By Daniel J. Phelan, Lisa D. Olsen, Martha L. Cashel, Judith L. Tegler, and Elizabeth H. 

Marchand

In cooperation with 

THE UNITED STATES AIR FORCE 

DOVER AIR FORCE BASE 

Hydrogeology and Simulation of Ground-Water Flow at 

Dover Air Force Base, Delaware 

Water-Resources Investigations Report 99-4224


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

distribution number 2958-A-4

U.S. Department of the Interior 

U.S. Geological Survey 

Hydrogeology and Simulation of Ground-Water Flow at 

Dover Air Force Base, Delaware 

By Kurt C. Hinaman and Frederick J. Tenbus 

Water-Resources Investigations Report 99-4224 

In cooperation with 

THE UNITED STATES AIR FORCE 

DOVER AIR FORCE BASE 

Baltimore Maryland 

2000 
U.S. DEPARTMENT OF THE INTERIOR 

Bruce Babbitt, Secretary 

U.S. Geological Survey 

Charles G. Groat, Director 

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

descriptive purposes only and does not imply endorsement 

by the U.S. Geological Survey. 

For additional information contact: 

District Chief 

U.S. Geological Survey, WRD 

8987 Yellow Brick Road 

Baltimore, Maryland 21237 

Copies of this report can be purchased from: 

U.S. Geological Survey 

Branch of Information Services 

Box 25286 

Denver, CO 80225-0286 
11

CONTENTS


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


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


Background....................................................................................................................................................................2


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


Description of investigation area...................................................................................................................................4


Acknowledgments. ........................................................................................................................................................7


Hydrogeology. ........................................................................................................................................................................7


Geologic framework. .....................................................................................................................................................7


Hydrologic framework. ...............................................................................................................................................10


Surficial aquifer...................................................................................................................................................14


Hydraulic-head distribution and fluctuations.............................................................................................14


Hydraulic conductivity. ..............................................................................................................................22


Upper confining unit of Calvert Formation. .......................................................................................................30


Frederica aquifer. ................................................................................................................................................30


Hydraulic-head distribution, fluctuation, and gradient. .............................................................................30


Hydraulic conductivity. ..............................................................................................................................35


Ground-water age. .......................................................................................................................................................35


Chlorofluorocarbon dates....................................................................................................................................35


Tritium dates. ......................................................................................................................................................35


Water budget................................................................................................................................................................40


Simulation of ground-water flow. ........................................................................................................................................40


Conceptual model. .......................................................................................................................................................40


Model design and boundary conditions.......................................................................................................................42


Calibration of the model. .............................................................................................................................................44


Simulated water budget. ..............................................................................................................................................53


Sensitivity of the model...............................................................................................................................................53


Comparison of simulated flow paths and ground-water recharge dates......................................................................57


Results of selected particle-tracking analyses. ............................................................................................................63


Natural Attenuation Project area.........................................................................................................................63


Long-term monitoring at OT-40..........................................................................................................................63


Contaminant plume at Area 6. ............................................................................................................................63


Selected limitations of the ground-water-flow model. ................................................................................................65


Summary...............................................................................................................................................................................69


References cited. ..................................................................................................................................................................70


FIGURES 

1.	 Maps showing location of:


1a. The Delmarva Peninsula and boundary of modeled area at Dover Air Force Base, Dover, Delaware. ............2


1b. Surface-water base-flow measurement sites and selected features near Dover Air Force Base, Delaware ......3


1c. Selected areas and monitor wells at Dover Air Force Base, Delaware..............................................................5


2. Map showing location of large areas with pavement and wetlands at and near Dover Air Force Base, Delaware ......6


3. Stratigraphic column and hydrogeologic units in the Dover Air Force Base area, Delaware.......................................8


4. Map showing geology of the Dover Air Force Base area, Delaware ..........................................................................11


iii 
1

1

FIGURES–Continued

 5. Map showing altitude of the base of the Columbia Formation in the Dover Air Force Base area, Delaware ...........12


 6.	 Map showing locations of gamma logs and thickness of the fine-grained sediments in the


Columbia Formation that are at, or below, the water table at Dover Air Force Base, Delaware .......................13


