS

GS045MLD 03/13/2000 < .02 < .02 NS <10.1 3.4 NS NS NS

GS045MLE 03/13/2000 .34 .40 NS <9.7 3.4 NS NS NS

GS045MLF 03/13/2000 2.8 .04 NS <9.4 2.6 NS NS NS

GS045MLG 03/13/2000 2.3 2.4 NS NS 2.7 NS NS NS

GS045MLH 03/13/2000 5.8 6.1 NS 481.9 2.2 NS NS NS

GS045MLH.DUP 03/13/2000 NS NS NS NS 1.7 NS NS NS

GS047MLA 03/14/2000 .27 .39 NS <8.9 1.8 NS NS NS

GS047MLB 03/14/2000 .37 .45 NS 13.4 .72 NS NS NS

GS047MLC 03/14/2000 1.0 1.1 NS 472.9 .59 NS NS NS

GS047MLD 03/14/2000 2.2 2.5 NS 704 .48 NS NS NS

GS047MLE 03/14/2000 1.6 2.4 NS 91.4 1.5 NS NS NS
22 Assessment of Natural Attenuation of Ground-Water Contamination at Dover Air Force Base, Delaware

 

Table 6. Concentrations of field constituents and methane in ground water from wells screened 

in the surficial aquifer at sites FT03, LF13, and WP14/LF15, Dover Air Force Base, Delaware, 

January–March 2000—Continued

[no., number; mg/L, milligrams per liter; μg/L, micrograms per liter; μS/cm, microsiemens per centimeter; °C, degrees Celsius; <, less than 

method detection level; DUP, duplicate sample; FDB, field blank; NS, not sampled; E, estimated result. Concentration exceeded highest 

calibration standard]

Sample

identification

number

Collection

date

Ferrous 

iron

(mg/L)

Total

iron

(mg/L)

Sulfide

(mg/L)

Methane

(μg/L)

Dissolved 

oxygen

(mg/L)

pH

(standard

units)

Specific

conductance

(μS/cm)

Temperature

(°C)

GSCP3M 02/15/2000 11 11 0.01 170 0.58 6.0 126 11.4

GSCP5M 03/06/2000 3.6 5.4 < .01 111.8 .32 5.3 105 14.8

GSCP6D 02/09/2000 5.6 6.6 < .01 <8.4 < .20 4.9 126 13.2

MW011 03/14/2000 3.0 4.7 < .01 430 .29 5.6 116 13.7

MW019 02/25/2000 3.8 4.4 < .01 1,100 < .20 5.8 101 14.8

MW037D 03/10/2000 < .02 < .02 < .01 <9.4 2.2 5.1 85 15.8

MW037S 03/10/2000 .50 .72 < .01 <9.5 1.7 5.4 110 13.3

MW038D 03/09/2000 .49 .51 < .01 <9.3 .67 6.3 81 14.9

MW038S 03/09/2000 1.5 1.8 < .01 <10.0 .19 5.1 110 14.9

MW039D 02/23/2000 < .02 < .02 < .01 <9.2 5.7 5.5 58 13.7

MW039S 03/14/2000 < .02 < .02 < .01 370 1.0 5.5 131 12.8

MW040D 03/07/2000 < .02 < .02 < .01 <9.5 4.8 5.4 72 14.4

MW040S 03/07/2000 < .02 < .02 < .01 <9.3 3.9 5.2 96 13.1

MW061S 02/29/2000 36 37 .13 >9,200 < .20 6.0 1,206 14.4

MW062D 03/10/2000 < .02 < .02 < .01 <9.5 6.6 5.5 70 14.0

MW062S 02/11/2000 3.8 6.2 < .01 190 < .02 4.9 147 13.2

MW064S 02/16/2000 19 21 < .01 34 < .02 4.8 678 13.4

MW206D 03/06/2000 < .02 < .02 < .01 <9.4 2.3 5.3 73 14.3

MW206M 03/06/2000 .30 .28 < .01 142.9 .45 5.4 97 14.2

MW206M.DUP 03/06/2000 .30 .27 < .01 NS NS NS NS NS

MW227D 03/07/2000 < .02 < .02 < .01 <9.6 2.8 5.2 80 14.4

MW227M 03/07/2000 .14 .51 < .01 <9.6 2.5 3.4 85 13.5

MW227S 03/07/2000 2.5 3.0 < .01 <9.5 < .20 5.3 265 11.9

MWD4B 03/06/2000 < .02 < .02 < .01 <9.5 4.4 6.0 150 10.2

MWD4H 03/15/2000 < .02 .05 < .01 NS NS 6.0 67 12.3

DM107S.FDB 02/24/2000 NS NS NS <9.3 NS NS NS NS

GS013D.FDB 02/29/2000 NS NS NS <9.6 NS NS NS NS

GSCP6D.FDB 02/09/2000 NS NS NS <9.5 NS NS NS NS

MW011.FDB 03/14/2000 NS NS NS <9.1 NS NS NS NS

MW019.FDB 02/25/2000 NS NS NS <9.5 NS NS NS NS

MW064.FDB 02/16/2000 NS NS NS <9.5 NS NS NS NS

MW227S.FDB 03/07/2000 < .02 < .02 < .01 <9.5 NS NS NS NS
Assessment of Natural Attenuation 23

the most heavily contaminated vertical interval, are plotted 

on these figures. These figures show the zones where DO is 

less than 1 mg/L, ferrous iron is greater than 1 mg/L, and 

methane is greater than 100 μg/L. Three discrete zones with 

depleted DO and elevated iron and methane coincide with 

the three source areas and the area downgradient of WP14/

LF15.

Using the TEAP definitions given above, mixed methanogenic/iron-reducing conditions have formed beneath all 

three source areas (fig. 10). Zones with iron reduction as the 

dominant TEAP surround the methanogenic zones beneath 

the WP14/LF15 and LF13 source areas. With the exception 

of higher methane concentrations at some of the wells downgradient of WP14/LF15, the distribution of dominant TEAPs 

is similar to the distribution in 1995–96 (Bachman and 

others, 1998, p. 24). This distribution indicates that the 

spatial patterns of redox conditions have not changed substantially.

The apparent co-existence of ferrous iron and methane is 

attributed to the length of the well screens. Long well 

screens, defined in this report as screens greater than 3 ft in 

length, likely produce samples that are a mixture of waters 

from vertically stratified redox environments (Smith and 

others, 1991). In actuality, the anaerobic reactions that 

produce ferrous iron and methane probably occur in discrete, 

narrow intervals that are controlled by the heterogeneity of 

the aquifer. Some ground-water samples also contain appreciable amounts of DO as well as ferrous iron and methane. 

This apparent redox disequilibrium also is attributed to a 

mixing of different redox environments. On a larger scale, 

however, the spatial patterns of depleted oxygen and 

elevated reduced species such as iron and methane are fairly 

consistent (figs. 9a, 9b, and 9c).
24 Assessment of Natural Attenuation of Ground-Water Contamination at Dover Air Force Base, Delaware

The presence of high concentrations of methane indicates 

that fermentation occurs near all of the contaminant source 

areas. Methane production often is used as an indicator of 

active fermentation because the hydrogen produced by the 

fermentation process is used as an electron donor by methanogens. If fermentable substrate is abundant within the 

plume, fermentation reactions will continue downgradient. 

