Hydrogeology and Water Quality of the 

Shallow Aquifer System at the Explosive 

Experimental Area, Naval Surface Warfare 

Center, Dahlgren Site, Dahlgren, Virginia

By Clifton F. Bell

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations Report 96-4209

Prepared in cooperation with the

ENVIRONMENTAL AND SAFETY OFFICE 

NAVAL SURFACE WARFARE CENTER, 

DAHLGREN SITE

Richmond, Virginia 

1996
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

The use of firm, trade, and brand names in this report is for identification purposes 

only and does not constitute endorsement by the U.S. Geological Survey

For additional information write to:

District Chief

U.S. Geological Survey

3600 West Broad Street, Room 606

Richmond, VA 23230-4916

Copies of this report can be purchased from:

U.S. Geological Survey 

Branch of Information Services 

Box 25286 

Denver, CO 80225-0286
CONTENTS

Abstract......................................................................................................................................................................^ 1

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

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

Description of study area............................................................................................................................................. 2

Previous investigations................................................................................................................................................ 2

Acknowledgments........................................................................................._^ 5

Methods of investigation....................................................................................................................................................... 5

Well installation...................................................................................................._^ 5

Analysis of pollen samples.......................................................................................................................................... 6

Collection of lithologic and borehole geophysical data.............................................................................................. 6

Water-level measurement............................................................................................................................................ 6

Slugtests......................................^ 7

Water-quality sampling and analysis........................................................................................................................... 7

Hydrogeology of the shallow aquifer system....................................................................................................................... 8

Geology....................................................................................................................................................................... 8

Stratigraphy....................................................................................................................................................... 8

Depositional history........................................................................................................................................... 10

Physical properties of hydrogeologic units ................................................................................................................. 10

Columbia aquifer............................................................................................................................................... 10

Upper confining unit.......................................................................................................................................... 11

Upper confined aquifer...................................................................................................................................... 15

Nanjemoy-Marlboro confining unit................................................................................................................... 15

Hydrologic characteristics of aquifers......................................................................................................................... 18

Columbia aquifer............................................................................................................................................... 18

Recharge.................................................................................................................................................. 18

Horizontal flow........................................................................................................................................ 22

Discharge................................................................................................................................................. 22

Hydraulic connection with Nanjemoy-Marlboro confining unit............................................................. 24

Upper confined aquifer...................................................................................................................................... 24

Recharge.................................................................................................................................................. 25

Horizontal flow and discharge................................................................................................................. 25

Water quality of the shallow aquifer system......................................................................................................................... 28

Quality assurance and quality control......................................................................................................................... 28

Occurrence and distribution of selected chemical constituents................................................................................... 29

Columbia aquifer............................................................................................................................................... 29

Field parameters and indicators............................................................................................................... 29

Major constituents................................................................................................................................... 29

Minor constituents................................................................................................................................... 31

Nanjemoy-Marlboro confining unit................................................................................................................... 32

Upper confined aquifer...................................................................................................................................... 34

Summary................................................................................................................................................................................ 35

References cited..................................................................................................................................................................... 36

FIGURES

1,2. Maps showing:

1. Location of study area.................................................................................................................................... 3

2. Locations of observation wells and surface-water sampling sites................................................................. 4

Contents III
3. Schematic diagram of geologic and hydrogeologic units of the shallow aquifer system....................................... 9

4. Hydrogeologic map showing altitude of bottom and thickness of the Columbia aquifer...................................... 12

5. Boxplots showing distribution of horizontal and vertical hydraulic conductivity of hydrogeologic units ............ 14

6. Hydrogeologic section A-A'.................................................................................................................................... 15

7-11. Hydrogeologic maps showing:

7. Thickness and altitude of upper confining unit.............................................................................................. 16

8. Altitude of top and thickness of upper confined aquifer................................................................................ 17

9. Altitude of top of Nanjemoy-Marlboro confining unit.................................................................................. 19

10. Potentiometric surface of the Columbia aquifer, March 1994....................................................................... 20

11. Potentiometric surface of the Columbia aquifer, December 1994................................................................. 21

12-14. Graphs showing maximum daily water levels in the Columbia aquifer for:

12. Well EEA-S1 and precipitation data, April 1994 through March 1995......................................................... 22

13. Well cluster 4 and synoptic water-level measurements for well clusters 5 and 7,

April 1994 through April 1995...................................................................................................................... 23

14. Wells EEA-S12 and EEA-S13, April 1994 through March 1995.................................................................. 24

15. Graph showing water level in the Columbia aquifer for well EEA-S1 July 11-15,1995...................................... 25

16. Hydrogeologic map showing potentiometric surface in the upper confined aquifer and the NanjemoyMarlboro confining unit, March 1994..................................................................................................................... 26

17. Hydrogeologic map showing potentiometric surface in the upper confined aquifer and the NanjemoyMarlboro confining unit, December 1994............................................................................................................... 27

18. Hydrograph showing water levels for wells EEA-M3, M6, M8, and tidal fluctuations in the Upper

Machodoc Creek, August 1-5,1995....................................................................................................................... 28

19. Boxplot summaries of water-quality analyses for selected constituents in ground water...................................... 30

20,21. Trilinear diagram showing major-ion composition of water from wells completed in the:

20. Columbia aquifer........................................................................................................................................... 33

21. Upper confined aquifer and Nanjemoy-Marlboro confining unit.................................................................. 34

TABLES

1. Horizontal hydraulic conductivities determined from slug tests at the Explosive Experimental Area,

Dahlgren, Virginia.................................................................................................................................................. 13

2. Summary statistics for water-quality analyses from the Columbia aquifer at the Explosive

Experimental Area, Dahlgren, Virginia.................................................................................................................. 32

IV Contents
CONVERSION FACTORS, VERTICAL DATUM, AND ABBREVIATED WATERQUALITY UNITS

Multiply By To obtain

Length

inch (in.) 

foot (ft) 

mile (mi)

25.4 

0.3048 

1.609

millimeter (mm) 

meter 

kilometer

Area

acre 4,047 square meter

Volume

gallon 

gallon

3.785 

3,785

liter (L) 

milliliter (mL)

Hydraulic conductivty

foot per day (ft/d)1 0.3048 meter per day

1A mathematical reduction of the unit cubic foot per day per square foot [(ft3/d)/ft2].

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

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

and Canada, formerly called Sea Level Datum of 1929.

Abbreviated water-quality units: Chemical concentration is reported in milligrams per liter 

(mg/L) or micrograms per liter (|ig/L). Milligrams per liter is a unit expressing the concentration of 

chemical constituents in solution as weight (milligrams) of solute per unit volume (liter) of water. 

One thousand micrograms per liter is equivalent to one milligram per liter. For concentrations less 

than 7,000 mg/L, the numerical value is the same as for concentrations in parts per million. Specific 

electrical conductance of water is reported in microsiemens per centimeter at 25 degrees Celsius 

(|iS/cm). Water temperature is reported in degree Celsius (°C), which can be converted to degree 

Fahrenheit (°F) by the following equation:

Contents
Hydrogeology and Water Quality of the Shallow 

Aquifer System at the Explosive Experimental 

Area, Naval Surface Warfare Center, Dahlgren 

Site, Dahlgren, Virginia

By Clifton F. Bell

Abstract

In October 1993, the U.S. Geological 

Survey began a study to characterize the hydrogeology of the shallow aquifer system at the 

Explosive Experimental Area, Naval Surface 

Warfare Center, Dahlgren Site, Dahlgren, 

Virginia, which is located on the Potomac River 

in the Coastal Plain Physiographic Province. The 

study provides a description of the hydrogeologic 

units, directions of ground-water flow, and background water quality in the study area to a depth 

of about 100 feet. Lithologic, geophysical, and 

hydrologic data were collected from 28 wells 

drilled for this study, from 3 existing wells, and 

from outcrops.

The shallow aquifer system at the Explosive Experimental Area consists of two finingupward sequences of Pleistocene fluvial-estuarine 

deposits that overlie Paleocene-Eocene marine 

deposits of the Nanjemoy-Marlboro confining 

unit. The surficial hydrogeologic unit is the 

Columbia aquifer. Horizontal linear flow of water 

in this aquifer generally responds to the surface 

topography, discharging to tidal creeks, marshes, 

and the Potomac River, and rates of flow in this 

aquifer range from 0.003 to 0.70 foot per day.

The Columbia aquifer unconformably overlies the upper confining unit an organic-rich 

clay that is 0 to 55 feet thick. The upper confining 

unit conformably overlies the upper confined 

aquifer, a 0- to 35-feet thick unit that consists of 

interbedded fine-grained to medium-grained 

sands and clay. The upper confined aquifer

probably receives most of its recharge from the 

adjacent and underlying Nanjemoy-Marlboro 

confining unit. Water in the upper confined aquifer generally flows eastward, northward, and 

northeastward at about 0.03 foot per day toward 

the Potomac River and Machodoc Creek.

The Nanjemoy-Marlboro confining unit 

consists of glauconitic, fossiliferous silty finegrained sands of the Nanjemoy Formation. Where 

the upper confined system is absent, the 

Nanjemoy-Marlboro confining unit is directly 

overlain by the Columbia aquifer. In some parts of 

the Explosive Experimental Area, horizontal 

hydraulic conductivities of the NanjemoyMarlboro confining unit and the Columbia aquifer 

are similar (from 10"4 to 10~2 foot per day), and 

these units effectively combine to form a thick 

(greater than 50 feet) aquifer.

The background water quality of the shallow aquifer system is characteristic of ground 

waters in the Virginia Coastal Plain Physiographic 

Province. Water in the Columbia aquifer is a 

mixed ionic type, has a median pH of 5.9, and a 

median total dissolved solids of 106 milligrams 

per liter. Water in the upper confined aquifer and 

Nanjemoy-Marlboro confining unit is a sodiumcalcium-bicarbonate type, and generally has 

higher pH, dissolved solids, and alkalinity than 

water in the Columbia aquifer. Water in the upper 

confined aquifer and some parts of the Columbia 

aquifer is anoxic, and it has high concentrations of 

dissolved iron, manganese, and sulfide.

Abstract
INTRODUCTION

The Naval Surface Warfare Center, Dahlgren 

Site (NSWCDL) at Dahlgren, Va. (fig. 1), was established in 1918 as a proving ground for naval ordnance. 

At present (1996) the NSWCDL is the U.S. Navy's 

primary center for research, development, and testing 

of offensive and defensive systems associated with 

surface/ship warfare. The Explosive Experimental 

Area (EEA) of the NSCWDL, located to the south of 

Machodoc Creek (fig. 2), is used primarily for testing 

and evaluation of naval ordnance. Through the Installation Restoration Program, the U.S. Navy currently is 

evaluating the EEA for potential contamination of 

ground water, soil, and surface water caused by various activities conducted during the history of the 

installation.

In 1993, the U.S. Geological Survey (USGS) 

began a hydrogeologic framework investigation of the 

shallow aquifer system underlying the EEA. The study 

was designed to provide the U.S. Navy with basic 

hydrogeologic and water-quality data and with a 

description of the natural properties and characteristics 

of the shallow aquifer system. Results from this investigation can be used by the NSWCDL to assist Installation Restoration activities and to provide a basis for 

future planning of operations that have the potential to 

affect the shallow aquifer system. Characterization of 

individual contaminated sites was not an objective of 

this study.

Purpose and Scope

The purpose of this report is to describe the 

hydrogeology and background water quality of the 

shallow aquifer system at the EEA, NSWCDL, 

Dahlgren, Va. The shallow aquifer system is defined in 

this report as the hydrogeologic units present beneath 

the study area to a depth of approximately 100 ft 

below land surface.