 7. Maps showing expected water table for the following conditions: 

(A) no recharge, (B) recharge, but no stream branches near Dover Air Force Base, 

(C) no recharge, with stream branches near Dover Air Force Base, and 

(D) with recharge and stream branches near Dover Air Force Base, Delaware ....................................................15


8.	 Map showing hydraulic head in the lower surficial aquifer in the 

Dover Air Force Base area under average recharge conditions, October 1959..................................................17 

9. 	Map showing average hydraulic head in the lower part of the surficial aquifer in the


detailed investigation area at Dover Air Force Base, Delaware, October 1959. ................................................18


10-11. Maps showing hydraulic head in the lower surficial aquifer in the detailed investigation area at

 Dover Air Force Base, Delaware: 

10. December 1993. ..............................................................................................................................................19


11. May 1994. .......................................................................................................................................................20


12. 	Map showing water levels for six wells completed in the surficial aquifer at


Dover Air Force Base and for one long-record well (Jd42-03) completed in the


surficial aquifer, Dover Air Force Base area, Delaware.....................................................................................21


 13. 	Graph showing water levels, screen intervals, and natural gamma logs for two well pairs


(locations shown in figure 12) at Dover Air Force Base, Delaware...................................................................22


14.	 Diagram showing screen interval in relation to a clay layer in the surficial aquifer, and the


recognition of local ground-water highs at Dover Air Force Base, Delaware....................................................23


15.	 Map showing average hydraulic head in the upper surficial aquifer in the detailed investigation


area at Dover Air Force Base, Delaware.............................................................................................................24


16-17. 	Maps showing hydraulic head in the upper surficial aquifer during a period of: 

16. Low recharge in the detailed investigation area at Dover Air Force Base, Delaware, December 1993.........25


17. High recharge in the detailed investigation area at Dover Air Force Base, Delaware, May 1994.................26


18-19. 	Maps showing approximate: 

18. Thickness of the upper confining unit of the Calvert Formation, Dover Air Force Base area, Delaware......31


19. Altitude of the top of the Frederica aquifer, Dover Air Force Base area, Delaware ......................................32


 20. Map showing potentiometric surface in the Frederica aquifer, Dover Air Force Base, Delaware...........................33


 21. 	Graph showing water levels in selected wells completed in the Frederica aquifer


(locations shown in figure 20), Dover Air Force Base, Delaware......................................................................34


 22. 	Map showing chlorofluorocarbon (CFC) recharge dates and tritium recharge dates


for water in selected wells, Dover Air Force Base area, Delaware ....................................................................36


iv 
3

3

3

FIGURES–Continued

 23. 	Diagram showing conceptual model of the upper ground-water-flow system at


Dover Air Force Base, Delaware ........................................................................................................................41


 24. 	Map showing model grid used in the simulation of ground-water flow in the


Dover Air Force Base area, Delaware................................................................................................................43


25-28. 	Maps showing boundary conditions of: 

25. 	Layer 1 (upper surficial aquifer), used to simulate ground-water flow in the


Dover Air Force Base area, Delaware .....................................................................................................45


26. 	Layer 2 (lower surficial aquifer), used to simulate ground-water flow in the


Dover Air Force Base area, Delaware .....................................................................................................46


27. 	Layer 3 (upper Calvert Formation), a confining unit used to simulate


ground-water flow in the Dover Air Force Base area, Delaware.............................................................47


28. 	Layer 4 (the Frederica aquifer), used to simulate ground-water flow in the


Dover Air Force Base area, Delaware. ....................................................................................................48


29-30. 	Map showing locations of model cells with rivers and drains in: 

29. 	Layer 1 (the upper surficial aquifer), used to simulate ground-water flow in the


Dover Air Force Base area, Delaware .....................................................................................................49


30. 	Layer 1 (the upper surficial aquifer), used to simulate ground-water flow in the


detailed investigation area, Dover Air Force Base, Delaware .................................................................50