On the basis of methane concentrations from samples 

downgradient of WP14/LF15 and LF13, fermentation is 

restricted to a zone that extends roughly 500 ft downgradient of the sources (fig. 9c). The available information 

indicates, therefore, that the dominant TEAP grades from 

methanogenic/iron-reducing to iron-reducing with distance 

from the source, and that the strongly reducing conditions 

required for dechlorination are present in ground water 

beneath the source areas.

The reaction footprints discussed in this section indicate

that electron acceptors are consumed in microbially mediated reactions with reactive organic contaminants. Aerobic 

conditions in the surficial aquifer indicate that the naturally 

occurring carbon is not sufficiently reactive to rapidly 

deplete the DO. As oxygenated water flows through the 

source areas, DO is consumed rapidly in reactions with 

labile compounds such as benzene, toluene, ethylbenzene, 

and the xylene isomers (BTEX), the most abundant soluble 

compounds of fuel hydrocarbons. If reactive carbon in the 

source areas is abundant, competing anaerobic electron 

acceptors also will be depleted and fermentation will be the 

dominant process. Fermentation reactions yield substantially less energy in comparison to oxidation reactions, so 

fermentation (and methane production) generally occur only 

when other electron acceptors are not available (Wiedemeier 

and others, 1999, p. 249). Benzene, toluene, ethylbenzene 

and the xylene isomers are known to biodegrade under a
Assessment of Natural Attenuation 25

wide range of anaerobic conditions, including methanogenic 

conditions (Grbic-Galic and Vogel, 1987; Cozzarelli and 

others, 1990).

Areal Distribution of Fuel Hydrocarbons Fuel 

hydrocar-bons are susceptible to natural attenuation and 

consequently form relatively short plumes (typically less 

than 400 ft) (Wiedemeier and others, 1999, p. 10). Thus, 

their main importance at the sites in the EMU is to create the 

strongly reducing conditions that are required for the 

biodegradation of the chlorinated solvents. The 

fermentation of the soluble fuel compounds produces 

hydrogen, which is a required electron donor for the 

halorespiring bacteria that carry out reductive dechlorination 

(Wiedemeier and others, 1999, 

p. 247). If the hydrocarbon source becomes depleted, hydrogen production from fermentation may cease, allowing the 

solvent plumes to resume downgradient migration (National 

Research Council, 2000, p. 73).

Soluble fuel compounds were detected in low concentrations in all three source areas, and in some downgradient 

locations. Benzene was detected in samples collected from 

wells GS003S (1.9 μg/L), GSCP3M (2.7 μg/L) and 

MW061S (8.5 μg/L) (table 7). The MCL for benzene 

(5 μg/L) was exceeded only at well MW061S, which is 

located in LF13. The other common soluble fuel compounds toluene, ethylbenzene, and the xylene isomers, were 

not detected in ground-water samples collected during 

January–March 2000. The MCLs for the soluble fuel compounds were not exceeded in any of the POC wells along the 

Base boundary.

Two processes may account for the sparse detections of 

BTEX in the natural attenuation study area. First, through 

the process of dissolution, the non-aqueous phase liquid 
26 Assessment of Natural Attenuation of Ground-Water Contamination at Dover Air Force Base, Delaware

(NAPL) presumed to be present in the source areas may have 

become depleted in these compounds. Measurements of a 

floating hydrocarbon layer in well MW013, located 140 ft 

southeast of well MW011, show that NAPL is present in 

WP14/LF15 (U.S. Army Corps of Engineers and Dames and 

Moore, Inc., 1997a). The monitoring network does not 

indicate that NAPL is present at the other two sites, but considering historical disposal practices, its presence is a possibility in the natural attenuation study area. The more 

water-soluble compounds of a multicomponent organic 

liquid such as jet fuel will partition relatively quickly to the 

ground water, so that the remaining NAPL consists mainly 

of the insoluble fraction (Borden and Kao, 1992). As 

NAPLs dissolve, the mass flux (mass per unit time) of contaminants to ground water will gradually decline. The rate 

of the decline depends on site-specific factors and cannot be 

determined with the available information. Because waste 

fuels and solvents have not been placed in these source areas 

recently, it is reasonable to expect that substantial depletion 

of BTEX in the source material has occurred.

Second, most of the BTEX mass that partitions to ground 

water may undergo biodegradation over short distances (tens 

of feet) from the source areas. These constituents are known 

to biodegrade under both iron-reducing and methanogenic 

conditions (Wiedemeier and others, 1999, p. 192), the two 

main anaerobic TEAPs at the site. Thus, rapid anaerobic 

biodegradation also may explain the low number of detections of BTEX in the contaminant plumes.

Areal Distribution of Chlorinated Solvents

Chlorinated solvents were detected in ground water beneath 

all three source areas, as well as downgradient of the WP14/

LF15 area. Because of their relatively widespread 

distribution, only the PCE → TCE → cis-1,2-DCE → VC 

reduction sequence is discussed here in detail; other 

chlorinated com-pounds such as chlorobenzene, 1,1,2,2-
Assessment of Natural Attenuation 27

TeCA, 1,2-dichloro-ethane (1,2-DCA), trans-1,2-dichloroethene (trans-1,2-DCE), and methylene chloride were detected less frequently in the natural attenuation study area 

(table 7; K.C. Alexander

Table 7. Concentrations of selected volatile organic compounds in ground water from wells screened in the surficial aquifer at sites 

FT03, LF13, and WP14/LF15, Dover Air Force Base, Delaware, January–March, 2000—Continued

Sampleidentificationnumber

Collectiondate

Benzene(μg/L)

Chlorobenzene(μg/L)

PCE(μg/L)

TCE(μg/L)

cis-1,2-DCE(μg/L)

VC(μg/L)

1,1,2,2,-TeCA(μg/L)

1,2-DCA(μg/L)

Methylene chloride(μg/L)

RL 1.0 1.0 1.0 1.0 1.0 2.0 1.0 1.0 1.0MCL 5 100 5 5 70 2 -- 5 5

DM101D 01/12/2000 ND ND ND ND ND ND ND ND NDDM101S 01/12/2000 ND ND ND ND ND ND ND ND NDDM107D 02/24/2000 ND ND 15 21 2.8 0.20 J ND 12 ND

DM107S 02/24/2000 ND ND 6.7 7.5 1.9 ND ND 1.3 ND

DM108D* 02/24/2000 0.24 J ND 40 110 16 1.6 J 5.6 55 0.67 J

DM108D.DUP* 02/24/2000 ND ND 32 88 13 1.0 J 4.6 41 .60 J

DM108S* 02/24/2000 ND ND 28 84 16 ND 7.2 ND .41 J

DM109D 01/19/2000 ND 0.29 J ND ND ND ND ND ND ND

DM109S 01/19/2000 ND ND ND ND ND ND ND ND NDDM110D 02/25/2000 ND ND ND ND ND ND ND ND NDDM110D.DUP 02/25/2000 ND ND ND ND ND ND ND ND NDDM110S* 02/29/2000 3.7 J 47 ND 1.4 J 270 130 ND ND 5.0 JDM110S.DUP* 02/29/2000 3.4 J 45 ND 1.4 J 270 120 ND ND 5.2 J

DM113D 03/09/2000 ND ND ND ND ND ND ND ND NDDM113S 03/09/2000 ND ND ND ND ND ND ND ND NDGS002D 02/16/2000 .18 J ND 8.3 24 5.6 .31 J 1.9 27 .46 J