The report includes a discussion of the (1) physical properties, (2) hydrology, and (3) water quality of 

the shallow aquifer system. Water-level measurements 

were collected monthly or continuously from March 

1994 through April 1995 at 28 wells drilled for this 

study and 1 existing well. Water-quality samples were 

collected from these 28 wells and from 3 surfacewater locations in June 1994. Other sources of data 

include lithologic samples, pollen samples, geophysical logs, and aquifer-test data from 28 wells drilled for

this study, and lithologic samples from outcrops and 

2 existing boreholes. The interpreted hydrogeologic 

framework is illustrated in maps and a hydrogeologic 

section, and an assessment of ground-water quality is 

provided by tabular and graphical summaries.

Description of Study Area

The NSWCDL is located along the Potomac 

River in King George County, Va. (fig. 1), and it is 

divided by Machodoc Creek into two parts: (1) the 

Mainside of 2,678 acres to the north; and (2) the EEA 

of 1,614 acres to the south of Machodoc Creek. The 

EEA is bounded to the north and west by Machodoc 

Creek and to the east by the Potomac River. The study 

area lies in the Coastal Plain Physiographic Province 

of Virginia.

Land surface is generally flat at the EEA, and 

elevations range from 0 to 30 ft above sea level. The 

EEA lies on a terrace on the riverward side of a major 

scarp, called the Dahlgren scarp in this report, that 

rises more than 100 ft above the elevation of the terrace. Land cover at the EEA is forest, open field, 

marsh, and military operations facilities. Surfacewater bodies on the EEA include Black Marsh and 

several unnamed tributaries to Machodoc Creek.

Previous Investigations

An Initial Assessment Study (IAS) of the 

NSWCDL was begun in 1981 (Fred C. Hart Associates, 1983). The IAS identified 10 potentially contaminated sites at the EEA; however, Confirmation Studies 

were not recommended for any of these sites (O'Brien 

and Gere, 1986). The U.S. Environmental Protection 

Agency has since designated nine sites at the EEA as 

Solid Waste Management Units (areas where solid 

wastes may have been released, even if the areas were 

not intended for the management of solid or hazardous 

wastes) or Areas of Concern. An annual Site Management Plan (Halliburton NUS, 1995) estimated schedules for conducting Comprehensive Environmental 

Response, Compensation, and Liability Act activities. 

The USGS began a hydrogeologic framework investigation of the Mainside of the NSWCDL in 1992; 

results from that study are presented in a data report 

(Bell and others, 1994) and in an interpretive report 

(Harlow and Bell, 1996).

2 Hydrogeology and Water Quality of the Shallow Aquifer System, Dahlgren, Virginia
77'30'

38'30'

38" 15'

38'

77' 15" ~^F 77' 76'45'

'T

PRINCE WILLIAM \ CALVERT

/x

STAFFORD

/ 

Base from U.S. Geological Survey 

State base map, 1:500,000 10 20 MILES

10

I 

20 KILOMETERS

NORTH CAROLINA

Figure 1. Location of study area at the Explosive Experimental Area, Dahlgren, Virginia.

Introduction 3
O

(D 

(0

(0

(3

Figure 2. Locations of observation wells and surface-water sampling sites at the Explosive Experimental Area, Dahlgren, Virg
Meng and Harsh (1988) define and provide an 

overview of the hydrogeologic framework of the 

Virginia Coastal Plain as part of the USGS Regional 

Aquifer System Analysis (RASA) of the northern 

Atlantic Coastal Plain. A subsequent RASA report 

(Harsh and Laczniak, 1990) describes the results of 

numerical modeling of the ground-water-flow system 

in the Virginia Coastal Plain. Mixon and others (1989) 

mapped the geology of the Virginia Coastal Plain and 

adjacent parts of the Piedmont. Numerous researchers 

(Schubel and Zabawa, 1973; Kerhin and others, 1980; 

Shideler and others, 1984; Colman and Mixon, 1988; 

Colman and others, 1990) have noted and described 

Pleistocene fluvial channel deposits associated with 

the ancestral Susquehanna River and its tributaries.

Acknowledgments

The author wishes to thank Ann Swope, Billie 

Weedon, Collins Johnson, and other members of the 

Safety and Environmental Office, NSWCDL, for their 

assistance in planning and implementation of this 

study. James DeShazo, EEA facilities manager, and 

Robert Hoyer assisted in the coordination of activities 

at the EEA. The U.S. Army COE completed the drilling operations at the EEA in a safe and efficient manner under the supervision of Charles Brown, and 

conducted the laboratory analysis of Shelby tube samples under the direction of David P. Hammer. The 

author also would like to thank Dr. Grace S. Brush of 

the Johns Hopkins University for the pollen analysis, 

and Dr. Richard Berquist of the Virginia Division of 

Mineral Resources and Dr. Gerald H. Johnson of the 

College of William and Mary whose geologic insights 

were much appreciated. Special thanks are extended to 

Daniel R. Conrad, formerly of the USGS, who performed much of the field work for this study.

METHODS OF INVESTIGATION

The hydrogeology and water quality of the shallow aquifer system at the EEA were investigated by 

use of field and laboratory procedures that are 

described in this section. Hydrogeologic and waterquality data collected for this investigation are presented in a separate data report (Hammond and Bell, 

1995). Well-construction data and periodic water-level 

measurements are stored in the USGS GroundWater-Site-Inventory file, a storage and retrieval

system that is part of the National Water Information 

System (NWIS). The continuous ground-water-level 

data are stored in the USGS Automated Data Processing System (ADAPS), which also is part of NWIS. 

The original field notebooks and hard copies of all 

well-construction, water-level, and water-quality data 

are stored at the Virginia District office of the USGS.

Well Installation

On the basis of a hydrogeologic assessment of 

the Mainside (Harlow and Bell, 1996), 20 wells were 

drilled and completed in the Columbia aquifer and 

8 wells were completed in the upper confined aquifer 

for this study. Each of the eight confined aquifer wells 

were to be clustered with a well in the shallow Columbia aquifer. Beneath four of the cluster sites, however, 

the thick sands typical of the upper confined aquifer on 

the Mainside were not present; therefore, at these clusters, the deeper wells were completed in the 

Nanjemoy-Marlboro confining unit. Well locations 

(fig. 2) were chosen to obtain maximum areal coverage of the EEA, to have drill sites accessible by drilling equipment, and to avoid areas of known or 

suspected contamination.

All 28 observation wells installed at the EEA for 

the hydrogeologic assessment were constructed of 

4-in. inside-diameter polyvinyl chloride (PVC) casing. 

Of the 20 wells completed in the Columbia aquifer 17 

were augered, and 3 (wells EEA-S6, -S8, -Sll) were 

drilled by the hydraulic rotary method. Four wells 

were completed in the upper confined aquifer and four 

wells were completed in the Nanjemoy-Marlboro confining unit by the hydraulic rotary method. In order to 

avoid possible contamination of the confined aquifers 

from the Columbia aquifer during drilling, 10-in. 

inside-diameter PVC casing was set and grouted into 

the upper confining unit before drilling continued 

through the aquifer below. An additional well, called 

FCOT-4, was completed in the Columbia aquifer for 

another project but was utilized for the hydrogeologic 

assessment described here.

Wells in the Columbia aquifer were constructed 

with 5-, 10-, and!5-ft screens; wells in the upper confined aquifer were constructed with 10-ft screens; and 

wells in the Nanjemoy-Marlboro confining unit were 

constructed with 10-ft screens. Centralizers made of 

stainless steel were placed at the top and bottom of the 

well screens. Details on the construction of individual 

wells are presented in Hammond and Bell (1995).

Methods of Investigation
Analysis of Pollen Samples

Pollen samples from selected core intervals 

were analyzed to distinguish among Pleistocene and 

older deposits. Because of the widely different depositional settings of Pleistocene and Tertiary marine sediments, age information is important in interpreting the 

lateral and vertical extent of the different geologic 

units.

Twelve sediment samples, representing all four 

hydrogeologic units observed in the shallow aquifer 

system, were collected by use of Shelby tubes or splitspoon samplers. The samples were sent to the Department of Geography and Environmental Engineering at 

the Johns Hopkins University for pollen analysis. In 

order to extract pollen from sediment, a measured volume of sediment was washed in strong acid, and 

acetylzed to remove carbonates, silicates, and organic 

material. The residue was washed in acetic acid, distilled water, and alcohol, and then it was stored in tertiary butyl alcohol. Aliquots of 0.2 mL were mounted 

in silicone oil on microscopic slides, and all pollen in 

the aliquot was identified and counted under X 400 

(magnification) (G. Brush, Johns Hopkins University, 

oral commun., 1995).

Collection of Lithologic and Borehole 

Geophysical Data

The extent and thickness of hydrogeologic units 

at the EEA were determined by interpretation of lithologic and borehole geophysical data. Sediment samples were recovered with split-spoon samplers, and 

lithologic descriptions were logged during drilling 

operations at 20 wells. Lithologic characteristics 

recorded include grain size, color, textures, sedimentary structures, degree of moistness, and the presence 

of organic material, shell material, indurated material, 

glauconite, or mica. In some intervals, macro-fossils 

were recovered and identified to aid in the geologic 

interpretation. In addition to split spoon samples, 36 

Shelby tube cores from 18 well sites were analyzed for 

vertical hydraulic conductivity and mineralogy by the 

U.S. Army Corps of Engineers (COE), Ohio River 

Division Laboratory, Cincinnati, Ohio. Vertical 

hydraulic conductivity was measured by the fallinghead permeameter method. Mineralogy was determined by visual inspection and x-ray diffraction. The 

core intervals were chosen to characterize vertical and 

area! variations in lithology.

Geophysical logs were recorded at all drill sites 

(Hammond and Bell, 1995). A probe was lowered to 

the bottom of the borehole and the geophysical 

response was recorded as the probe was raised. Natural gamma-radiation, spontaneous potential, singlepoint resistance, 16-in. normal resistivity, and 64-in. 

normal resistivity logs were run at boreholes for wells 

completed in the upper confined aquifer and Nanjemoy-Marlboro confining unit, prior to installation of 

the casing. Natural gamma-radiation and electromagnetic (EM) induction conductivity logs were run at all 

wells after installation of the casing. Along certain 

intervals in the wells, the EM induction conductivity 

signal was affected by metal centralizers.

Water-Level Measurement

Continuous water-level data were collected 

from March 1994 through April 1995 and used to 

observe seasonal and short-term variations in water 

levels. The water-level data allow direct comparison 

of the effects of tidal influences, precipitation events, 

and evapotranspiration on water levels in different 

aquifers. Analog-to-digital recorders (ADR's) were 

completed on 10 wells (EEA-M1, SI, M3, S3, M4, S4, 

M6, S6, M8, and S8) to monitor the water levels. The 

ADR's recorded measurements every hour. Measurements of tidal fluctuations were collected by the USGS 

from a tide gage completed on the north side of Upper 

Machodoc Creek.

Water levels were measured monthly at each of 

the 28 wells completed for this study (Hammond and 

Bell, 1995) and at well FCOTS-4. Measuring points 

were established for each well on the top of the casing; 

the elevations of these wells were surveyed by the 

USGS from a first-order bench mark to an accuracy of 

0.01 ft. Water-levels in wells were measured to an 

accuracy of 0.01 ft by chalked steel tape. These measurements were then subtracted from the elevations of 

the respective measuring points to obtain the elevations of the water levels. Maps were constructed that 

depict hydraulic-head contours for the high-water and 

low-water periods of the year. Water-level data also 

were used to determine horizontal and vertical 

hydraulic-head gradients.