31. 	 Graph showing relation between measured heads (September 1997)


and simulated heads in the (A) upper part of the surficial aquifer, and


(B) lower part of the surficial aquifer, Dover Air Force Base, Delaware...........................................................52


32- 34. 	Maps showing simulated heads and average heads in the: 

32. Lower part of the surficial aquifer, Dover Air Force Base area, Delaware ....................................................54


33. Upper part of the surficial aquifer, Dover Air Force Base area, Delaware ....................................................55


34. Frederica aquifer, Dover Air Force Base area, Delaware...............................................................................56


35.	 Graph showing sensitivity of simulated head in the upper surficial aquifer to changes in recharge,


horizontal hydraulic conductivity of the upper surficial aquifer,


horizontal hydraulic conductivity of the lower surficial aquifer, and


horizontal hydraulic conductivity of clays in the surficial aquifer,


Dover Air Force Base, Delaware ........................................................................................................................58


36. 	Graph showing sensitivity of simulated head in the lower surficial aquifer to changes in recharge,


horizontal hydraulic conductivity of the upper surficial aquifer,


horizontal hydraulic conductivity of the lower surficial aquifer,


horizontal hydraulic conductivity of the upper Calvert Formation confining unit, and


horizontal hydraulic conductivity of the Frederica aquifer,


Dover Air Force Base, Delaware ........................................................................................................................59


37.	 Graph showing sensitivity of simulated potentiometric surface of the Frederica aquifer to variation of


horizontal hydraulic conductivity of the Frederica aquifer,


horizontal hydraulic conductivity of the upper Calvert Formation confining unit, recharge,


horizontal hydraulic conductivity of the lower surficial aquifer, and


horizontal hydraulic conductivity of the upper surficial aquifer,


Dover Air Force Base, Delaware ........................................................................................................................60


v 
41-

4424242( ( 

FIGURES–Continued 

38.	 Graph showing sensitivity of simulated ground-water discharge to Pipe Elm Branch


to changes in recharge,


horizontal hydraulic conductivity of the upper surficial aquifer,


horizontal hydraulic conductivity of clays in the surficial aquifer,


horizontal hydraulic conductivity of the lower surficial aquifer,


horizontal hydraulic conductivity of the upper Calvert Formation confining unit, and


horizontal hydraulic conductivity of the Frederica aquifer for the


Dover Air Force Base area, Delaware ................................................................................................................61


39-40.	 Maps showing pathlines and traveltimes for advective transport of ground-water particles starting at: 

39. 	LF13 in the Natural Attenuation Project Area (location shown in figure 1c),


Dover Air Force Base, Delaware .............................................................................................................64


40. 	WP14/LF15 in the Natural Attenuation Project Area (location shown in figure 1c),


Dover Air Force Base, Delaware .............................................................................................................65


41. 	Map showing pathlines and traveltimes for ground-water particles starting at the


OT-40 site (location shown in figure 1c), Dover Air Force Base, Delaware .....................................................66


42a-e. 	 Maps showing: 

a. Composite pathlines and traveltimes for ground-water particles released in Area 6


 (location shown in figure 1c), Dover Air Force Base, Delaware.................................................................67


b-e. 	Pathlines and traveltimes for ground-water particles released in Area 6


(location shown in figure 1c), Dover Air Force Base, Delaware, for the following layers:


(B) 	Layer 1 (the upper surficial aquifer), (C) Layer 2 (the lower surficial aquifer) 

(D)	 Layer 3 (the upper Calvert confining unit), and (E) Layer 4 (the Frederica aquifer)............................68


TABLES 

1.	 Generalized stratigraphic, lithologic, and hydrologic characteristics of geologic formations


underlying the Dover Air Force Base area, Delaware. ........................................................................................9


2. Base flow at surface-water sites at Dover Air Force Base, Delaware .....................................................................14


3. Hydraulic properties for selected aquifers and confining units in the Dover Air Force Base area, Delaware ........27


4.	 Hydraulic gradient between the Frederica aquifer and the surficial aquifer at well pair


DM376D and DM376F (location shown on figure 1C) at Dover Air Force Base, Delaware ...........................34