GS002M 02/10/2000 ND ND .60 J .83 J .24 J ND ND ND .22 J

GS003M 02/23/2000 ND ND .30 J ND 6.6 ND ND ND NDGS003S 02/23/2000 1.9 ND .19 J 1.1 15 8.8 ND ND ND

GS004D 02/25/2000 ND ND ND .17 J .78 J ND ND ND ND

GS004D.DUP 02/25/2000 ND ND ND .14 J .78 J ND ND ND ND

GS004M 02/15/2000 .16 J ND .58 J .29 J 1.4 ND ND .22 J NDGS004S 02/15/2000 .20 J ND .41 J .48 J 1.1 ND ND ND NDGS010D 02/17/2000 ND ND .38 J 4.2 2.6 ND 2.1 ND ND

GS010S 02/17/2000 .61 J ND .44 J 2.9 2.5 ND .90 J ND ND

GS010S.DUP 02/17/2000 .63 J ND .42 J 2.8 2.5 ND .95 J ND ND

GS011D 01/13/2000 ND ND ND ND ND ND ND ND ND

Table 7. Concentrations of selected volatile organic compounds in ground water from wells screened in the surficial aquifer at sites FT03, 

LF13, and WP14/LF15, Dover Air Force Base, Delaware, January–March, 2000

[μg/L, micrograms per liter; ND, not detected; RL, reporting limit; J, estimated result; result is less than the reporting limit; MCL, maximum contaminant level; PCE, tetrachloroethene; 

 TCE, trichloroethene; cis-1,2-DCE, cis-1,2-dichloroethene; VC, vinyl chloride; 1,2-DCA, 1,2-dichloroethane; 1,1,2,2-TeCA, 1,1,2,2-tetrachloroethane; concentration listed in bold when MCL

 exceeded; DUP, duplicate sample; AMB, ambient blank; TPB, trip blank; FDB, field blank; EQB, equipment blank; SWB, source-water blank]
28 Assessment of Natural Attenuation of Ground-Water Contamination at Dover Air Force Base, Delaware GS011S 01/13/2000 ND ND ND ND ND ND ND ND ND GS012D 02/17/2000 ND ND ND ND ND ND ND ND ND GS012S 02/17/2000 ND ND ND ND ND ND ND ND ND GS013D 2/29/20000 ND ND ND ND ND ND ND ND ND GS014D 02/16/2000 ND ND ND ND ND ND ND ND ND GS045MLA 03/13/2000 ND ND 12 15 6.3 ND ND ND ND GS045MLB 03/13/2000 ND ND 00.88 J 1.1 1.8 ND ND ND ND GS045MLC 03/13/2000 ND ND 0.50 J 0.45 J 0.23 J ND ND ND ND GS045MLD 03/13/2000 ND ND 0.34 J 0.36 J .20 J ND ND ND ND GS045MLE 03/13/2000 ND ND 03.6 2.9 .79 J ND ND ND ND GS045MLF 03/13/2000 ND ND 2.2 1.9 0.68 J ND ND ND ND GS045MLG* 03/13/2000 0.54 J ND 63 49 43 0.73 J ND ND ND GS045MLH* 03/13/2000 ND ND 1,100 600 400 ND ND ND ND GS045MLH.DUP* 03/13/2000 ND ND 960 520 340 ND ND ND ND GS047MLA* 03/14/2000 ND ND 180 45 18 ND ND ND 2.6 J GS047MLB* 03/14/2000 ND ND 430 110 46 ND ND ND 11 GS047MLC* 03/14/2000 ND ND 1,100 290 130 12 J ND 17 J 20 GS047MLD* 03/14/2000 4.6 J ND 430 160 79 16 J ND 21 4.5 J GS047MLE 03/14/2000 ND ND 1.9 1.6 1.3 ND ND ND 1.2 GSCP3M 02/15/2000 2.7 ND ND .56 J .12 J ND ND 0.24 J ND GSCP5M 03/06/2000 ND ND ND 7.2 4.5 .85 J 5.1 .39 J ND GSCP6D 02/09/2000 ND ND ND ND ND ND ND ND ND

MW011* 03/14/2000 ND ND 3,000 1,400 450 ND ND ND 42 J

MW011.DUP* 03/14/2000 ND ND 2,900 1,200 420 ND ND ND 43 J

MW019 02/25/2000 ND ND ND ND 6.9 11 ND .39 J ND

MW037D 03/10/2000 ND ND ND ND ND ND ND ND NDMW037S 03/10/2000 ND ND ND 1.5 1.1 ND .95 J ND .20 JMW038D 03/09/2000 ND ND ND ND ND ND ND ND NDMW038S 03/09/2000 ND ND ND ND ND ND ND ND NDMW039D 02/23/2000 ND ND ND ND ND ND ND ND NDMW039S* 03/14/2000 ND ND 310 120 51 9.1 J ND 7.5 J 4.0 J

MW039S.DUP* 03/14/2000 ND ND 300 110 49 8.9 J ND 7.3 J 11

Table 7. Concentrations of selected volatile organic compounds in ground water from wells screened in the surficial aquifer at sites 

FT03, LF13, and WP14/LF15, Dover Air Force Base, Delaware, January–March, 2000—Continued

Sampleidentificationnumber

Collectiondate

Benzene(μg/L)

Chlorobenzene(μg/L)

PCE(μg/L)

TCE(μg/L)

cis-1,2-DCE(μg/L)

VC(μg/L)

1,1,2,2,-TeCA(μg/L)

1,2-DCA(μg/L)

Methylene chloride(μg/L)

RL 1.0 1.0 1.0 1.0 1.0 2.0 1.0 1.0 1.0MCL 5 100 5 5 70 2 -- 5 5
Assessment of Natural Attenuation 29

MW040D 03/07/2000 ND ND ND ND ND ND ND ND NDMW040S 03/07/2000 ND ND ND ND ND ND ND ND NDMW061S* 02/29/2000 8.5 75 ND ND 0.30 J ND ND 0.89 J 0.42 JMW062D 03/10/2000 ND ND ND ND ND ND ND ND .20 JMW062S 02/11/2000 ND 0.46 J ND ND ND ND ND ND ND

MW064S 02/16/2000 ND ND ND ND ND ND ND ND NDMW206D 03/06/2000 ND ND ND ND ND ND ND ND NDMW206M 03/06/2000 0.18 J ND 34 34 26 0.32 J ND 10 NDMW206M.DUP 03/06/2000 .20 J ND 38 37 28 .38 J ND 12 ND

MW227D 03/07/2000 ND ND 1.3 7.5 4.3 ND 2.8 ND .21 JMW227M 02/10/2000 ND ND ND ND ND ND ND ND NDMW227S 03/07/2000 ND ND ND ND ND ND ND ND NDMWD4B 03/06/2000 ND ND 0.11 J ND ND ND ND ND ND

MWD4H 03/15/2000 ND ND ND ND ND ND ND ND .23 J

JAN0500.EQB 01/05/2000 ND ND ND ND ND ND ND ND 2.0JAN0500.SWB 01/05/2000 ND ND ND ND ND ND ND ND 1.8JAN1300.TPB 01/13/2000 ND ND ND ND ND ND ND ND .28 JFEB1500.TPB 02/15/2000 ND ND ND ND ND ND ND ND .34 JFEB2300.TPB 02/23/2000 ND ND ND ND ND ND ND ND .35 JFEB2900.TPB 02/29/2000 ND ND ND ND ND ND ND ND .54 JMAR0600.TPB 03/06/2000 ND .21 J ND ND ND ND ND .30 J .67 J