6 Hydrogeology and Water Quality of the Shallow Aquifer System, Dahlgren, Virginia
Slug Tests

Slug tests, by the volume-displacement method, 

were conducted on all 28 wells to determine horizontal 

hydraulic conductivity. An In-Situ Hermit (1000B) 

data logger and pressure transducer were used to 

record the water levels. Prior to the introduction of the 

slug, the water level was monitored for 10 minutes so 

that any rising or falling background trend could be 

established and subtracted from the water-level recovery. The slug was introduced by a sand-filled PVC 

cylinder of known volume. The cylinder was rapidly 

lowered into the well, causing a temporary rise in the 

water level. The data logger recorded the rise and the 

ensuing decline of the water level over time until it 

returned to equilibrium. Instantaneous water levels 

were recorded at intervals that varied from every 

0.2 seconds at the beginning of the test to every 

30 seconds at the end of the test. The decline in water 

level after removal of the slug from the well and the 

subsequent rise to equilibrium were also recorded on 

some wells.

For wells completed in the Columbia aquifer, 

slug-test data were analyzed by the Bouwer and Rice 

method (Bouwer and Rice, 1976; Bouwer, 1989), 

which is considered appropriate for unconfined conditions and can be applied to partially screened aquifers. 

Slug-test data from wells completed in the upper confined aquifer and the Nanjemoy-Marlboro confining 

unit were analyzed using a method described by 

Cooper, Bredehoeft, and Papadopulos (1967), which 

is considered appropriate for wells that are fully 

screened in confined aquifers. If available, data from 

slug-removal tests were used to obtain the hydraulic 

conductivities presented in this report; however, conductivities were calculated from both slug-addition 

and slug-removal tests to ensure that they differed by 

no more than one order of magnitude.

Water-Quality Sampling and Analysis

Water-quality samples were collected from the 

28 wells, from 2 surface-water sites in Machodoc 

Creek, and from 1 surface-water site in Black Marsh 

(fig. 2) in July 1994. Field parameters and the concentrations of selected chemical constituents, including 

major ions, were determined for all water-quality samples. Water-quality data were used to assess the background geochemistry of ground water and surface 

water at the EEA, and to determine the major hydrogeochemical processes that affect ground-water quality at the EEA. Water-quality data are presented in 

Hammond and Bell (1995).

Prior to the collection of water-quality samples, 

all equipment was cleaned thoroughly with laboratory 

detergent, and rinsed using distilled water. The outside 

of the pump and Teflon tubing were rinsed using distilled water between sampling, and the entire system 

was flushed using distilled water after each day of 

sampling. Ground-water samples were collected by 

use of a stainless steel submersible pump with a Teflon 

discharge tube. Each well was purged of at least three 

well volumes of water prior to sampling, unless the 

well went dry in which case the water level was 

allowed to recover completely prior to sampling. 

Surface-water samples were collected by use of a 

weighted-bottle sampler and 2-L polyethylene bottle 

using the equal-width-increment method (Edwards 

and Glysson, 1988), and composited in a Teflon chum 

splitter.

Temperature, dissolved oxygen, specific conductance, pH, and alkalinity were measured in the 

field. Temperature and dissolved oxygen were measured in wells and surface water after sampling by use 

of a YSI (model 54A) dissolved oxygen meter with 

cable and submersible stirrer. The dissolved oxygen 

meter was calibrated to air at least once per day. Specific conductance and pH were measured on unfiltered 

water samples by use of an Orion (model 122) specific 

conductance meter with glass conductivity cell and 

Orion (S A 250) pH meter with gel-filled pH electrode, 

respectively. The calibration of the specific conductance meter was checked each day using three solutions of known conductance. The pH meters were 

calibrated each day using two solutions of known pH 

values (usually pH 4 and 7). Alkalinities were determined by incremental titration of 100-mL filtered 

water samples using sulfuric acid.

Water samples collected for dissolved inorganic 

analyses were filtered immediately after collection to 

minimize oxidation of any unstable and chemically 

reduced solutes and then placed in acid-rinsed bottles. 

The filtering was accomplished by use of a peristaltic 

pump with silicon tubing and a 142-mm-diameter 

filter-plate assembly with 0.45-urn pore-size filter 

membrane. The filter membranes were flushed with 

200 mL of American Society of Testing and Materials 

Type II water before filtration of the sample. Water 

samples collected for major cations and metals were 

acidified to pH 2 using concentrated nitric acid. All

Methods of Investigation
equipment used for sampling dissolved organic carbon 

was first rinsed using carbon-free water; sample water 

was then passed through a 0.45-um silver filter and 

placed into clear glass bottles. All sample bottles were 

placed in sealed plastic bags, stored on ice, and 

shipped overnight in high-impact plastic ice chests to 

the laboratories.

Quality controls included duplicate samples, an 

equipment blank, and a carbon-free deionized water 

blank. Duplicate samples were collected at approximately 10 percent of the sampling sites, including one 

duplicate surface-water sample to determine the reproducibility of water-quality data. The equipment blank 

was collected by passing carbon-free deionized water 

through the pump and tubing.

All water-quality samples were sent to the 

USGS National Water Quality Laboratory in Arvada, 

Colo., for determination of inorganic constituents and 

organic carbon. Metals and major cations were measured by use of inductively coupled argon plasma 

spectroscopy. Chloride and fluoride were measured by 

a colorimetric method, and sulfate concentrations were 

determined by a turbidimetric method. Total dissolved 

solids were measured as residue-on-evaporation at 

180°C. The analytical methods are described in 

Wershaw and others (1987) and Fishman and Friedman (1989). Laboratory quality-assurance procedures 

are described in Friedman and Erdmann (1982) and 

Jones (1987).

HYDROGEOLOGY OF THE SHALLOW 

AQUIFER SYSTEM

The shallow aquifer system underlying the 

EEA, as defined in this report, comprises the uppermost 100 ft of sediments, and has been divided into 

four hydrogeologic units: (1) the Columbia aquifer; 

(2) the upper confining unit; (3) the upper confined 

aquifer; and (4) the Nanjemoy-Marlboro confining 

unit (fig. 3). In some areas of the EEA, the upper confining unit and upper confined aquifer are missing and 

the Columbia aquifer lies directly on the NanjemoyMarlboro confining unit. Lithologic data and geophysical borehole logs collected for this study and the 

hydrogeologic assessment of the Mainside (Harlow 

and others, 1996) provided the primary basis for 

framework interpretation. Other sources of hydrogeologic data included outcrops along Machodoc Creek 

and the Potomac River, lithologic logs of a borehole at 

Little Ferry Landing, and lithologic and geophysical

logs of a water-supply well at the EEA (unpublished 

data on file in the Virginia District office of the U.S. 

Geological Survey).

Geology

Several thousand feet of unconsolidated sedimentary deposits of the Coastal Plain Physiographic 

Province underlie the NSWCDL. These deposits consist of tabular bodies of interbedded sand, silt, and 

clay, and range in age from Cretaceous to Quaternary 

(Meng and Harsh, 1988). The stratigraphy and 

inferred depositional environments of the sediments 

that constitute the shallow aquifer system at the EEA 

are derived from previous investigations (Ward, 1985; 

Meng and Harsh, 1988; Mixon and others, 1989; 

Harlow and Bell, 1996) and from interpretations of 

lithologic and geophysical data collected for this 

investigation.

Stratigraphy

The basal geologic unit of the shallow aquifer 

system is the Nanjemoy Formation of Eocene age 

(fig. 3). The Nanjemoy Formation is divided formally 

into the lower Potapaco Member, which consists of 

glauconitic sandy clay; and the upper Woodstock 

Member, which consists of very glauconitic, micaceous silty sand. At the EEA, the Potapaco Member 

crops out at several locations along the shores of the 

Machodoc Creek and the Potomac River. Die Potapaco Member can be recognized by its distinctive 

lithology (glauconitic silty fine sands) and the presence of abundant bivalve fossils, including Venericardia ascia and Macrocallista subimpressa (Ward, 

1985).

The Nanjemoy Formation is unconformably 

overlain by deposits of Pleistocene age, which are 

divided, in this report, into two geologic units: undifferentiated Pleistocene deposits and the Tabb Formation of late Pleistocene age. The undifferentiated 

Pleistocene deposits consist of coarse-grained sand 

and pebbles that fine upward to interbedded fine- to 

medium-grained sand, clay, and peat, and then to clay 

that contains abundant wood fragments at the top of 

the unit. Pollen analysis of cores from this unit confirmed the age of this unit as Pleistocene, but it currently is unknown with which formally defined 

geologic formation these deposits correlate. Possibilities include the Shirley and Charles City Formations.

8 Hydrogeology and Water Quality of the Shallow Aquifer System, Dahlgren, Virginia
QUATENRARY

TERITIARY

GEOLOGIC UNIT

HOLOCENE

PLEISTOCENE

EOCENE

PALEOCENE

Alluvial, paludal, and fill deposits

Tabb Formation (Sedgefield Member)

Pleistocene deposits, undifferentiated

Nanjemoy 

Formation

Woodstock 

Member

Potapaco 

Member

Marlboro Clay

HYDROGEOLOGIC UNIT

Columbia aquifer

upper confining unit

upper confined aquifer

Nanjemoy-Marlboro 

confining unit

NOT TO SCALE

Figure 3. Schematic diagram of geologic and hydrogeologic units of the shallow aquifer system at the 

Explosive Experimental Area, Dahlgren, Virginia.

Hydrogeology of the Shallow Aquifer System 9
The Charles City Formation has been mapped at the 

top of the Dahlgren scarp south of the eastern part of 

the EEA (Mixon and others, 1989).

The Sedgefield Member of the Tabb Formation 

overlies the undifferentiated Pleistocene deposits, and 

consists of fine- to coarse-grained sand and pebbles 

that fine upward into silt and clay. The Sedgefield 

Member of the Tabb Formation has been mapped as 

the surficial geologic unit across the entire EEA 

(Mixon and others, 1989); no deposits of the 

Lynnhaven or Poquoson Members of the Tabb Formation have been mapped at the study area. Uraniumseries age dating of corals from the Sedgefield Member have yielded an average age of 71,000 ± 7,000 

years before present (B.P.) (Mixon and others, 1989); 

however, basal sands may be as old as 100,000 years 

B.P. (R. Berquist, Virginia Division of Mineral 

Resources, oral commun., 1995). Organic-rich mud, 

sand, and peat of Holocene age are found in intertidal 

areas at the EEA.

Depositional History

Sediments of the Nanjemoy Formation were 

deposited in a protected marine shelf with water levels 

that ranged from 50 to 230 ft (Meng and Harsh, 1988). 

During several Miocene, Pliocene, and Pleistocene 

marine transgressions, additional sediments were 

deposited over the study area that have been subsequently removed by erosion during the late Pleistocene. For example, Miocene and Pliocene sediments 

of the Chesapeake Group crop out at the face of the 

Dahlgren scarp, south and west of the EEA, however, 

they are no. longer present at the EEA.

During a Pleistocene glacial period, fluvial erosion incised deep (greater than 80 ft) paleochannels 

into the Nanjemoy Formation, which filled with the 

undifferentiated Pleistocene sediments during the subsequent interglacial period as sea level rose. These 

sediments were deposited in a high-energy fluvial 

environment that gradually changed to a low-energy 

estuarine-paludal (marsh) environment during the 

course of the transgression. The undifferentiated Pleistocene sediments may be contemporaneous with the 

Charles City Formation that has been mapped at the 

top of the Dahlgren scarp (Mixon and others, 1989). 

Alternatively, these sediments may correlate with the 

Shirley Formation, which is one of the most extensive 

Pleistocene units in the Virginia Coastal Plain Physiographic Province.