5.	 Age of ground water from analysis of chlorofluorocarbons (CFCs) in selected wells at


Dover Air Force Base, Delaware .......................................................................................................................37


6. Water budget for the Dover Air Force Base area, Delaware ...................................................................................40


7.	 Observed and simulated ground-water discharge to stream reaches in the


Dover Air Force Base area, Delaware................................................................................................................53


vi 
TABLES–Continued 

8. Water budget for simulation of ground-water flow in the Dover Air Force Base area, Delaware ..........................53


9.	 Comparison of chlorofluorocarbons (CFCs) recharge dates to recharge dates from the 

ground-water particle-tracking simulations at Dover Air Force Base, Delaware. .............................................62 

Abbreviations 

CFC Chlorofluorocarbon 

CPS counts per second 

DAFB Dover Air Force Base 

DNREC Department of Natural Resources and Environmental Control, State of Delaware 

d day 

ft foot 

GIS Geographic Information System 

GRFL Ground-Water Remediation Field Laboratory at Dover Air Force Base 

HAZWRAP Hazardous Waste Remedial Actions Program 

in/yr inch per year 

IRP Installation Restoration Program 

K hydraulic conductivity 

LF landfill 

mi mile 

MODFLOW A modular computer code that uses finite-difference numerical techniques for the simulation of groundwater flow. 

MODPATH A particle-tracking computer program for use with MODFLOW. 

RI Remedial Investigation; also used to refer to the "Remedial Investigation" Report 

 (U.S. Army Corps of Engineers and Dames & Moore, Inc., 1994, 1997a, 1997b, and 1997c) 

RMSE root-mean-squared error 

RTDF Remediation Technology Development Forum 

436 SPTG/CEV 436th Support Group, Civil Engineer Squadron, Environmental Flight 

USACE U.S. Army Corps of Engineers 

USGS U.S. Geological Survey 

WP waste pit 

yr year 

vii 
_____________________________________________________________________________________________________________________________________ 

_____________________________________________________________________________________________________________________________________ 

Conversion Factors and Vertical Datum 

Multiply By To obtain 

__________________________________________________________

inch (in.) 

_______________________

2.54 

____________________________________________________ 

centimeter 

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

foot (ft) 0.3048 meter 

foot per day (ft/d)

square foot (ft2)

foot squared per day (ft2/d) 

cubic foot per day (ft3/d) 

 0.3048 

 0.09290 

0.09290 

0.02832 

meter per day 

square meter 

meter squared per day 

cubic meter per day 

mile (mi) 1.609 kilometer 

acre 4,047.0 square meter 

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

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

of 1929. 

Temperature in degrees Fahrenheit (o

F) can be converted to degrees Celsius (oC) by using the following equation: 

o

C = 5/9 x (oF - 32) 

viii 
Hydrogeology and Simulation of Ground-Water Flow at 

Dover Air Force Base, Delaware 

By Kurt C. Hinaman and Frederick J. Tenbus 

Abstract 

Dover Air Force Base in Kent County, 

Delaware, has many contaminated sites that are in 

active remediation. To assist in this remediation, a 

steady-state model of ground-water flow was 

developed to aid in understanding the hydrology 

of the system, and for use as a ground-watermanagement tool. This report describes the 

hydrology on which the model is based, a 

description of the model itself, and some 

applications of the model. 

Dover Air Force Base is underlain by 

unconsolidated sediments of the Atlantic Coastal 

Plain. The primary units that were investigated 

include the upper Calvert Formation and the 

overlying Columbia Formation. The uppermost 

sand unit in the Calvert Formation at Dover Air 

Force Base is the Frederica aquifer, which is the 

deepest unit investigated in this report. A 

confining unit of clayey silt in the upper Calvert 

Formation separates the Frederica aquifer from the 

lower surficial aquifer, which is the basal 

Columbia Formation. North and northwest of 

Dover Air Force Base, the Frederica aquifer 

subcrops beneath the Columbia Formation and the 

upper Calvert Formation confining unit is absent. 