MAR1400.TPB 03/14/2000 ND ND ND ND ND ND ND ND .72 JDM108D.AMB 02/24/2000 ND ND ND ND ND ND ND ND .30 JDM110D.AMB 02/25/2000 ND ND ND ND ND ND ND ND .38 JMW039S.AMB 03/14/2000 ND ND ND ND ND ND ND ND .92 JDM107S.FDB 02/24/2000 ND ND ND ND ND ND ND ND .34 JGSCP6D.FDB 02/09/2000 ND ND ND ND ND ND ND ND .59 JGS013D.FDB 02/29/2000 ND ND ND ND ND ND ND ND NDMW011.FDB 03/14/2000 ND ND ND ND ND ND ND ND 1.1MW019.FDB 02/25/2000 ND ND ND ND ND ND ND ND .22 JMW064S.FDB 02/16/2000 ND ND ND ND ND ND ND ND .44 J

Table 7

MW227S.FDB 03/07/2000 ND ND ND ND ND ND ND ND ND

. Concentrations of selected volatile organic compounds in ground water from wells screened in the surficial aquifer at sites 

FT03, LF13, and WP14/LF15, Dover Air Force Base, Delaware, January–March, 2000—Continued

Sampleidentificationnumber

Collectiondate

Benzene(μg/L)

Chlorobenzene(μg/L)

PCE(μg/L)

TCE(μg/L)

cis-1,2-DCE(μg/L)

VC(μg/L)

1,1,2,2,-TeCA(μg/L)

1,2-DCA(μg/L)

Methylene chloride(μg/L)

RL 1.0 1.0 1.0 1.0 1.0 2.0 1.0 1.0 1.0MCL 5 100 5 5 70 2 -- 5 5
30 Assessment of Natural Attenuation of Ground-Water Contamination at Dover Air Force Base, Delaware

 

and others, U.S. Geological Survey, written commun., 2000). 

With the exception of 1,1,2,2-TeCA, which was detected in 

well GS010D at a concentration of 2.1 μg/L, these compounds were not detected in the POC wells near the DAFB 

boundary. 

Distributions of PCE, TCE, cis-1,2-DCE, and VC in the 

upper part of the surficial aquifer, the most heavily contaminated vertical interval, are shown in figures 11a, 11b, 11c, 

and 11d. The measured concentrations of these constituents 

throughout the full thickness of the aquifer ranged from 

3,000 μg/L (PCE in well MW011) (table 7) to below the 

reporting limit of 1.0 μg/L. The zones in the upper part of 

the surficial aquifer where the compounds were detected 

above 1 μg/L, and above their respective MCLs, are shown 

in figures 11a through 11d.

In general, VOC concentrations were highest beneath 

source areas and declined sharply with distance from the 

sources. The parent compounds PCE and TCE are prevalent

only in the WP14/LF15 source area and plume. Both of 

these compounds were detected in wells substantial disese samples are elevated due to high analyte levels.

volatile organic compounds in ground water from wells screened in the surficial aquifer at sites 

F15, Dover Air Force Base, Delaware, January–March, 2000—Continued

Chlorobenzene

(μg/L)

PCE

(μg/L)

TCE

(μg/L)

cis-1,2-DCE

(μg/L)

VC

(μg/L)

1,1,2,2,-TeCA

(μg/L)

1,2-DCA

(μg/L)

Methylene 

chloride

(μg/L)

 1.0 1.0 1.0 1.0 2.0 1.0 1.0 1.0

100 5 5 70 2 -- 5 5
Assessment of Natural Attenuation 31

tances (thousands of feet) downgradient of the known source 

areas. For example, PCE was detected at 28 μg/L in well 

DM108S, which is located 1,000 feet downgradient of the 

source areas. TCE was detected at 2.9 μg/L in well GS010S 

and at 4.2 μg/L in well GS010D. This well cluster is located 

1,800 ft downgradient of WP14/LF15. The MCL of 5 μg/L 

for TCE was not exceeded at these wells. With the exception 

of low concentrations in the GS003 well cluster, PCE and 

TCE were not detected in FT03 or LF13. By comparison, 

the breakdown products cis-1,2-DCE and VC were detected 

in all three source areas. The distribution of cis-1,2-DCE in 

the WP14/LF15 area is similar to that of TCE, although 

samples with concentrations exceeding the MCL of 70 μg/L 

are confined to the source area. VC was detected only in

the monitor wells in the immediate vicinity of the source 

areas.

Site FT03—The three ground-water samples collected in 

FT03 indicate that ground water is reducing, and that dechlorination reactions are occurring in this area. The concentrations of the breakdown products cis-1,2-DCE and VC 

are substantially higher than the parent compounds (table 7). 

The MCL of 2 μg/L for VC was exceeded in samples from 

wells GS003S and MW019. Shallow ground water in FT03 

migrates over relatively short flow paths of approximately 

100 to 500 ft before reaching the adjacent drainage ditch or 

Pipe Elm Branch (fig. 5). Flow paths could be shorter 

depending on the specific location of the source area. Using 

the calculated mean ground-water flow velocity of 0.4 ft/d, 

traveltimes over 100–500 ft range from 250 d (days) to

3.4 yrs.

Intrinsic biodegradation in combination with the proximity of hydrologic discharge boundaries appears to be 

effective in controlling contaminant migration at this site. 
32 Assessment of Natural Attenuation of Ground-Water Contamination at Dover Air Force Base, Delaware

The fate of contaminated water in the discharge areas is not 

well understood, however. Samples from the piezometers 

installed in the drainage ditch (GS003S and GS003M) contain parent compounds and breakdown products, indicating 

that the plume reaches the discharge area prior to complete 

dechlorination at this location. Because piezometer GS003S 

is screened 6 to 9 ft below the streambed (table 1), it is not 

known whether the contaminated water actually discharges 

to surface water, or migrates under the drainage ditch. If the 

plume does discharge to the drainage ditch, as indicated by 

upward hydraulic-head gradients, further anaerobic biodegradation may occur during migration through the anaerobic organic-rich deposits that underlie the ditch. If the 

contaminants reach the streambed surface-water interface, 

additional aerobic biodegradation of cis-1,2-DCE and VC 

may occur in this environment. Consequently, degradation 

reactions in the discharge areas may strongly affect the fate

of contaminated water at this site.

Site LF13—With the exception of well DM110S, the 

wells along the perimeter of LF13 did not contain VOCs. 

High concentrations of methane, ferrous iron, chloride, and 

the breakdown products cis-1,2-DCE and VC in the sample 

from well DM110S indicate that extensive dechlorination is 

occurring near this well. However, the location of the 

source area within the landfill and the direction of plume 

migration currently are not well understood. The direction 

of plume migration in the upper part of the surficial aquifer 

could vary by 180 degrees (fig. 5), depending on the specific 

location of the source area. The steep downward vertical 

gradients in this area (fig. 7) also indicate that a plume 

migrating from a shallow source area has a downward trajectory along the first part of the flow path. If the plume 

migrating from the vicinity of well DM110S flows in a
Assessment of Natural Attenuation 33

northerly direction (the regional flow direction), wells 

MW062 and DM113, and possibly MW040, are located 

downgradient of the source area. Results from the January–

March 2000 sampling indicate that the plume has not 

reached these locations. Another possibility is that the 

plume flows to the south, toward well cluster MW064. Considering the possible ground-water flow directions and the 

location of LF13 relative to the site boundary, it is unlikely 

that this plume migrates toward the DAFB boundary under 

either flow scenario described above.