The Sedgefield Member of the Tabb Formation 

was deposited in a fluvial-estuarine setting during a 

later Pleistocene marine transgression. Sedgefield depositional environments at the EEA range vertically and 

laterally from high-energy fluvial environments to 

low-energy floodplain and estuarine environments. 

Maximum sea level during the deposition of the 

Sedgefield Member was approximately 30 ft above 

present sea level, and thus the Sedgefield member was 

not deposited on top of the Dahlgren scarp.

The Holocene Epoch began approximately 

10,000 years ago as sea level began to rise following 

the Wisconsinan glacial period. Low areas at the EEA, 

such as Black Marsh and unnamed tributaries to 

Machodoc Creek and the Potomac River, were inundated during the Holocene, resulting in the deposition 

of paludal (swamp and marsh) mud, sand, and peat.

Physical Properties of Hydrogeologic 

Units

The following sections describe the lithology, 

hydraulic characteristics, extent, and thickness of 

hydrogeologic units beneath the EEA. The interpreted 

hydrogeologic framework is illustrated by use of maps 

and hydrogeologic sections. Hydrogeologic data 

collected for this investigation are published in 

Hammond and Bell (1995).

Columbia Aquifer

The Columbia aquifer is present across the 

entire EEA, but it has highly variable physical properties that correspond to the range of depositional environments in which it was formed. The Columbia 

aquifer consists of sand, silt, and clay, and a pebble lag 

deposit present at the base of the aquifer in most locations. The Columbia aquifer correlates with the Sedgefield Member of the Tabb Formation and Holocene 

paludal deposits (fig. 3), however, it also contains artificial fill material at some locations. The finingupward nature of the Tabb Formation causes the aquifer generally to have the most permeable sediments at 

its base, and the least permeable sediments near land 

surface. At any one location, however, multiple finingupward sequences may be present in the aquifer. Rates 

and directions of flow in the Columbia aquifer are 

affected strongly by local lithologic heterogeneities, 

such as thin layers of clay or coarse-grained paleochannel deposits.

10 Hydrogeology and Water Quality of the Shallow Aquifer System, Dahlgren, Virginia
The depth from land surface to the top of the 

first confining unit represents the maximum potential 

thickness of the Columbia aquifer; this thickness varies from less than 8 ft at well EEA-S4 to approximately 34 ft at well EEA-S13 (fig. 4). A hydrogeologic map of the altitude of the bottom of the 

Columbia aquifer (fig. 4) illustrates the erosional surface formed prior to deposition of the Tabb Formation. 

The actual saturated thickness of the Columbia aquifer 

varies seasonally as water levels rise and fall. In general, the depth to the first confining unit is greatest in 

the southern part of the EEA, where Pleistocene 

streams have eroded older material and deposited 

thick beds of sand and pebbles. These paleochannel 

deposits were noted during construction of wells EEAS3, Sll, S12, S13, and S15, where the depth to the 

first confining unit exceeds 20 ft. Coarse-grained 

deposits also were noted during construction of wells 

EEA-S6 and EEA-S8, suggesting that paleochannels 

also underlie the northern part of the EEA.

The depth to the first confining unit is generally 

less than 15 ft in the central area of the EEA, as indicated by borehole logs from wells EEA-S2, S4, S5, 

S7, S9, S10, S16, S17, S18, and S19. This central area 

of the EEA roughly corresponds to an east-west trending band, where the upper confining unit is missing 

and the Columbia aquifer directly overlies the 

Nanjemoy-Marlboro confining unit. In such areas, the 

contact between the Columbia aquifer and the 

Nanjemoy-Marlboro confining unit lies above sea 

level and can be viewed in outcrops along the 

Machodoc Creek and the Potomac River. North and 

south of this area, the Columbia aquifer overlies the 

upper confining unit.

The variable lithology of the Columbia aquifer 

results in a wide range of hydraulic properties. Horizontal hydraulic conductivities in the Columbia aquifer, calculated from slug tests of 18 wells, ranged from 

0.10 ft/d (well EEA-S10) to 21 ft/d (well EEA-S17), 

with a median value of 1.4 ft/d (table 1, fig. 5). The 

base of the Columbia aquifer, where wells completed 

for this study were screened, contains the coarsestgrained deposits of the aquifer; therefore, horizontal 

hydraulic conductivities calculated from slug tests represent some of the most permeable sediments of the 

aquifer.

In the center of the EEA, at wells EEA-S4, S9, 

S10, and S18, horizontal hydraulic conductivities generally are less than 1 ft/d; whereas in the northern and 

southern areas of the EEA, horizontal hydraulic

conductivities generally are greater than 1 ft/d. This 

pattern does not hold everywhere, as indicated by low 

horizontal hydraulic conductivities at wells EEA-S1 

and Sll (0.20 and 0.19 ft/d, respectively). The Columbia aquifer, however, is thicker in the northern and 

southern parts of the EEA than in the central area, and 

transmissivity would be greater in those areas even 

where the horizontal hydraulic conductivity is not significantly higher.

Vertical hydraulic conductivities in the Columbia aquifer, measured by falling-head permeameter 

tests of seven Shelby tube cores, ranged from a minimum of 0.0023 ft/d in a sandy clay (well EEA-S3) to a 

maximum of 4.3 ft/d in a sand (well EEA-M2), with a 

median value of 0.14 ft/d (fig. 5). Shelby tube cores 

cannot penetrate very pebbly sediments or very hard 

clays; therefore, vertical hydraulic conductivities 

obtained from Shelby tube cores do not represent the 

upper and lower extreme vertical permeabilities for 

the Columbia aquifer.

At some locations, the Columbia aquifer contains thick deposits of clay that can significantly 

impede downward movement of water. A highly plastic, stiff, mottled clay, for example, underlies much of 

the northern area of the EEA (EEA-S6, S7, S8, S17, 

SI8, and S20). This clay lies at elevations ranging 

from 3 to 17 ft above sea level, and varies in thickness 

from about 0.2 ft at well EEA-S7 to more than 10 ft at 

well EEA-S20. Because the clay pinches out, or was 

eroded to the south, it was not observed at wells EEAS5,S9,orS16.

Upper Confining Unit

The upper confining unit consists of plastic clay, 

silty clay, and clayey sand that is grey in color and 

contains abundant organic material, including wood 

fragments. This unit is overlain by the Columbia aquifer and has a gradational contact with the underlying 

upper confined aquifer. In some areas, the upper confining unit may directly overlie the NanjemoyMarlboro confining unit. The upper confining unit correlates with the upper, fine-grained, undifferentiated 

Pleistocene deposits (fig. 3).

The upper confining unit is not present across 

the entire EEA; either it was not deposited or it was 

eroded from an east-west trending band across the 

center of the EEA. In this area, the Columbia aquifer 

directly overlies the Nanjemoy-Marlboro confining 

unit, as illustrated in hydrogeologic section A-A' 

(fig. 6). The altitude of the top of the upper confining

Hydrogeology of the Shallow Aquifer System 11
77

3 

5'

38°19'00"

s.

$

§SL

 31

a

<D

0>

1

1

a

i

\ > .- /

**y

Wood 

A ' "\ : island

.-$ -AEEA-M7 .

t ' 1 ' \

V?r°\x^_ . <

' - \ " *v

/ ^ ,,

x";" -X

:Jlv^-.;-

*"^. *-:: .

A BaDer Point 

1 KILOME

Figure 4. Altitude of bottom and thickness of the Columbia aquifer at the Explosive Experimental Area, Dahlgren, Virginia.
Table 1. Horizontal hydraulic conductivities determined from slug tests at the Explosive Experimental Area, 

Dahlgren, Virginia

[for locations of wells, see figure 2; slug-test data from wells completed in the Columbia aquifer were analyzed by the Bouwer and Rice method 

(Bouwer and Rice, 1976; Bouwer, 1989); slug-test data from wells completed in the upper confined aquifer and the Nanjemoy-Marlboro 

confining unit were analyzed using a method described by Cooper, Bredehoeft, and Papadopulos (1967)]

Well number

EEA-S1

EEA-S3

EEA-S4

EEA-S6

EEA-S7

EEA-S8

EEA-S9

EEA-S10

EEA-S11

EEA-S12

EEA-S13

EEA-S14

EEA-S15

EEA-S16

EEA-S17

EEA-S18

EEA-S19

EEA-S20

EEA-M1

EEA-M2

EEA-M3

EEA-M4

EEA-M5

EEA-M6

EEA-M7

EEA-M8

Hydrogeologic unit

Columbia aquifer

Columbia aquifer

Columbia aquifer

Columbia aquifer

Columbia aquifer

Columbia aquifer

Columbia aquifer

Columbia aquifer

Columbia aquifer

Columbia aquifer

Columbia aquifer

Columbia aquifer

Columbia aquifer

Columbia aquifer

Columbia aquifer

Columbia aquifer

Columbia aquifer

Columbia aquifer

upper confined aquifer

Nanjemoy-Marlboro confining unit

upper confined aquifer

Nanjemoy-Marlboro confining unit

Nanjemoy-Marlboro confining unit

upper confined aquifer

Nanjemoy-Marlboro confining unit

upper confined aquifer

Type of slug test

removal

removal

addition

removal

addition

removal

addition

addition

addition

removal

addition

addition

removal

removal

removal

addition

addition

removal

removal

addition

removal

addition

removal

removal

removal

removal

Horizontal hydraulic 

conductivity 

(feet per day)

0.20

12

.63

6.8

1.8

1.5

.58

.10

.19

1.4

5.7

9.5

8.7

1.6

21

.63

1.2

1.0

1.1

.0086

3.3

.043

.026

5.1

.0032

23

Hydrogeology of the Shallow Aquifer System 13
IV»

o D 1 °1

< °~

Q UJ 10° >- u_

i 5-

I > 10'1

NH

O D

§ 1 °"2

10'3

- 1

 

(18)

-

-

EXPLANATION

MAXIMUM

j I 75

uj - 50

UJ 9(; 

Q. tO

MINIMUM

(16) NUMBER OF

SAMPLES

COLUMBIA UPPER NANJEMOYAQUIFER CONFINED MARLBORO

AQUIFER CONFINING

101 

% 10°

Q

| S 1C'1

Q it! 10'2

< ^" 10'3

UJ O 10-4

8 i°-5

10'6

UNIT

!

-

-

-

-

I

^

.

-

-

(16)

(13)

COLUMBIA UPPER NANJEMOYAQUIFER CONFINING MARLBORO 

UNIT CONFINING

UNIT

Figure 5. Distribution of horizontal and vertical hydraulic conductivity of hydrogeologic 

units at the Explosive Experimental Area, Dahlgren, Virginia. (Horizontal hydraulic 

conductivity was calculated from slug tests; vertical hydraulic conductivity was measured 

from falling-head permeameter tests of Shelby tube samples.)

14 Hydrogeology and Water Quality of the Shallow Aquifer System, Dahlgren, Virginia
unit ranges from 21.8 ft below sea level (well BEAMS) to approximately 13 ft above sea level (well 

54Q12) (fig. 7), and reaches about 21.0 ft above sea 

level just outside the EEA at Little Ferry Landing 

(fig. 7) (unpublished data on file at the Virginia District office of the U.S. Geological Survey).

Pleistocene streams eroded a high-relief surface 

into the top of the upper confining unit. Correspondingly, the altitude of the top of the upper confining unit 

is lowest where the Columbia aquifer has the coarsest, 

most permeable sediments, such as at well EEA-S3 

and EEA-S6. The upper confining unit at the EEA 

ranges in thickness from about 18 ft (well 54Q12) to 

about 51 ft (well EEA-S6) (fig. 7) and reaches 61 ft at 

Little Ferry Landing (unpublished data on file at the 

Virginia District office of the U.S. Geological Survey). 