The Calvert Formation dips to the southeast. The 

Columbia Formation consists predominately of 

sands, silts, and gravels, although in places there 

are clay layers that separate the surficial aquifer 

into an upper and lower surficial aquifer. The 

areal extent of these clay layers has been mapped 

by use of gamma logs. 

Long-term hydrographs reveal substantial 

changes in both seasonal and annual ground-water 

recharge. These variations in recharge are related 

to temporal changes in evaporation, transpiration, 

and precipitation. The hydrographs show areas 

where extensive silts and clays are present in the 

surficial aquifer. In these areas, the vertical 

gradient between water levels in wells screened 

above and below the clays can be as large as 

several feet, and local ground-water highs 

typically form during normal recharge conditions. 

When drought conditions persist, water drains off 

these highs and the vertical gradients decrease. At 

the south end of Dover Air Force Base, 

hydrographs of water levels in the Frederica 

aquifer show that off-Base pumping can cause the 

water levels to decline below sea level during part 

of the year. 

A 4-layer, steady-state numerical model of 

ground-water flow was developed for Dover Air 

Force Base and the surrounding area. The upper 

two layers represent the upper and lower surficial 

aquifers, which are in the Columbia Formation. In 

some areas of the model, a semi-confining unit is 

used to represent an intermittent clay layer 

between the upper and lower surficial aquifer. 

This semi-confining unit causes the local groundwater highs in the surficial aquifer. The third 

model layer represents the upper part of the 

Calvert Formation, a confining unit. The fourth 

model layer represents the Frederica aquifer. The 

model was calibrated to hydraulic heads and to 

ground-water discharge in Pipe Elm Branch, both 

of which were measured in September 1997. For 

the calibrated model, the root-mean-squared errors 

for the hydraulic heads and the ground-water 

discharge in the Pipe Elm Branch were 9 percent 

of the range of head and 3 percent of discharge, 

respectively. Heads simulated by use of the model 

were consistent with a map showing average water 

levels in the region. 

The U.S. Geological Survey’s MODPATH 

program was used to simulate ground-water-flow 

directions for several areas on the Base. This 

analysis showed the effects of the local groundwater highs. In these areas, ground water can flow 

Abstract 1 
from the highs and then dramatically change flow 

direction as it enters the lower surficial aquifer. 

The steady-state model has several limitations. 

The entire ground-water system is under transient 

hydraulic conditions, due mainly to seasonal and 

yearly changes in recharge and to withdrawal from

irrigation wells. Yet this steady-state model is still 

considered to be an effective tool for 

understanding the ground-water-flow system 

underlying the Base for average conditions. If the

ground-water system undergoes changes, such as 

an increase in pumping from existing or new wells 

in the surficial aquifer or in the Frederica aquifer 

at or near the Base, then the model may need to be

verified for these conditions and, if necessary, 

recalibrated. Nevertheless, the model can be used 

to determine ground-water-flow pathlines in areas 

of the Base where flow directions are constant. In 

addition, the steady-state model is a necessary step

in the development of transient models and solutetransport models, which are planned for future

ground-water monitoring on the Base. 

Introduction 

Dover Air Force Base (DAFB), located in Kent County,

Delaware (fig. 1a), has been in operation almost continuously since 1941. Various activities in support of the 

military mission have resulted in contamination of shallow 

ground water underlying the Base by synthetic organic 

compounds (Bachman and others, 1998). As a result, DAFB 

is now actively engaged in an Installation Restoration 

Program (IRP) to assess and remediate contaminated ground 

water underlying the Base. 

Background 

DAFB is an active military installation that covers an

area of approximately 4,000 acres (fig. 1b). Ground-water 

contamination has been found in several areas on the Base. 

Some of these areas are adjacent to one another, some are 

adjacent to the Base boundary, some are affected by a unique 

geologic or hydrologic setting, and some are difficult to

characterize because of physical-access problems. In 1995, 

the U.S. Geological Survey (USGS) in cooperation with the 

DAFB, and as part of a long-term-monitoring project, began 

work on a Base-wide ground-water-flow model to help

assess the ground-water-contamination issue. 