Despite the incomplete knowledge of ground-water flow 

directions and contaminant distributions, analysis of the 

hydrologic and geochemical data collected at this site 

indicates that natural attenuation is adequate for controlling 

off-site migration of contaminated water. This conclusion is

based on the following factors: (1) potential plume 

migration directions relative to the DAFB boundary, (2) the 

presence of methanogenic conditions, (3) the formation of 

chlorinated-solvent breakdown products, and (4) declining 

contaminant concentrations in well DM110S (fig. 12).

Site WP14/LF15—Results of ground-water samples show 

that intrinsic biodegradation of the chlorinated solvents is 

occurring at WP14/LF15. This conclusion is based on the 

presence of chlorinated-solvent breakdown products within 

and downgradient of the source areas, concentration reductions in the general direction of ground-water flow, and the 

presence of the reducing conditions required for reductive 

dechlorination. Concentrations still decrease sharply downgradient of the source areas, as documented in 1995–96 

(Bachman and others, 1998, p. 37). For example, the concentration of PCE drops from 1,100 μg/L in piezometer 

GS047MLC to 40 μg/L in well DM108D. MCLs were not 

exceeded at any POC well along the DAFB boundary. 
34 Assessment of Natural Attenuation of Ground-Water Contamination at Dover Air Force Base, Delaware
Assessment of Natural Attenuation 35

Despite concentration decreases, three observations based on 

the January–March 2000 monitoring data raise concerns 

about the effectiveness of natural attenuation for controlling 

off-site migration at this site.

First, redox conditions downgradient of the source areas 

may not be appropriate for reductive dechlorination of the 

chlorinated solvents. Conditions generally are mixed between iron-reducing and methanogenic (fig. 10). As discussed previously, the appearance of mixed conditions probably 

results from sampling a relatively small number of longscreened wells. Reductive dechlorination of the highly 

chlorinated parent compounds (PCE and TCE) is known to 

occur in the anaerobic core of a plume, but not in the lessreducing zones near the plume fringes (Wiedemeier and 

others, 1999, p. 242). Moreover, reductive dechlorination 

depends on a continuous supply of electron donors in the 

contaminant plume. Because a viable electron donor has not 

yet been identified and the distribution of redox conditions

has not been defined in sufficient detail, it currently is not 

possible to determine the sustainability of reductive dechlorination in this plume. If reductive dechlorination is not 

occurring downgradient of the source areas, the parent compounds may persist. Other anaerobic biodegradation processes such as oxidation under iron-reducing conditions are 

possible, but have not been clearly demonstrated under field 

conditions. Moreover, these oxidation reactions are favorable only for the breakdown products. Cis-1,2-DCE and VC 

potentially biodegrade under both iron-reducing and aerobic 

conditions (Bradley and Chapelle, 1998; Bradley, 1997). 

Thus, the spatial distributions of redox conditions and 

specific reaction mechanisms probably have an effect on 

intrinsic biodegradation in this plume.

Second, detections of low levels of contaminants in the 

streambed piezometers and monitor wells near Pipe Elm 

Branch indicate that contaminated water is reaching and 

potentially discharging to this hydrologic boundary along the 

entire length of the plume. As reported in Bachman and 

others (1998, p. 14), and shown by the configuration of the 

potentiometric surface in the upper part of the surficial 

aquifer during March 2000, shallow contamination from 

WP14 migrates over a short flow path in the shallow flow 

system and discharges into adjacent Pipe Elm Branch. Contamination from LF15, on the other hand, migrates over a 

longer flow path, potentially discharging to Pipe Elm Branch 

further north near the DAFB boundary. The fate of the contaminated water in these discharge areas probably affects 

plume length and the potential for off-site movement.

Third, temporal variability in source concentrations may 

occur and affect contaminant concentrations in the plume. 

In some locations, for example, contaminant concentrations 

increase with distance downgradient of the source. These 

increases may result from collecting samples from wells 

with long screens, or from comparing concentrations near 

the center of the plume to concentrations on the fringe of the 

plume. For example, concentrations of PCE and TCE at the 

DM108 well cluster are higher than concentrations at the 

DM107 well cluster, even though well DM108 appears to be 

further downgradient, indicating that well DM107 is located 

closer to the fringe of the plume. Alternatively, this concentration distribution could occur when the source concentrations change over time. Under this scenario, higher 

concentrations intermittently leached from the source area 

would migrate downgradient as slugs of high-concentration 

water. These slugs may influence the effectiveness of 

natural attenuation in reducing contaminant concentrations.

Temporal Variability The record of reliable VOC data 

in the EMU extends back only to the early 1990s, when the 

majority of the wells were installed for previous investigations. Available VOC concentration data from selected 

source-area wells are shown on figure 12. Sampling done at 

a WP14/LF15 source-area well, MW039S, shows no clear 

concentration trends over the period 1994–2000. PCE concentrations were sharply higher in samples collected during 

the most recent sampling event (January–March 2000), however. In contrast, time-series sampling at well DM110S 
36 Assessment of Natural Attenuation of Ground-Water Contamination at Dover Air Force Base, Delaware

shows that the concentrations of total 1,2-DCE, and VC have 

declined steadily since 1995.

Although the data from the WP14/LF15 source area do 

not show much temporal variability over the 6-yr period, 

data from downgradient well DM108D indicate that a pulse 

of higher-concentration water began flowing past this 

location sometime during 1994–96 (fig. 13). Using the 

average ground-water velocity of 0.4 ft/d, this water 

originated from the source area roughly 7 yrs prior to arrival, 

or during 1987–89. Methane also may be increasing (fig. 

13), indicating that source-strength variations are the cause 

of the VOC concentration variability, rather than a lack of 

highly reducing conditions within the plume.

Vertical Distribution of Selected Compounds Most of 

the ground-water contamination in the natural attenuation 

study area is in the upper part of the surficial aquifer. The 

maj-ority of the samples collected from the lower part of the 

surficial aquifer did not contain detectable VOCs (table 7). 

Notable exceptions are samples from wells GS010D, 

MW227D, DM107D, and DM108D, which contained TCE 

and PCE at detectable concentrations up to 110 μg/L (TCE at 

well DM108D), and the deepest port from piezometer 

GS045ML, which contained PCE, TCE, and cis-1,2-DCE at 

detectable concentrations up to 15 μg/L (TCE). The deep 

contamination may be associated with WP14/LF15, or with 

an unidentified source further upgradient.

Data from multilevel piezometers GS045ML and 

GS047ML show the vertical distributions of VOCs, electron 

acceptors, and metabolic by-products near the source 

areas in WP14/LF15. Vertical profiles of PCE, TCE, cis-1,2-

DCE, VC, DO, ferrous iron, and methane are shown in 

figure 14. At GS045ML, VOC concentrations are highest 

near the water table and decrease sharply with depth. At 

GS047ML, the highest VOC concentrations are in a narrow 

vertical interval about 10 ft below the water table. At both 

locations, methane and ferrous-iron concentrations also are 

highest, and dissolved oxygen lowest, in the sample ports 

with the highest VOC concentrations. These footprints 

clearly show the vertical extent of the contaminated intervals, and demonstrate that conditions for efficient chlorinated-solvent biodegradation, such as low DO and elevated 

ferrous iron and methane, have developed in these intervals. 