The unit is absent in the center of the study area.

The upper confining unit has the lowest permeability values in the shallow aquifer system (fig. 5), as 

indicated by falling-head permeameter test of 13 

Shelby tube cores (Hammond and Bell, 1995). Vertical 

hydraulic conductivities ranged from 6.5XKT6 ft/d 

(well EEA-M8) to 0.71 ft/d (EEA-M6), with a median 

value of 2.0X10"4 ft/d (fig. 5).

Upper Confined Aquifer

The upper confined aquifer consists of interbedded sand and clay grading downward into sand and 

pebbles. The sand is typically glauconitic, and the 

entire aquifer contains abundant organic material, 

including wood fragments and tree stumps. The upper 

confined aquifer has a gradational contact with the 

overlying upper confining unit, and unconformably 

overlies the Nanjemoy-Marlboro confining unit. Geologically, this unit correlates with the lower, coarsegrained part of the undifferentiated Pleistocene deposits (fig. 3). At some locations, notably well EEA-S2, 

fine sands of the Nanjemoy Formation also are 

included as part of the upper confined aquifer.

The upper confined aquifer was deposited in 

paleochannels and, thus, is not present across the 

entire EEA. As expected, the upper confined aquifer 

has a similar areal distribution to the upper confining 

unit; the aquifer is present in the northern and southern 

parts of the EEA, but it was not deposited over an eastwest trending band across the center of the study area 

(fig. 8). The altitude of the top of the aquifer decreases 

to the north and to the south of this area, ranging from 

17 ft below sea level (well EEA-M2) to 57 ft below 

sea level (well EEA-M3) (fig. 8). The observed thickVERT1CAL SCALE GREATLY EXAGGERATED

EXPLANATION

i

COLUMBIA AQUIFER 

UPPER CONFINED AQUIFER

UPPER CONFINING UNIT

NANJEMOY-MARLBORO 

CONFINING UNIT

HYDROGEOLOGIC CONTACT 

TOTAL DEPTH OF WELL

Figure 6. Hydrogeologic section A-A'at the Explosive 

Experimental Area, Dahlgren, Virginia. (Line of section 

shown in figure 2.)

ness of the upper confined aquifer varies from about 

11 ft (well EEA-M2) to about 33 ft (well EEA-M3) 

(fig. 8). The base of the upper confined aquifer, like 

that of many fluvial deposits, exhibits high relief, 

ranging from 90 ft below sea level (well EEA-S3) to 

about 28 ft below sea level (well EEA-M2). The upper 

confined aquifer is the most permeable unit in the shallow aquifer system (fig. 5). Horizontal hydraulic conductivities from 4 slug tests range from 1.1 ft/d (well 

EEA-M1) to 23 ft/d (well EEA-M8) (table 1).

Nanjemoy-Marlboro Confining Unit

The Nanjemoy-Marlboro confining unit consists 

of greenish-black, silty, fine-grained sand and sandy 

clay. This unit is very glauconitic and contains abundant bivalve fossils. Geologically, the NanjemoyMarlboro confining unit correlates with the Nanjemoy 

Formation of Eocene age and the Marlboro Clay of

Hydrogeology of the Shallow Aquifer System 15
0) 77'03'30" 77'02'30% 77°01'30"

\o

EXPLANATION

APPROXIMATE AREA WHERE UPPER CONFINING 

RMi£ UNIT IS MISSING

 10 LINE OF EQUAL THICKNESS OF THE UPPER 

CONFINING UNIT-Dashed where approximately 

located. Contour interval is 10 feet

WELL CLUSTER AND WELL NUMBERALTITUDE OF TOP OF UNIT. 

3.44 DATUM IS SEA LEVEL

22 THICKNESS OF UNIT IN FEET 

M§12 WATER-SUPPLY WELL AND WELL NUMBER

38"17'00"

Figure 7. Thickness and altitude of upper confining unit at the Explosive Experimental Area, Dahlgren, Virginia.
77"03'30"

38"19'00"

38"17'00'

APPROXIMATE AREA WHERE UPPER CONFINED 

AQUIFER IS MISSING

 20 -- LINE OF EQUAL ALTITUDE OF THE TOP OF THE 

UPPER CONFINED AQUIFER-Dashed where 

approximately located. Datum is sea level. Contour 

interval is 20 feet

EEA-M1 WELL CLUSTER AND WELL NUMBER

ALTITUDE OF TOP OF UNIT 

DATUM IS SEA LEVEL

21 THICKNESS OF UNIT IN FEET

Figure 8. Altitude of top and thickness of upper confined aquifer at the Explosive Experimental Area, Dahlgren, Virginia.
Paleocene age, although wells drilled for this study did 

not reach the latter unit. Locally, glauconitic silty 

sands from the Nanjemoy Formation that were 

reworked during the Pleistocene epoch are included in 

the Nanjemoy-Marlboro confining unit. These Pleistocene sediments are lithologically very similar to the 

Nanjemoy Formation but contain Pleistocene pollen 

and lack Eocene bivalves.

Pleistocene erosion carved a surface of high 

(greater than 100 ft) relief on the top of the NanjemoyMarlboro confining unit (figs. 6,9). In the northern 

and southern areas of the EEA, fluvial erosion incised 

Nanjemoy-Marlboro deposits to a maximum depth of 

90 ft below sea level (well EEA-M3). An east-west 

trending band across the center of the EEA, however, 

was a paleointerfluve; in this area the top of the 

Nanjemoy-Marlboro confining unit lies above sea 

level and can be seen as outcrop along the shores of 

the Machodoc Creek and the Potomac River.

The Nanjemoy-Marlboro confining unit is more 

permeable than the upper confining unit (fig. 5). Horizontal hydraulic conductivities from 4 slug tests 

ranged from 3.2xlO~3 fl/d (well EEA-M7) to 4.3xlO~2 

ft/d (well EEA-M4) (table 1). Vertical hydraulic conductivities from falling-head permeameter tests of 

16 Shelby tube cores ranged from 5.9xlO~5 ft/d to 

7.1xlO~2 ft/d, with a median value of 8.8xlO~3 ft/d 

(fig. 5).

Hydrologic Characteristics of Aquifers

Ground-water recharge, horizontal flow, and 

discharge pathways for the Columbia aquifer and the 

upper confined aquifer were interpreted from hydrogeologic data. Contour maps were drawn to illustrate 

the hydraulic head distribution of both aquifers during 

March and December 1994, which represent highwater and low-water conditions, respectively. Interpreted directions of ground-water flow are orthogonal 

to the contours of hydraulic head. Water-level data 

from the Nanjemoy-Marlboro confining unit were 

used to interpret the hydraulic connection of this unit 

with the two aquifers. Hydrographs were constructed 

to illustrate the responses of wells to precipitation, 

evapotranspiration, and tides.

Columbia aquifer

Hydraulic heads in the Columbia aquifer fluctuate as much as 10 ft annually in response to changing

rates of precipitation and evapotranspiration (figs. 10 

and 11). The highest water levels were measured in the 

center of the EEA, where land-surface elevations 

exceed 20 ft (wells EEA-S1, S4, S5, S9, S10, and 

SI 1). Water levels decrease toward major surfacewater drainages, such as Machodoc Creek, Black 

Marsh, and the Potomac River.

During high-water conditions, water levels in 

the Columbia aquifer are within 2 ft of land surface 

across most of the EEA, and sometimes water levels 

rise above land surface, forming ponds in flat, poorly 

drained areas, such as at wells EEA-S4, S5, S9, S10, 

S16, S19, and S20. In areas of high relief and sandy 

Columbia sediments, such as along the southern fence 

(wells EEA-S12, S13, S14, and S15), the water table 

usually remains 4 ft or more beneath land surface. 

During low-water conditions, water levels approach or 

fall below sea level in wells EEA-S3 and S7, which lie 

very close to major surface-water bodies, and also in 

well EEA-S15, which drains rapidly because of high 

relief and permeable Columbia sediments.

Recharge

The Columbia aquifer is recharged directly by 

precipitation across most of the EEA. Water levels in 

most of the wells in the Columbia aquifer rise significantly and quickly after precipitation events (fig. 12), 

indicating a direct connection to the surface. Locally, 

water in the Columbia aquifer may be perched on top 

of clay layers or semi-confined beneath them. In particular, the plastic clay that lies beneath much of the 

northern area of the EEA could significantly impede 

vertical movement of ground water. Hydrographs at 

well clusters 4,5, and 7 (fig. 13) indicate that during 

parts of the year there is an upward gradient between 

the Columbia aquifer and the underlying NanjemoyMarlboro confining unit, especially at well cluster 5. 

In the Churchill Test Area, and possibly other areas, 

upward hydraulic gradients indicate that the Columbia 

aquifer can receive recharge from the underlying 

Nanjemoy-Marlboro confining unit.

Water-level maps indicate that the Columbia 

aquifer at the EEA is not significantly recharged from 

areas outside the EEA, with the exception of some 

areas along the southern boundary. Ground water 

flowing beneath the EEA from the south tends to discharge to Black Marsh, its tributaries, and a small tributary of Machodoc Creek before reaching the center of 

the study area. At wells located south of Black Marsh, 

(EEA-S13, S14, and S15) hydraulic heads are

18 Hydrogeology and Water Quality of the Shallow Aquifer System, Dahlgren, Virginia
77C'Q3'30" 77"01'30"

ws

I

I 

fCO

38'; 19'00'

EXPLANATION

10 LINE OF EQUAL ALTITUDE OF THE TOP OF THE 

NANJEMOY-MARLBORO CONFINING UNIT-Dashed 

where approximately located. Datum is sea level. 

Contour interval is variable

EEA^S! 0 WELL AND WELL NUMBER

MA-MI WELL CLUSTER AND WELL NUMBER (Altitude of

(-40) top of Nanjemoy-Marlboro confining unit in feet above

A sea level)

38'1700'

 5J 

«£.

$

^ Figure 9. Altitude of top of Nanjemoy-Marlboro confining unit at the Explosive Experimental Area, Dahlgren, Virginia. to
TTOS'SO" ~1

§o.

8=a

o

! i

If

i

10

Is*

38" 19-00"

38'17'OC1

10 - - POTENTIOMETRIC CONTOUR OF THE COLUMBIA

AQUIFER, MARCH 1994--0ashed where

Baber Point approximately located. Datum is sea level. Contour 

interval is 5 feet

WELL AND WELL NUMBER

EEA-M1 WELL CLUSTER AND WELL NUMBER (Attitude of (23.9) water level in Columbia aquifer in feet above sea level)

CHURCHILL TEST AREA

Figure 10. Potentiometric surface of the Columbia aquifer, March 1994, at the Explosive Experimental Area, Dahlgren, Virginia.
77'03'30"

| 

I

9 to

I

38°19'0oi *

I '

&/ ' 9

Baher Point

\

EXPLANATION

 10 POTENTIOMETRIC CONTOUR OF THE COLUMBIA 

AQUIFER, DECEMBER 1994-Dashed where approximately located. Datum is eea level. Contour interval is 5 feat

EEA-S11 ;AJS11 WELL AND WELL NUMBER

EEA-M1 WELL CLUSTER AND WELL NUMBER (Altitude of 

(18.5) water level in Columbia aquifer in feet above sea level)

38° t TOO"

jo Figure 11 . Potentio metric surface of the Columbia aquifer, December 1994, at the Explosive Experimental Area, Dahlgren, Virginia.
LU

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Figure 12. Maximum daily water level in the Columbia aquifer for well EEA-S1 and precipitation data, April 1994 

through March 1995, at the Explosive Experimental Area, Dahlgren, Virginia.

significantly higher than those at nearby wells north of 

Black Marsh because ground water was recharged at 

higher elevations on the Dahlgren scarp or the upland 

terrace. For example, despite the close proximity of 

wells EEA-S12 and S13 (fig. 2), hydraulic head at well 

EEA-S13 is significantly higher than at well EEA-S12 

(fig. 14) because these wells are located on opposite 

sides of a tributary to Black Marsh.