A significant amount of information about the 

environmental setting and contamination at and near DAFB

has been collected and synthesized. Most of the work has

been compiled in a summary by Dames & Moore, Inc., and 

HAZWRAP (Hazardous Waste Remedial Actions Program)

(1993). Other environmental investigations with ground2 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Introduction 3 
water components have been conducted near DAFB (CH2M 

Hill Southeast, Inc., 1988a, 1988b). A Base-wide remedial 

investigation (RI) has been completed recently (Dames & 

Moore, Inc., and HAZWRAP, 1993; U.S. Army Corps of 

Engineers and Dames & Moore, Inc., 1994; 1997a, 1997b, 

1997c). In addition, DAFB has been selected as a groundwater remediation field laboratory (GRFL), where new 

technologies in ground-water remediation are tested 

(Applied Research Associates, Inc., 1996). An industrial 

and government consortium, Remediation Technology 

Development Forum (RTDF), has studied contamination at 

DAFB in order to develop other remediation technologies 

(U.S. Environmental Protection Agency, 1996a, 1996b, 

1996c). Other groups also have studied ground-water 

contamination at DAFB (Ball and others, 1997; Eng, 1995; 

Johnston, 1996). The USGS recently investigated natural 

attenuation at several sites on the eastern side of the Base 

(Bachman and others, 1998) (fig. 1c). 

In addition to the environmental investigations already 

conducted, DAFB and the surrounding area have been the 

subject of numerous geologic and hydrologic investigations, 

only a few of which are cited in this report. In the 1950’s, 

Marine and Rasmussen (1955) studied the ground-water 

resources of Delaware. In the mid–1950's, DAFB drilled a 

high-capacity water-supply test well, which was documented 

in two reports (Rasmussen and others, 1958; Benson and 

others, 1985). Jordan (1962, 1964) and Johnston (1973) 

studied the geologic formations in the area. Several studies 

(Boggess and Adams, 1965; Adams and others, 1964; Davis 

and others, 1965; Boggess and others, 1965) compiled maps 

of the water table and soil-engineering characteristics. In the 

mid- to -late 1960's, the water resources of the Delmarva 

Peninsula were investigated (Cushing and others, 1973). In 

the 1970’s, Leahy (1976 and 1979) determined the hydraulic 

characteristics of the Piney Point aquifer, which underlies 

the Calvert Formation, and the overlying confining units. 

During the 1970's, regional numerical simulations of 

ground-water flow in the Dover area were done for the 

unconfined aquifer (Johnston, 1976), the Piney Point aquifer 

(Leahy, 1979), and the Piney Point and Cheswold aquifers, 

which are in the lower part of the Calvert Formation (Leahy, 

1982). In the early 1980's, geologic maps of the area were 

published (Pickett and Benson, 1983; Benson and Pickett, 

1986). Spoljaric (1988, 1989a, 1989b, and 1991) compiled 

geologic and hydrologic information of the area. Spoljaric 

(1986) also studied the concentration of sodium in the Piney 

Point Formation. Vroblesky and Fleck (1991) reported the 

results of the USGS Regional Aquifer-System Analysis of 

the Northern Atlantic Coastal Plain, which included the 

Coastal Plain of Delaware. Phelan (1990) described the 

water use in the St. Jones River Basin, which includes 

DAFB. 

As remedial activities proceed at DAFB, many factors 

need to be considered. It is important to know if remedial 

activities at one site will affect other sites, and which sites 

need to be remediated first. If long-term monitoring is used 

with the remediation process, it is essential that groundwater-monitoring wells be correctly placed to intercept flow 

from contaminated areas, and to select an appropriate 

monitoring frequency. It is useful to know the likelihood of 

contaminant transport in areas where physical access for 

installing monitoring wells is difficult. It is also important to 

know the likelihood of contaminant transport off Base or to 

deeper aquifers. An examination of the hydrogeology at 

DAFB, coupled with a numerical model of ground-water 

flow, is helpful in addressing these concerns. 