The profiles also indicate that the vertical distributions 

of PCE, TCE, cis-1,2-DCE, and VC are highly variable, 

with large concentration changes over small vertical intervals. Concentrations of PCE, for example, ranged from 

below the detection limit to over 1,000 μg/L over vertical 

intervals of 2 to 5 ft. Comparisons of concentrations of a representative organic constituent and redox-sensitive species, 

PCE and methane, from the multilevel piezometers and the 

adjacent long-screened wells are shown in figure 15. These 

representative data show that water from vertically stratified 

zones with substantially different geochemical characteristics mixes in the wells. In one location, GS047ML/ 

MW039S, the value from the well was reasonably representative of the vertical profile, but in another location,

GS045ML/MW011, the value was not representative of the 

profile and could lead to incorrect conclusions about natural 

attenuation in this area. In this case, most of the ground 

water in the sample from well MW011 appeared to come 

from a contaminated interval near the top of the well screen. 

These results clearly show the effectiveness of piezometers 

in accurately defining the location and vertical extent of the 

contaminants and redox-sensitive species.

Revised Conceptual Model and Possible Further Study

The information presented in this report and new results 

in the ground-water literature have been used to develop an 

improved conceptual model of natural attenuation of the 

contaminant plumes in the EMU. The improved conceptual 

model can be expressed as a series of hypotheses:

(1) Intrinsic biodegradation is occurring in all three 

plumes, but because of spatial variability in redox 

conditions and a sparse monitoring network of longscreened wells, the reaction mechanisms and spatial 

distribution of reactions are unclear.

(2) The fate of contaminated water at the discharge 

boundaries in FT03 and WP14/LF15 is important for 

determining the effect on surface water, and the offsite migration potential of the plumes.

(3) Temporal variability in source-area concentrations and 

shallow flow directions is present, and has an 

appreciable effect on the fate of contaminants 

migrating from WP14/LF15.

(4) Mixed redox conditions observed in the aquifer result 

from sample collection from long-screened wells.

Four issues that currently are unclear and may require 

further study have been identified from the improved conceptual model. Most of these issues are related to WP14/

LF15 because of the greater off-site migration potential at 

this site.

(1) What is the areal and vertical extent of the contaminant plumes migrating from FT03, LF13, and WP14/

LF15? With better plume definition, future assessments potentially can be based on plume stabilization, 

quantitative mass budgeting, or numerical solutetransport modeling.

(2) To what extent does intrinsic biodegradation reduce 

the concentrations of the chlorinated solvents downgradient of WP14/LF15? Do anaerobic conditions 

persist downgradient? What are the key reaction 

mechanisms and reactants and where do these reactions occur in the surficial aquifer? The resolution 

of the spatial distributions of contaminants and redoxsensitive species can be improved by reducing the use 

of long-screened wells. Long-screened wells mix 

water from vertically stratified zones, which potentially can lead to incorrect conclusions about natural 

attenuation.
Assessment of Natural Attenuation 37
38 Assessment of Natural Attenuation of Ground-Water Contamination at Dover Air Force Base, Delaware
Summary and Conclusions 39
40 Assessment of Natural Attenuation of Ground-Water Contamination at Dover Air Force Base, Delaware

(3) What is the fate of contaminants migrating from FT03 

and WP14/LF15 at the ground-water discharge 

boundaries? Do contaminants discharge to surface 

water or biodegrade extensively in the streambed 

sediments prior to discharge? Does deeper contamination migrate past these discharge areas? 

Hydrologic boundaries may be important at these sites 

by physically intercepting contaminant plumes and 

preventing off-site movement in ground water.

(4) Has temporal variability in source concentrations and 

ground-water flow patterns occurred in WP14/LF15? 

If so, what is the magnitude of the variability and the 

effect on the fate of contaminants migrating from 

WP14/LF15? Is seasonal variation in heads an important factor in controlling contaminant concentrations and migration pathways?

Summary and Conclusions

Fire training and waste disposal in the East Management 

Unit of Dover Air Force Base, Delaware have resulted in 

subsurface contamination with chlorinated solvents and fuel 

hydrocarbons. Four source areas, Fire Training Area Three, 

the Rubble Area Landfill, the Liquid Waste Disposal Landfill, and the Receiver Station Landfill, are present in the East 

Management Unit. Natural attenuation of ground-water 

contamination is the remediation method selected by the 

U.S. Environmental Protection Agency and the Delaware 

Department of Natural Resources and Environmental 

Control. As part of the selected remediation, the U.S. 

Geological Survey, in cooperation with the U.S. Air Force, is 

conducting a long-term monitoring program and an assessment of the effectiveness of natural attenuation in the 

surficial aquifer. This report presents the first assessment of 

the effectiveness of natural attenuation since long-term 

monitoring began in 1999. The conclusions in this report are 

based on ground-water samples collected from January 

through March 2000, previous analytical results from 

selected wells, available geologic and geophysical well logs, 

and newly acquired information such as sediment organiccarbon measurements, hydraulic-conductivity measurements 

from slug tests on wells in the natural attenuation study area, 

and water-level measurements from wells screened in the 

surficial aquifer. The results of this assessment are in 

general agreement with the results of a preliminary study by 

Bachman and others in 1998.

Contamination occurs in the surficial aquifer, which is a 

single hydrostratigraphic unit bounded on the top by the 

water table and on the bottom by a regional confining unit, 

the Calvert Formation. Ground water contains chlorinated 

solvents, such as chlorinated ethenes and ethanes, and fuel 

hydrocarbons. Ground water in the natural attenuation study 

area recharges at topographically high areas and discharges 

to Pipe Elm Branch and local drainage ditches. The general 

ground-water flow direction is to the northwest. Hydraulicconductivity values in the natural attenuation study area 

range from 3 feet per day to 313 feet per day, with a geometric mean value of 43 feet per day. The ground-water 

velocity in the area is approximately 0.4 feet per day. The 

residence time for ground water in the natural attenuation 

study area ranges from weeks to decades, depending on the 

length of the flow path.

The organic-carbon content of the surficial aquifer sediment is low, with an arithmetic mean value of 0.00029 grams 

of carbon per gram dry sediment. To estimate the effect of 

adsorption on contaminant migration velocities, the organiccarbon results were used to calculate retardation factors for 

the compounds that are prevalent in ground water. The 

retardation factors range from 1.00 for vinyl chloride to 1.35 

for tetrachloroethene, indicating that contaminant velocities 

are similar to ground-water velocities, and that adsorption to 

aquifer solids is not a strong natural-attenuation mechanism. 

Similarly, abiotic reactions and dispersion also are not 

expected to be important mechanisms. Consequently, the 

most important natural attenuation mechanism in the surficial aquifer is intrinsic biodegradation.

Fifty-three monitor wells and multilevel piezometers 

completed in the surficial aquifer were sampled to determine 

the distributions of volatile organic compounds, electron 

acceptors, and metabolic by-products in the natural 

attenuation study area. The water-quality data indicate that 

intrinsic biodegradation is occurring at all four sites in the 

East Management Unit. This conclusion is based on the 

spatial distributions of volatile organic compounds, electron 

acceptors, and metabolic by-products in ground water. The 

strongest evidence for intrinsic biodegradation is the 

detection of tetrachloroethene and trichloroethene breakdown products within and downgradient of the source areas. 