At well EEA-S3, the Columbia aquifer has a 

direct hydraulic connection with the Potomac River, as 

indicated by tidal oscillations in ground-water level 

and by water quality. The connection is enhanced by 

very permeable Columbia aquifer sediments, and isolation of the Columbia aquifer from upland areas. Well 

EEA-S3 is located on a small area of land surrounded 

on all sides by surface water (fig. 2), and thus shallow 

ground water is recharged only by direct precipitation 

and surface water.

Horizontal Flow

The water table is shallow and subparallel to 

land surface (figs. 10,11). Water in the Columbia 

aquifer generally flows from topographically high 

areas beneath the western and central EEA toward 

topographically low areas, such as the Potomac River,

Machodoc Creek, Black Marsh, and unnamed tributaries of these surface-water bodies, as well as to drainage ditches and swales.

The horizontal hydraulic gradient is greater in 

the western third of the EEA than the eastern part 

because of the steep land-surface slope along the shore 

of Machodoc Creek upstream of Rowland Point. The 

average horizontal hydraulic gradient across the entire 

study area is approximately 0.01. Using an estimated 

aquifer porosity of 0.30 and a range in horizontal 

hydraulic conductivity of 0.10 to 21 ftVd (table 1), linear rates of ground-water flow in the Columbia aquifer 

are estimated to range from about 0.003 to 0.70 ft/d or 

about 1 to 300 ft/year on average. These low rates of 

flow are expected for an area such as the EEA that 

generally has flat topography and relatively finegrained sediments. Rates of ground-water flow 

beneath topographically flat areas are less than the 

average, whereas flow rates beneath steep areas, such 

as near discharge areas, are greater than the average.

Discharge

Ground-water discharges from the Columbia 

aquifer by: (1) evapotranspiration, (2) springs and 

seeps, and (3) downward leakage to the Nanjemoy22 Hydrogeology and Water Quality of the Shallow Aquifer System, Dahlgren, Virginia
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 EEA-S7

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1994 1995

Figure 13. Maximum daily water level in the Columbia aquifer for well cluster 4 and synoptic waterlevel measurements for well clusters 5 and 7, April 1994 through April 1995, at the Explosive 

Experimental Area, Dahlgren, Virginia.

Hydrogeology of the Shallow Aquifer System 23
A M J JASOND

1994 1995

Figure 14. Maximum daily water levels in the Columbia 

aquifer for wells EEA-S12 and EEA-S13, April 1994 through 

March 1995, at the Explosive Experimental Area, Dahlgren, 

Virginia.

Marlboro confining unit. The water table lies within 

10 ft of land surface across most of the EEA, and 

ground water can evaporate directly to the atmosphere 

or can be taken up by plant roots. Evapotranspiration 

is most dramatic in forested areas, such as near well 

EEA-S1, where it causes a marked diurnal fluctuation 

in water level from late May until early October 

(fig. 15). Evapotranspiration is a major ground-waterdischarge pathway in flat, forested areas of the EEA. 

Evapotranspiration also occurs in the wetland areas 

fringing surface-water bodies, such as Black Marsh 

and Machodoc Creek, and their tributaries, which 

receive ground water from the upland areas of the 

EEA.

Erosion by the Machodoc Creek and the Potomac River has resulted in low (less than 30 ft) bluffs 

along the shoreline of the EEA. At these locations, 

ground water can discharge from springs and seeps as 

it flows along the top of the Nanjemoy-Marlboro confining unit, upper confining unit, or clay beds in the 

Columbia aquifer. A spring was observed discharging 

ground water to Machodoc Creek near well cluster 7 

during October 1993, but such springs probably flow 

only during very wet periods. At other times, ground 

water discharges by evaporation from the seepage face 

of the bluff. Ground water also likely discharges from 

the Columbia aquifer directly to surface water by way 

of submarine seeps and springs.

Hydraulic Connection with Nanjemoy-Marlboro 

Confining Unit

In some areas where the Columbia aquifer 

directly overlies the Nanjemoy-Marlboro confining

unit, there is an exchange of water between the two 

units. Hydraulic properties of the units are similar in 

areas where the Columbia aquifer consists of silty 

sand and finer material; for example, only about one 

order-of-magnitude difference in horizontal hydraulic 

conductivity was estimated between wells EEA-S4 

and M4 (0.63 and 0.043 ft/d, respectively) (table 1). 

Similarly, there is overlap in the ranges of vertical 

hydraulic conductivity for the two units (fig. 5). In 

these areas of direct contact, the Columbia aquifer and 

Nanjemoy-Marlboro confining unit can effectively 

combine to form a thick (greater than 50 ft) aquifer, 

and the Columbia aquifer can either contribute to, or 

receive ground water from, the Nanjemoy-Marlboro 

confining unit, depending on the direction and magnitude of the vertical hydraulic gradient. Such a hydraulic connection is indicated at cluster 4 by the fact that 

water levels in the two units are similar and respond to 

precipitation events in a parallel manner (fig. 13). 

Well clusters 4, 5, and 7 are located in areas 

where the Columbia aquifer directly overlies the 

Nanjemoy-Marlboro confining unit. Cluster 4 (fig. 13) 

showed a slight downward gradient between the two 

screened zones of about 0.01 during the low-water 

period (for this study period, approximately June 1994 

to January 1995), and little vertical gradient during the 

rest of the year. The water level data suggest that in 

areas of high land-surface elevation (greater than 

20 ft) in the western area of the EEA, the Columbia 

aquifer can recharge the Nanjemoy-Marlboro confining unit. In contrast, well cluster 5 (fig. 13) showed a 

slight upward gradient of 0.005 to 0.01 over most of 

the study period, indicating that beneath the Churchill 

Test Area the Nanjemoy-Marlboro confining unit can 

discharge ground water to the Columbia aquifer. However, well cluster 7 (fig. 13) showed little to no vertical 

gradient for most of the study period except for a 

downward gradient of 0.005 to 0.05 in the spring and 

early summer of 1994.

Upper Confined Aquifer

Water levels in the upper confined aquifer generally are lower than those in the Columbia aquifer 

and the Nanjemoy-Marlboro confining unit, ranging 

from about 17 ft above sea level during the high-water 

period in well EEA-M1 to just above sea level in well 

EEA-M3 (figs. 16 and 17). Well EEA-S2 is completed 

in the Nanjemoy-Marlboro confining unit just below 

the upper confined aquifer, but the water level at this 

well is probably close to the water level in the upper

24 Hydrogeology and Water Quality of the Shallow Aquifer System, Dahlgren, Virginia
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Figure 15. Water level in the Columbia aquifer for well EEAS1, July 11-15, 1995, at the Explosive Experimental Area, 

Dahlgren, Virginia.

confined aquifer. Water levels in the three upper confined aquifer wells near the shore of Machodoc Creek 

and the Potomac River (EEA-M3, M6, and M8) 

remained within a few feet of sea level throughout the 

year.

Recharge

Sediments of the upper confined aquifer occupy 

paleochannels cut into the Nanjemoy-Marlboro confining unit, and the upper confining unit overlies this 

aquifer at all well cluster locations, including those on 

the Mainside of the NSWCDL. Water-level data indicate that downward gradients exist between the 

Columbia aquifer and the upper confined aquifer at 

well clusters 1, 6, and 8 throughout the year, and at 

well cluster 2 during high-water conditions. Although 

the magnitude of these gradients ranges from <0.001 

to as high as 0.20, the low vertical permeability of the 

upper confining unit (median 2.0x10~4 ft/d) inhibits 

recharge across this unit. The upper confined aquifer, 

therefore, is probably recharged by a combination of 

flow across the upper confining unit and flow from the 

adjacent and underlying Nanjemoy-Marlboro confining unit.

A horizontal hydraulic gradient of 0.001 to 0.01 

is estimated between the Nanjemoy-Marlboro confining unit beneath the central and western EEA and the 

upper confined aquifer beneath the southern part of the 

EEA. In high-permeability zones of the NanjemoyMarlboro confining unit (horizontal hydraulic conductivity of 0.5 ft/d, assumed porosity of 0.30), these gradients could result in horizontal ground-water-flow 

rates of 0.6 to 6 ft/year from the Nanjemoy-Marlboro

confining unit to the upper confined aquifer, which are 

sufficient to maintain the observed hydraulic gradient 

in the upper confined aquifer. Horizontal hydraulic 

gradients between the two units are probably even 

higher south of the EEA because of the higher landsurface elevations on top of the Dahlgren scarp. The 

southern arm of the upper confined aquifer may act as 

a large subsurface drain to intercept flow from the 

uplands and divert it toward the Potomac River.

Horizontal Flow and Discharge

Water in the southern arm of the upper confined 

aquifer generally flows eastward-northeastward 

toward the Potomac River, as indicated by decreasing 

hydraulic head from well EEA-M1 to M2 to M3 

(fig. 17); some ground water also may flow west from 

the vicinity of well EEA-M1 toward Machodoc Creek. 

The horizontal hydraulic gradient is about 0.002 to the 

east, which would result in ground-water-flow rates of 

about 0.03 ft/d assuming an average horizontal 

hydraulic conductivity of 4.0 ft/d and a porosity of 

0.30. The low hydraulic gradient results in relatively 

low ground-water-flow rates despite the permeable 

nature of the upper confined aquifer, and ground-water 

discharges directly to the Potomac River and 

Machodoc Creek at an undetermined distance offshore 

where the river has breached the upper confining unit. 

Bathymetric contours indicate that the upper confining 

unit has been eroded away in some parts of the Potomac River channel, and possibly parts of the 

Machodoc Creek channel (U.S. Geological Survey 

and National Ocean Service, 1983).

Because only two wells are completed in the 

upper confined aquifer in the northern area of the 

EEA, exact ground-water-flow directions in this area 

are difficult to determine; however, ground water 

probably flows northward and eastward toward 

Machodoc Creek and the Potomac River. In the upper 

confined aquifer, the horizontal hydraulic gradient in 

the northern part of the EEA is much lower than that in 

the southern area because of lower rates of recharge 

through the Nanjemoy-Marlboro confining unit. 

Ground water discharges directly to the Potomac 

River and probably to Machodoc Creek where these 

surface-water bodies have breached the upper confining unit.