This report is part of the Long-Term-Monitoring Project. 

This project is managed by the USGS for the 436th Support 

Group, Civil Engineer Squadron Environmental Flight (436 

SPTG/CEV) of DAFB. 

Purpose and Scope 

This report describes the hydrogeology of DAFB and the 

development and use of a numerical model that simulates 

steady-state ground-water flow at DAFB. The report 

includes a compilation of recharge, as well as a compilation 

of hydraulic conductivities for hydrogeologic units above the 

base of the Frederica aquifer. Also included are data, such as 

gamma logs and stream discharge, that were collected as part 

of this investigation, and an analysis of these data to describe 

the ground-water-flow system. A conceptual model 

developed on the basis of the compiled hydrogeologic data is 

presented. The report details the assembly of a numerical 

model and provides examples of the use of the model as a 

management tool for environmental work at DAFB, 

including simulations of ground-water pathways for several 

contamination sites. 

Description of Investigation Area 

DAFB is located in Kent County, Delaware, about 

3.5 miles southeast of the center of Dover, Delaware (fig. 

1b). It encompasses approximately 4,000 acres, including 

annexes, easements, and leased property (U.S. Army Corps 

of Engineers and Dames & Moore, Inc., 1994). Land use in 

the surrounding area is primarily cropland and wetlands, 

with some rural residential development. 

Land use at DAFB can be divided into two main 

categories: areas with no manmade structures and areas with 

manmade structures (fig. 2). Most of the land areas with no 

manmade structures are located in the eastern, southern, and 

northern parts of the Base. These areas are mainly forests or 

fields. The areas with structures include runways and 

tarmacs, hangars and industrial buildings, offices, and 

on-Base housing east of US Route 113 and Delaware 

Route 1. Figure 2 shows the major areas in the vicinity of 

DAFB that are covered by pavement, such as runways and 

large parking lots (fig. 2). West of US Route 113 and 

Delaware Route 1, manmade structures include off-Base 

housing, the Base elementary school, and a golf course. 

DAFB is located on the Delmarva Peninsula, within the 

Atlantic Coastal Plain. It is underlain by unconsolidated 

deposits of clay, silt, sand, and gravel that lie unconformably 

on crystalline basement. The sediments range in age from 

Early Cretaceous to Holocene (Benson and Spoljaric, 1996). 

The Coastal Plain sediments thicken to the southeast, with 

4 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Introduction 5 
6 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
progressively younger units subcropping beneath a surficial 

blanket of Pleistocene deposits (Benson and Spoljaric, 

1996). 

The topography of DAFB is relatively flat with little 

spatial variation. The surface elevations range from about 

5 ft above sea level near the St. Jones River, to about 30 ft 

above sea level at the northwestern boundary of the Base. 

The northwest-southeast trending runway has an elevation of 

28 ft above sea level, which is higher than most of the 

surrounding area (U.S. Army Corps of Engineers and 

Dames & Moore, Inc., 1997a). 

Delaware has a humid, continental climate with welldefined seasons. Delaware Bay, Chesapeake Bay, and the 

Atlantic Ocean have a considerable effect on the climate 

because winds from the bays and ocean tend to moderate 

temperatures. Summers are warm and humid. The winters 

are mild and there are few prolonged periods of freezing 

weather. Freezing of soils is rare and ground-water recharge 

occurs throughout most of the year. The proximity of large 

bodies of water and the inflow of southerly winds cause high 

relative humidity throughout the year (Rasmussen and 

others, 1958; U.S. Department of Agriculture, 1941; Wood, 

1996). 

Precipitation near DAFB averages about 46 in. per year 

and is distributed fairly uniformly, with the greatest amount 

during the summer. Monthly precipitation ranges from an 

average minimum of less than 3 in. in February, to an 

average maximum of more than 5 in. in August (National 

Oceanic and Atmospheric Administration, 1996; Wood, 

1996). Mather (1969) estimated annual evapotranspiration 

losses in central Delaware to be about 25 in. 