The distributions of electron acceptors and metabolic byproducts indicate that contaminant biodegradation has 

changed the prevailing geochemistry of the surficial aquifer, 

creating the strongly reducing conditions necessary for 

chlorinated-solvent biodegradation. Geochemical changes 

include depleted dissolved oxygen and elevated ferrous iron 

and methane relative to concentrations in uncontaminated 

zones of the surficial aquifer. Redox conditions are mixed 

methanogenic/iron-reducing near the source areas. Ironreducing conditions appear to dominate further downgradient.

Natural attenuation appears to be adequate for controlling the migration of the contaminant plumes at Fire Training Area Three and the Rubble Area Landfill. The potential 

for off-site migration is low at these sites because reductive 

dechlorination reduces contaminant concentrations and 

plume lengths, and the sources are relatively far from the 

Dover Air Force Base boundary. At Fire Training Area 

Three, geochemical and hydrologic results indicate that 

intrinsic biodegradation in combination with the proximity 

of hydrologic discharge boundaries limits contaminant
References Cited 41

migration. At this site, contaminated ground water migrates 

over relatively short flow paths (tens to hundreds of feet) to 

nearby discharge boundaries where further biodegradation 

may occur. At the Rubble Area Landfill, geochemical 

results indicate that reductive dechlorination of the chlorinated solvents is extensive, and that source-area concentrations have been declining steadily since 1995. These observations in combination with the potential ground-water flow 

directions relative to the Base boundary indicate that off-site 

migration is unlikely.

In the Liquid Waste Disposal Landfill and Receiver 

Station Landfill area, intrinsic biodegradation is occurring, 

but the plume is larger, and the uncertainty about the effectiveness of natural attenuation in reducing contaminant 

concentrations and controlling plume migration is greater. 

Maximum contaminant levels were not exceeded in any of 

the January–March 2000 samples collected from point-ofcompliance wells, however. Three observations from the 

January–March 2000 ground-water data raise concerns about 

the effectiveness of natural attenuation at this site. First, 

redox conditions downgradient of the source area may not be 

appropriate for reductive dechlorination. Second, contaminated water potentially is discharging to Pipe Elm Branch. 

Third, temporal variability in source concentrations may 

occur and have an effect on the fate of contaminants migrating from the source area.

 The new results presented in this report led to the 

development of improved conceptual models for Fire Training Area Three, the Rubble Area Landfill, the Liquid Waste 

Disposal Landfill, and the Receiver Station Landfill, and to 

the recognition of four issues that currently are unclear and 

may need further study. These issues include delineating the 

areal and vertical extent of the contaminant plumes in greater 

detail, determining the extent of intrinsic biodegradation 

downgradient of the Liquid Waste Disposal and Receiver 

Station Landfills, determining the fate of contaminants in the 

ground-water discharge areas, and determining the effect of 

temporal variability in source concentrations and groundwater flow patterns on the plume migrating from the Liquid 

Waste Disposal and Receiver Station Landfills. Some of 

these issues have been addressed with an additional study 

performed since the January–March 2000 sampling round. 

In this study, ground-water samples were collected with a 

Geoprobe direct-push drilling rig and a mini-piezometer to 

map the contaminant plumes in greater detail and measure 

contaminant concentrations in the ground-water discharge 

areas. This study, and the ongoing long-term monitoring 

program, also provided information on temporal variability 

in the source areas and plumes. Further study is planned to 

better understand the fate of the volatile organic compounds 

in the ground-water discharge areas, and the spatial distribution of redox conditions and reaction mechanisms in the 

contaminant plumes.

Abdul, S.A., Gibson, T.L., and Rai, D.N., 1987, Statistical 

correlations for predicting the partition coefficient for 

nonpolar organic contaminants between aquifer organic 

carbon and water: Hazardous Waste and Hazardous 

Materials, v. 4, no. 3, p. 211–222.

Bachman, L.J., Cashel, M.L., and Bekins, B.A., 1998, 

Assessment of natural attenuation of contamination 

from three source areas in the East Management Unit, 

Dover Air Force Base, Kent County, Delaware: U.S. 

Geological Survey Water-Resources Investigations 

Report 98–4153, 46 p.

Baedecker, M.J., and Cozzarelli, I.M., 1992, The 

determination and fate of unstable constituents of 

contaminated groundwater, in Lesage, S., and Jackson, 

R.E., eds., Groundwater contamination and analysis at 

hazardous waste sites: New York, Marcel Dekker, 

p. 425–461.

Beman, J.E., Phelan, D.J., Dysart, J.E., Cashel, M.E., and 

Smith, V.C., 1999, Hydrogeologic and water-quality 

data for the East Management Unit of Dover Air Force 

Base, Kent County, Delaware, 1995–1996: U.S. 

Geological Survey Open-File Report 99–253, 95 p.

Borden, R.C., and Kao, C., 1992, Evaluation of 

groundwater extraction for remediation of petroleum 

contaminated groundwater: Water and Environmental 

Research, v. 64, no. 1, p. 28–36.

Bouwer, H., and Rice, R.C., 1976, A slug test method for 

determining hydraulic conductivity of unconfined 

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Bradley, P.M., 1997, Kinetics of DCE and VC 

mineralization under methanogenic and Fe(III)-reducing 

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v. 31, no. 9, p. 2,692–2,696.

Bradley, P.M., and Chapelle, F.H., 1998, Effect of 

contaminant concentration on aerobic microbial 

mineralization of DCE and VC in stream-bed sediments: 

Environmental Science and Technology, v. 32, no. 5, 

p. 553–557.

Chapelle, F.H., McMahon, P.B., Dubrovsky, N.M., Fujii, 

R.F., Oaksford, E.T., and Vroblesky, D.A., 1995, 

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1990, Transformation of monoaromatic hydrocarbons to 

organic acids in an anoxic groundwater environment: 

Environmental Geology and Water Science, v. 16, no. 2, 

p. 135–141.

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42 Assessment of Natural Attenuation of Ground-Water Contamination at Dover Air Force Base, Delaware

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Phillips, P.J., Denver, J.M., Hamilton, P.A., Michel, 

R.L., and Coplen, T.B., 1992, Chlorofluorocarbons 

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tracers in shallow groundwater of the Delmarva 

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Grbic-Galic, D., and Vogel, T.M., 1987, Transformation of 

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Applied and Environmental Microbiology, v. 53, 

p. 254–260.

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conducting air-pressurized slug tests and computation of 

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Installation restoration program, draft technical 

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N.L., 1989, Homogeneous hydrolysis rate constants for 

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the subsurface: New York, Wiley, 617 p.

Appendix A. Hydraulic-head measurements from wells screened in the surficial aquifer, 

Dover Air Force Base, Delaware, March 15–16, 2000—Continued

Well

identification

number

Top of 

casing

(ft s.l.)

Total

depth

(ft)

Screen

length

(ft)

Depth

to water

(ft)

Hydraulic

head

(ft s.l.)