Wells EEA-M3, M6, and M8, all close to the 

shoreline of the EEA, show a marked tidal affect on 

the upper confined aquifer (fig. 18). These groundwater-level oscillations are caused by the transmission

Hydrogeology of the Shallow Aquifer System 25
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EXPLANATION

APPROXIMATE AREA WHERE UPPER CONFINED AQUIFER IS MISSING

 10 POTENTIOMETRIC CONTOUR OF THE UPPER

CONFINED AQUIFER AND NANJEMOY-MARLBORO 

CONFINING UNIT MARCH 1994--Dashed where 

approximately located. Datum is sea level. Contour interval is 5 feet

EEA-M11 WELL CLUSTER AND WELL NUMBER (Altitude of (16.9) water level In upper confined aquifer and 

* Nanjemoy-Marlboro confining unit in feet above 

sea level

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Dahlgren, Virginia.
77<-03'30'' 77'02'30" 77»01'30"

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APPROXIMATE AREA WHERE UPPER CONFINED 

AQUIFER IS MISSING

 10 POTENTIOMETRIC CONTOUR OF THE UPPER

CONFINED AQUIFER AND NANJEMOY-MARLBORO 

CONFINING UNIT DECEMBER! 994--Dashed where 

approximately located. Datum is sea level. Contour 

interval is 5 feet

EEA-S1

EEA-M1 WELL CLUSTER AND WELL NUMBER (Altitude of 

(13.9) water level in upper confined aquifer and 

* Nanjemoy-Marlboro confining unit in feet above 

sea level

Figure 17. Potentiometric surface in the upper confined aquifer and the Nanjemoy-Marlboro confining unit, December 1994, at the Explosive Experimental Area, 

Dahlgren, Virginia.
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234 

AUGUST 1995

EXPLANATION

 EEA-M3

- - - EEA-M6

 EEA-M8

 TIDE GAGE

Figure 18. Water levels for wells EEA-M3, M6, M8, and tidal fluctuations in 

the Upper Machodoc Creek, August 1-5,1995, at the Explosive 

Experimental Area, Dahlgren, Virginia.

of large tidally induced pressure-head changes across 

the upper confining unit. It should be noted, however, 

that flow rates across the upper confining unit are still 

controlled by the low permeability of this unit, and 

there is little tidally induced flow between surface 

water and the upper confined aquifer at the EEA 

except where the upper confining unit is very thin or 

has been removed by erosion.

WATER QUALITY OF THE SHALLOW 

AQUIFER SYSTEM

The background quality of water in the shallow 

aquifer system at the EEA is typical of shallow ground 

water in the Virginia Atlantic Coastal Plain Physiographic Province (Virginia State Water Control Board, 

1973; Richardson and Brockman, 1992), and is similar 

to the quality of water in the shallow aquifer system at 

the Mainside (Harlow and Bell, 1996). The following 

section describes the interpretation of water-quality 

data collected at the EEA, including quality assurance/ 

quality control (QA/QC) results and distribution of 

selected constituents.

Quality Assurance and Quality Control

Water-quality data were verified by examination 

of laboratory quality-assurance (QA) criteria, qualitycontrol (QC) samples, and cation-anion balances. Laboratory QA procedures are described in Friedman and

Erdmann (1982) and Jones (1987). QC samples collected in the field include duplicate samples, equipment blanks, and deionized water blanks. At least 1 

QC sample was collected in duplicate for every 10 

water-quality samples collected. QA goals were that 

the relative percent difference (RPD) of constituent 

concentrations in duplicate samples were not to 

exceed 20 percent. Chemical constituents were not to 

be present in the equipment and deionized water 

blanks at concentrations above the reporting limits. 

Cation-anion imbalances were not to be greater than 

10 percent.

The only duplicate analyses of ground water 

that did not fall within 20 RPD were those for dissolved iron (well EEA-S6) and dissolved aluminum 

(well EEA-M6). Duplicate samples of surface water at 

site EEA-SW2 exceeded 20 RPD for field alkalinity 

and all three trace metals. These differences in measured concentrations were not large enough to affect 

interpretation of water-quality data.

Small concentrations of dissolved organic carbon (0.3 mg/L) were measured in the deionized water 

blank. Maximum concentrations of constituents 

detected in five equipment blank samples collected 

throughout the sampling period were 0.6 mg/L dissolved organic carbon, 0.2 mg/L suspended organic 

carbon, 0.03 mg/L dissolved calcium, 2.3 mg/L dissolved chloride, 0.50 mg/L dissolved silica, 48 

dissolved iron, 2.1 mg/L alkalinity as CaCC>3, 2 

dissolved manganese, and 20 ug/L dissolved aluminum. Although these concentrations were not high 

enough to affect interpretation of water-quality data, it

28 Hydrogeology and Water Quality of the Shallow Aquifer System, Dahlgren, Virginia
should be recognized that water-quality constituents 

at, or below, these concentrations could have been 

derived from the sampling pump or tubing.

Of 28 wells sampled for water-quality, only 2 

had cation-anion balances that exceeded 10 percent: 

wells EEA-S11 and S15 had balances of 17.3 percent 

and -10.6 percent, respectively. Cation-anion imbalances indicate an error in the measurement of some 

ion(s), or that an unmeasured ion (for example, nitrate) 

was present in a significant concentration. Waterquality analyses from these two wells should be interpreted with caution, but the difference does not affect 

the overall interpretation of water-quality data. Cationanion balances did not exceed 10 percent in any of the 

three surface-water samples.

Occurrence and Distribution of Selected 

Chemical Constituents

The range in values of selected chemical constituents in the Columbia aquifer, upper confined aquifer, and the Nanjemoy-Marlboro confining unit is 

discussed with regard to spatial distribution on the 

NSWCDL and U.S. Environmental Protection Agency 

(1995) secondary maximum contaminant levels 

(SMCL's) or Virginia Department of Environmental 

Quality Ground-Water Standards (VGWS's) for drinking water, which are nonenforceable standards established on the basis of aesthetic concerns, such as taste 

and odor. Reporting limits and water-quality data for 

wells and surface-water sampling sites at the EEA are 

published in Hammond and Bell (1995).

Columbia Aquifer

Maximum, minimum, and median values of 

constituent concentrations and field parameters in the 

Columbia aquifer are presented in table 2, and boxplot 

summaries of water-quality data from three hydrogeologic units are shown in figure 19. A trilinear plot 

(fig. 20) shows the relative major-ion distribution of 

water from all 20 wells completed in the Columbia 

aquifer.

Field Parameters and Indicators

Water in the Columbia aquifer is characterized 

by low pH; samples from 18 of 20 wells had pH below 

the SMCL range of 6.5 to 8.5. Low pH in shallow 

ground water commonly is a result of precipitation 

reacting with carbon dioxide in the atmosphere and

soil zone, which produces carbonic acid (Hem, 1989). 

Deeper, older water usually has reacted with buffering 

minerals in aquifers, such as calcite, which raise pH 

(and alkalinity). Dissolved oxygen concentration varied from less than 1 to 9.0 mg/L across the EEA, and 

represent local differences in atmospheric connection, 

recharge rate, and organic content of Columbia aquifer 

sediments.

Other field parameters and indicators, such as 

specific conductance, total dissolved solids (TDS), and 

alkalinity, are highly variable in the Columbia aquifer. 

At well EEA-S3, which has a direct connection with 

the Potomac River, much higher specific conductance, 

TDS, and alkalinity values are present than at any 

other well (7,600 fiS/cm, 4,510 mg/L, and 489 mg/L 

as CaCO3 , respectively). The highest concentrations 

of all major ions also were measured at well EEA-S3. 

Water in most wells completed in the Columbia aquifer had specific conductance below 300 fiS/cm, TDS 

less than 200 mg/L, and alkalinity below 100 mg/L as 

CaCO3 (fig.l9).

Three of the six alkalinity values greater than 

100 mg/L as CaCO3 were measured at wells EEA-S4, 

S9, and S10, which correspond to an area where the 

Nanjemoy-Marlboro confining unit lies within 10 ft of 

land surface. Dissolution of fossil shell material in this 

unit caused the high alkalinities. Relatively high alkalinities at wells EEA-S6 and S19 could be due to dissolution of shell material in artificial fill sediments.

Major Constituents

Water from the Columbia aquifer has variable 

major-ion composition (fig. 20). Dominant cations 

measured were calcium and sodium, with lesser percentages of magnesium, whereas dominant anions 

were bicarbonate and sulfate, with low to moderate 

percentages of chloride.

The most common single water type in the 

Columbia aquifer was calcium-bicarbonate, measured 

at four wells (EEA-S4, S9, S18, and S19) where the 

Columbia aquifer overlies the Nanjemoy-Marlboro 

confining unit. Most Columbia aquifer wells yielded 

water of a mixed-ionic character, however, and the 

median concentrations of all major ions were less than 

20 mg/L. Low-ionic strength water of mixed-ionic 

composition is common in shallow ground water in 

the Coastal Plain Physiographic Province of Virginia 

(Richardson and Brockman, 1992; Harlow and Bell, 

1996). Major ions, such as calcium, sodium, bicarbonate, sulfate, and chloride, are derived mainly from

Water Quality of the Shallow Aquifer System 29
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EXPLANATION

MAXIMUM

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- - - U.S. ENVIRONMENTAL PROTECTION AGENCY 

SECONDARY MAXIMUM CONTAMINANT LEVEL 

OR VIRGINIA DEPARTMENT OF ENVIRONMENTAL 

QUALITY GROUND-WATER STANDARD

U.S. GEOLOGICAL SURVEY REPORTING LIMIT

CA= Columbia aquifer

UCA= upper confined aquifer

NMCU= Nanjemoy-Marlboro confining unit

Figure 19. Summaries of water-quality analyses for selected constituents in ground water at the Explosive 

Experimental Area, Dahlgren, Virginia. (Results from 20 analyses were used for all boxplots except that of aluminum, 

which used 19.)

30 Hydrogeology and Water Quality of the Shallow Aquifer System, Dahlgren, Virginia
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Figure 19. Continued.

precipitation, mineral dissolution, and degradation of 

organic material in the subsurface (Hem, 1989; 

Drever, 1988).

EEA-S3 was the only well with a sodiumchloride water type, and the only well to exceed the 

VGWS for sodium and the SMCL for chloride. The 

major-ion composition, specific conductance, and concentration of total dissolved solids of water from well 

EEA-S3 is very similar to that from the surface-water 

site, SW-1 (fig. 21), providing more evidence of a 

direct connection of the Columbia aquifer and surface 

water at this location. No other wells (even those that 

are close to the shoreline such as EEA-S6, S7, and S8) 

contained water that was similar in chemical composition to surface-water samples. It appears, therefore,

that surface water has little direct effect on groundwater quality at the EEA except possibly in a narrow 

zone adjacent to the shoreline. The Potomac River, 

however, may affect ground-water quality indirectly 

by generating aerosols (sea spray) and, thereby, 

increasing the sodium and chloride concentrations of 

local precipitation.

Minor Constituents

Dissolved iron and manganese displayed a wide 

range in concentration in the Columbia aquifer 

(fig. 19); iron ranged from 10 to 40,000 u,g/L, whereas 

manganese ranged from 9 to 550 jig/L. Iron and manganese concentrations are controlled by local redox

Water Quality of the Shallow Aquifer System 31
conditions. In oxidizing conditions, both iron and 

manganese exist in relatively insoluble forms. Where 

oxygen has been depleted by microbial degradation of 

organic matter, however, iron and manganese in aquifer minerals can be reduced to much more soluble 

forms. High concentrations of these constituents indicate local reducing conditions in the Columbia aquifer. 

Dissolved aluminum also showed a wide range in concentration, ranging from less than 10 to 1,700 ug/L. 

Aluminum is derived from dissolution of several silicate minerals, including clays, and aluminum concentrations are controlled largely by local differences in 

pH (Hem, 1989).

Both dissolved organic carbon (DOC) and suspended organic carbon (SOC) were present in water at

all wells completed in the Columbia aquifer, mostly at 

relatively low (less than 5 mg/L) concentrations. Naturally occurring DOC and SOC are derived from 

organic material both in the soil zone and aquifer sediments, so detection of these constituents does not necessarily indicate ground-water contamination. 

Anthropogenic contamination might be indicated by 

concentrations of DOC and (or) SOC that are significantly higher than background levels.