DM02S 26.24 21.80 10 14.01 12.23

DM03S 24.88 16.77 10 6.11 18.77

DM04S 26.87 16.66 10 4.67 22.20

DM05S 24.36 15.68 10 7.58 16.78

DM06S 24.12 16.06 10 5.38 18.74

Appendix A. Hydraulic-head measurements from wells screened in the surficial aquifer,

Dover Air Force Base, Delaware, March 15–16, 2000

[ft, feet; ft s.l., feet above sea level; total depth measured from top of well casing to bottom of well]
44 Assessment of Natural Attenuation of Ground-Water Contamination at Dover Air Force Base, Delaware

DM101D 25.84 55.95 10 13.93 11.91

DM101S 25.70 18.78 10 5.37 20.33

DM102D 21.44 61.56 10 8.51 12.93

DM102S 21.59 18.72 10 8.62 12.97

DM103D 26.80 77.97 10 14.56 12.24

DM103S 27.00 21.89 10 9.16 17.84

DM104D 24.68 82.11 10 12.71 11.97

DM104S 24.71 19.58 10 7.65 17.06

DM105D 21.65 82.16 10 9.78 11.87

DM105S 21.44 19.59 10 7.57 13.87

DM106D 26.07 84.92 10 14.03 12.04

DM106S 26.05 20.12 10 3.90 22.15

DM107D 15.96 41.11 10 6.37 9.59

DM107S 16.38 18.91 10 6.65 9.73

DM108D 13.80 35.13 10 5.06 8.74

DM108S 13.99 19.24 10 5.16 8.83

DM109D 23.87 46.41 10 11.58 12.29

DM109S 24.18 19.30 10 4.04 20.14

DM110D 29.43 77.43 10 17.51 11.92

DM110S 29.36 24.03 10 9.41 19.95

DM111D 20.32 43.85 10 10.23 10.09

DM111S 20.20 19.29 10 10.09 10.11

DM113D 23.34 66.02 10 11.89 11.45

DM113S 23.20 21.14 10 11.31 11.89

DM114D 26.51 81.17 10 14.50 12.01

DM115DA 24.58 85.88 10 12.71 11.87

DM115S 24.56 18.89 10 3.66 20.90

DM116D 19.70 48.67 10 12.43 7.27

DM116S 20.02 17.69 10 9.11 10.91

DM117D 16.95 45.56 10 9.88 7.07

DM117S 16.52 17.69 10 11.35 5.17

DM118D 16.42 44.76 10 9.70 6.72

DM118S 16.55 20.05 10 10.59 5.96

DM119D 17.27 41.34 10 10.65 6.62

DM119S 15.72 20.31 10 11.16 4.56

GS001D 13.59 28.68 3 2.53 11.06

GS001M 15.82 19.16 1 5.22 10.60

GS001S 14.26 8.54 .50 4.56 9.70

GS002D 10.79 26.39 3 1.00 9.79

GS002M 10.36 15.85 1 .55 9.81

GS003M 8.27 18.80 3 1.31 6.96

GS003S 7.56 11.84 3 1.05 6.51

GS004M 10.33 16.40 1 3.03 7.30

GS010D 16.48 48.37 5 8.85 7.63

GS010S 16.46 24.75 5 9.30 7.16

GS011D 21.77 45.11 5 11.02 10.75

Appendix A. Hydraulic-head measurements from wells screened in the surficial aquifer, 

Dover Air Force Base, Delaware, March 15–16, 2000—Continued

Well

identification

number

Top of 

casing

(ft s.l.)

Total

depth

(ft)

Screen

length

(ft)

Depth

to water

(ft)

Hydraulic

head

(ft s.l.)
Appendixes 45

GS011S 22.59 19.86 5 2.68 19.91

GS012D 22.83 53.92 5 9.82 13.01

GS012S 23.08 17.45 10 3.82 19.26

GS013D 9.34 37.30 10 2.98 6.36

GS014D 22.97 64.48 10 10.20 12.77

GS303D 22.19 59.40 10 9.55 12.64

GSCP3D 16.26 45.75 3 8.16 8.10

GSCP3M 16.30 22.30 3 8.18 8.12

GSCP4M 18.33 23.15 3 9.75 8.58

GSCP5M 17.92 29.26 3 7.73 10.19

GSCP6D 18.44 40.14 3 7.67 10.77

MW010 22.38 59.02 46 10.90 11.48

MW011 21.76 57.02 44 10.49 11.27

MW012 21.81 57.08 44 10.37 11.44

MW018 19.73 50.21 38 10.87 8.86

MW019 16.27 46.01 35 8.59 7.68

MW020 12.34 42.11 33 4.92 7.42

MW021 18.03 62.03 50 6.69 11.34

MW022 20.19 62.03 50 8.78 11.41

MW023 21.39 59.09 47 9.86 11.53

MW036D 17.53 56.64 8 6.40 11.13

MW037D 16.28 49.89 5 5.16 11.12

MW037S 16.29 22.60 10 5.77 10.52

MW038D 19.64 59.60 5 8.30 11.34

MW038S 19.05 31.76 10 7.69 11.36

MW039D 19.55 59.25 5 8.27 11.28

MW039S 19.57 22.16 10 8.35 11.22

MW040D 20.51 48.78 5 9.01 11.50

MW040S 20.68 24.11 10 9.41 11.27

MW041D 23.45 65.00 5 11.88 11.57

MW041S 23.61 25.01 10 12.00 11.61

MW042P 19.41 59.99 33 8.02 11.39

MW043D 14.06 22.37 5 6.53 7.53

MW043S 14.15 44.64 10 6.59 7.56

MW044D 20.99 52.92 5 12.23 8.76

MW044S 20.61 24.76 10 11.80 8.81

MW061S 26.77 29.56 10 14.55 12.22

MW062D 22.14 70.08 5 10.43 11.71

MW062S 22.27 27.78 10 10.57 11.70

MW064D 23.92 66.40 5 11.75 12.17

MW064S 24.14 37.23 10 11.86 12.28

MW206D 16.85 46.90 10 5.82 11.03

MW207D 20.75 50.23 10 9.63 11.12

MW207M 20.82 37.46 10 9.71 11.11

Appendix A. Hydraulic-head measurements from wells screened in the surficial aquifer, 

Dover Air Force Base, Delaware, March 15–16, 2000—Continued

Well

identification

number

Top of 

casing

(ft s.l.)

Total

depth

(ft)

Screen

length

(ft)

Depth

to water

(ft)

Hydraulic

head

(ft s.l.)
References Cited 46

MW226D 24.31 79.30 10 12.73 11.58

MW226M 24.71 49.22 10 13.09 11.62

MW226S 24.81 22.28 10 13.94 10.87

MW227D 22.86 49.71 10 13.85 9.01

MW227M 21.45 35.43 10 12.46 8.99

MW227S 21.39 24.98 10 11.83 9.56

MWD4A 20.93 17.21 10 8.25 12.68

MWD4B 17.29 16.04 10 6.29 11.00

MWD4C 18.81 16.81 10 5.61 13.20

MWD4D 23.34 16.99 10 7.29 16.05

MWD4E 24.01 20.13 3 12.42 11.59

MWD4F 20.38 17.13 3 10.28 10.10

MWD4G 21.68 18.36 3 12.32 9.36

MWD4I 21.87 25.32 3 14.33 7.54

Appendix A. Hydraulic-head measurements from wells screened in the surficial aquifer, 

Dover Air Force Base, Delaware, March 15–16, 2000—Continued

Well

identification

number

Top of 

casing

(ft s.l.)

Total

depth

(ft)

Screen

length

(ft)

Depth

to water

(ft)

Hydraulic

head

(ft s.l.)