Nanjemoy-Marlboro Confining Unit

Water from the Nanjemoy-Marlboro confining 

unit generally exhibited higher pH, specific conductance, TDS, and alkalinity than water from the Columbia aquifer (fig. 19). Water samples from all four wells

Table 2. Summary statistics for water-quality analyses from the Columbia aquifer at the Explosive Experimental Area, 

Dahlgren, Virginia

[all analyses are for the dissolved constituent; results from 20 analyses were used to calculate all statistics unless otherwise noted; , no SMCL exists 

for this constituent; °C, degrees Celsius; Jlg/L, micrograms per liter, mg/L, milligrams per liter; uS/cm, microsiemens per centimeter at 25°C; <, less 

than; C, carbon; CaCO3, calcium carbonate; SMCL, U.S. Environmental Protection Agency, (1995) secondary maximum contaminant level; VGWS, 

Virginia Department of Environmental Quality ground-water standard]

Water-quality constituent

Specific conductance, [iS/cm

pH, standard units

Dissolved oxygen, mg/L

Calcium, mg/L

Magnesium, mg/L

Sodium, mg/L

Potassium, mg/L

Alkalinity, mg/L as CaCO3

Sulfate, mg/L

Chloride, mg/L

Fluoride, mg/L

Silica, mg/L

Dissolved solids, residue at 180°C, mg/L

f\

Aluminum, jxg/L

Iron, |ig/L

Manganese, ng/L

3Dissolved organic carbon, mg/L as C

Maximum 

concentration

7,600

6.8

9.0

220

160

1,100

27

489

130

2,100

1.5

46

4,510

1,700

40,000

550

14

Minimum 

concentration

48

4.2

.2

1.4

.82

1.8

<.10

4

.60

1.9

<.10

9.9

37

<10

9

10

.6

Median 

concentration

151

5.9

4.3

13

2.2

6.2

1.6

22

16

4.1

.10

20

106

20

230

59

1.4

SMCL 

or 

VGWS

-

6.5-8.5

--

--

--

270

-

--

250

250

--

--

500

--

300

50

1

Number of 

samples 

exceeding 

SMCL/ 

VGWS

--

18

--

--

--

1

--

--

0

1

--

--

1

--

9

11

16

Results from 18 analyses were used to calculate statistics for dissolved oxygen. 

Results from 19 analyses were used to calculate statistics for aluminum. 

Results from 17 analyses were used to calculate statistics for dissolved organic carbon.

32 Hydrogeology and Water Quality of the Shallow Aquifer System, Dahlgren, Virginia
EXPLANATION

 EEA-S1 + EEA-S13

O EEA-S2 -< >- EEA-S14

 EEA-S3 @ EEA-S15

D EEA-S4 © EEA-S16

A EEA-S5 # EEA-S17

A EEA-S6 © EEA-S18

T EEA-S7 O- EEA-S19

V EEA-S8 - EEA-S20

4 EEA-S8-d

A EEA-S9

 EEA-S10

& EEA-S11

X EEA-S12

'O *0

CALCIUM CHLORIDE 

PERCENT OF TOTAL MILLIEQUIVALENTS PER LITER

Figure 20. Major-ion composition of water from wells completed in the Columbia aquifer at the Explosive 

Experimental Area, Dahlgren, Virginia.

completed in this unit were within the SMCL pH 

range of 6.5 to 8.5. Specific conductance ranged from 

535 M<S/cm at well EEA-M2 to 1,330 \iS/cm at well 

EEA-M7, and TDS ranged from 304 mg/L at well 

EEA-M2 to 1,010 mg/L at well EEA-M7. Alkalinity 

was less variable, and ranged from 198 mg/L as 

CaCO3 at well EEA-M7 to 285 mg/L as CaCO3 at 

well EEA-M4. Water in two of the four wells completed in the Nanjemoy-Marlboro confining unit 

(EEA-M4 and M5) contained significant concentrations of dissolved oxygen (2.3 and 6.0 mg/L, respectively), possibly due to the downward flow of oxygencontaining water from the overlying Columbia aquifer. 

Concentrations of trace metals such as iron, manganese, and aluminum were generally similar to, or 

lower than, the Columbia aquifer medians. Concentrations of DOC were slightly higher than the Columbia 

aquifer median, ranging from 3.8 mg/L at well EEAM7 to 7.7 mg/L at well EEA-M2.

A trilinear plot of water-quality data from the 

Nanjemoy-Marlboro confining unit (fig. 21) shows 

that sodium, or a combination of sodium and calcium, 

are the dominant cations at all four wells completed in 

this unit; however, no samples exceeded the VGWS

Water Quality of the Shallow Aquifer System 33
CALCIUM CHLORIDE

PERCENT OF TOTAL MILLIEQUIVALENTS PER LITER

Figure 21. Major-ion composition of water from wells completed in the upper confined aquifer and 

Nanjemoy-Marlboro confining unit at the Explosive Experimental Area, Dahlgren, Virginia.

for sodium of 270 mg/L. Bicarbonate was the major 

anion at three of the wells (EEA-M2, M4, and M5), 

whereas sulfate was the dominant anion at well 

EEA-M7. The sulfate concentration of 510 mg/L at 

well EEA-M7 exceeded the SMCL of 250 mg/L.

The water quality of the Nanjemoy-Marlboro 

confining unit is controlled by dissolution of carbonate 

minerals and cation exchange. As low-pH, lowalkalinity water from the Columbia aquifer moves 

downward into the underlying Nanjemoy-Marlboro 

confining unit, it dissolves fossil shell material composed of calcium carbonate (CaCO3), which raises the 

pH, specific conductance, TDS, and alkalinity. The

resulting calcium-bicarbonate water is further modified by exchange of sodium for calcium on surfaceactive minerals such as glauconite (Chapelle and 

Knobel, 1983), resulting in a sodium-calciumbicarbonate water type. High concentration of sulfate 

at well EEA-M7 may have been derived from microbial oxidation of organic matter or sulfur-containing 

minerals such as pyrite (FeS2).

Upper Confined Aquifer

The water quality of the upper confined aquifer 

is similar to that of the Nanjemoy-Marlboro confining 

unit for many constituents (fig. 19). Values of pH were

34 Hydrogeology and Water Quality of the Shallow Aquifer System, Dahlgren, Virginia
slightly lower than those of the confining unit, with 

measurements from one (EEA-M1) out of the four 

wells falling below the lower SMCL of 6.5. Specific 

conductance ranged from 338 |LiS/cm at well EEA-M1 

to 600 n,S/cm at well EEA-M3, and TDS ranged from 

210 mg/L at well EEA-M1 to 368 mg/L at well EEAM6. Alkalinity in the upper confined aquifer was very 

similar to that of the Nanjemoy-Marlboro confining 

unit, ranging from 146 mg/L at well EEA-M1 to 

279 mg/L at well EEA-M8. Unlike the wells completed in the confining unit, however, none of the four 

wells completed in the upper confined aquifer contained concentrations of dissolved oxygen greater than 

Img/L.

The major-ion composition of water in the 

upper confined aquifer is similar to that of the 

Nanjemoy-Marlboro confining unit. Concentrations of 

calcium, magnesium, sodium, potassium, and chloride 

are in the same range as that of the NanjemoyMarlboro confining unit and are significantly higher 

than Columbia aquifer medians (fig. 19). A trilinear 

plot indicates that water from wells in the upper confined aquifer has a sodium-calcium-bicarbonate water 

type, with the exception of water at well EEA-M8, 

which has a sodium-bicarbonate water type (fig. 21).

Two important differences stand out between 

the water quality of the upper confined aquifer and that 

of the Nanjemoy-Marlboro confining unit. The aquifer 

has much lower concentrations of sulfate and much 

higher concentrations of the redox-sensitive trace metals, iron and manganese. In fact, all four wells in the 

upper confined aquifer exceeded the SMCL's for iron 

and manganese of 300 and 50 mg/L, respectively.

The water quality of the upper confined aquifer 

is consistent with the previously described conceptual 

flow system in which water recharges the upper confined aquifer through the Nanjemoy-Marlboro confining unit. High-alkalinity sodium-calcium-bicarbonate 

water from the Nanjemoy-Marlboro confining unit 

moves laterally into the upper confined aquifer, which 

itself contains little shell material. Microbial oxidation 

of the abundant organic matter in the upper confined 

aquifer, however, consumes any dissolved oxygen 

present in water from the confining unit, reduces sulfate to sulfide, and reduces iron and manganese to 

more soluble forms.

SUMMARY

The shallow aquifer system at the EEA consists 

of two fining-upward sequences of Pleistocene fluvial - 

estuarine deposits that overlie marine deposits of the 

Nanjemoy-Marlboro confining unit of Eocene age. 

The surficial hydrogeologic unit is the Columbia aquifer that generally is unconfined and consists of deposits of the Sedgefield Member of the Tabb Formation. 

These fluvial-estuarine deposits range from clay to 

coarse-grained sand and pebbles and have varied 

hydraulic properties. Horizontal linear flow of water in 

the Columbia aquifer is affected strongly by lithologic 

heterogeneities, such as paleochannels and thin beds 

of clay. The Columbia aquifer receives recharge from 

infiltration of precipitation, and water generally flows 

according to surface topography, discharging to tidal 

creeks, marshes and the Potomac River. Locally, 

ground water discharges to Machodoc Creek by way 

of surface springs. Horizontal linear flow rates of 

water in the Columbia aquifer range from 0.003 to 

0.70 ft/d on average, but can vary greatly depending 

on the local gradient and hydraulic characteristics.

The Columbia aquifer unconformably overlies 

the upper confining unit an organic-rich clay having 

low permeability. The upper confining unit conformably overlies the upper confined aquifer, which consists of interbedded fine-grained to medium-grained 

sands and clay with a pebble lag. The upper confining 

unit and upper confined aquifer represent a single 

Pleistocene channel-fill sequence, and a gradational 

contact between the two units. The erosional surface 

incised by the paleochannel into the underlying 

Eocene deposits has more than 80 ft of relief across 

the EEA, and the upper confined system is not present 

in the paleointerfluvial areas. The upper confined aquifer probably receives most of its recharge from the 

adjacent and underlying Nanjemoy-Marlboro confining unit. Water in the upper confined aquifer generally 

flows about 0.03 ft/d toward the Potomac River and 

Machodoc Creek. Near the western shoreline of the 

EEA, water in the upper confined aquifer may flow 

westward toward Machodoc Creek.

The upper confined aquifer unconformably 

overlies the Nanjemoy-Marlboro confining unit, which 

consists of glauconitic, fossiliferous silty fine-grained 

sands of the Nanjemoy Formation of Eocene age. The 

upper confined system is absent in an eastern-western 

trending band across the center of the EEA; in this 

area the Columbia aquifer directly overlies the 

Nanjemoy-Marlboro confining unit. In some areas of

Summary 35
the EEA, vertical hydraulic conductivities of the 

Nanjemoy-Marlboro confining unit and Columbia 

aquifer are similar. In these areas, the Columbia aquifer and Nanjemoy-Marlboro confining unit can effectively combine to form a thick (greater than 50 ft) 

unconfined aquifer.

Water quality of the shallow aquifer system is 

typical of shallow ground water elsewhere in the 

Virginia Coastal Plain Physiographic Province. Water 

in the Columbia aquifer is a mixed-ionic type characterized by low pH, alkalinity, and TDS. This composition is indicative of infiltrated precipitation that has 

reacted with carbon dioxide in the soil zone and aquifer sediments, but has not been significantly buffered 

by dissolution of carbonate or silicate minerals. Water 

in the upper confined aquifer and Nanjemoy-Marlboro 

confining unit is a sodium-calcium-bicarbonate water 

type characterized by moderate to high pH, TDS, and 

alkalinity. This composition is indicative of ground 

water that has reacted with carbonate minerals and 

undergone ion-exchange processes. Anoxic conditions 

in the upper confined aquifer and parts of the Columbia aquifer have resulted in high concentrations of dissolved iron and manganese, and low concentrations of 

sulfate.

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