Acknowledgments 

The authors thank: 

Charles Mikula, Jo Anne Deramo, Gregory Jackson, and 

Robert Wickso, of the 436 SPTG/CEV, for the timely 

manner in which they dealt with data requests and with 

requests to install equipment on the Base. 

Robert Lyon and John Osgood of Dames & Moore, Inc., 

who provided electronic forms of well logs, wellconstruction information, and information on the 

hydrogeology of the Base. 

Mike Apgar of the Delaware Department of Natural 

Resources and Environmental Control (DNREC), who 

provided a copy of the Wildcat Landfill report. 

Craig Bartlett and Edward Lutz of DuPont, working as 

part of the Remediation Technology Development 

Forum (RTDF), who provided advice on the 

hydrogeology and the modeling approach as did 

Mark Noll and Steven Farrington of Applied Research 

Associates, Inc. 

Scott Andres, Kelvin Ramsey, and John Talley of the 

Delaware Geological Survey (DGS), who provided 

geological insights into the area.

 Michael Doughton of the U.S. Geological Survey 

(USGS), who measured dissolved gas concentrations 

and ground-water samples, and Ed Busenberg and 

Niel Plummer of the USGS, who performed 

chlorofluorocarbon (CFC) dating. 

Hydrogeology 

Ground-water flow and solute transport are strongly 

influenced by the hydrogeologic framework of the DAFB 

area. The accuracy of results from model simulation of 

ground-water flow and the usefulness of these results to 

subsequently determine future ground-water conditions is 

dependent upon how well the ground-water-flow system is 

understood. For this reason, it was essential to develop an 

accurate conceptual model of the hydrogeology at DAFB. 

One of the first tasks in the project was to review the 

historical literature for a description of the hydrogeologic 

system. This literature and records from regulatory 

agencies, primarily DNREC, provided historical hydrogeological data, such as well records. Concurrently, a 

geographical information system (GIS) data base of hydrogeological data was assembled. Use of a GIS allowed 

different types of data, such as well records and geologic 

maps, to be compiled and combined into one source. A 

conceptual model of the hydrogeologic system was 

developed on the basis of data and information located in the 

literature survey, and gaps in hydrogeologic data were 

identified. To fill these gaps, several types of data were 

collected. Synoptic ground-water levels were collected 

concurrently with measurements of ground-water discharge 

measured at streams and drains at and near DAFB. These 

data were necessary to calibrate the model. Continuous 

recorders were installed in many ground-water wells and on 

some surface-water bodies to determine the water-level 

fluctuations. Analyses of the concentrations of chlorofluorocarbon and tritium in ground water was used to 

estimate ages of the water. These ages were then used to 

define the conceptual model of the ground-water-flow 

system, and to help calibrate the ground-water-flow model. 

Gamma logs were collected to determine the thickness of 

fine-grained sediments in the surficial aquifer. These logs 

also were used to determine the thicknesses and infer the 

lithology of the sediments in the upper Calvert Formation 

confining unit. 

Geologic Framework 

The stratigraphic units that have been identified in the 

DAFB area (Benson and Spoljaric, 1996) are shown in 

figure 3 and in table 1. In this area, the geologic formations 

dip gently and thicken to the southeast (Pickett and Benson, 

1983; Benson and Pickett, 1986). North of DAFB, the 

Hydrogeology 7 
8 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Table 1. Generalized stratigraphic, lithologic, and hydrologic characteristics of geologic formations 

underlying the Dover Air Force Base area, Delaware

 [Modified from Benson and Spoljaric, 1996; Benson, Jordan, and Spoljaric, 1985; DAFB, Dover Air Force Base; >, greater than] 

Approximate 

thickness at 

System Series Formation 

DAFB 

(feet) 

General 

lithology 

Hydrogeologic 

unit 

Quaternary Pleistocene Columbia A 35-85 Sand, silt, gravel, clay Surficial aquifer 

Tertiary Middle to Calvert 0 at DAFB Sand Choptank aquifer

 Lower Miocene subcrops south grouped with

 of DAFB Surficial aquifer 

15-40 Silt Confining unit

 25 Sand Frederica aquifer

 90 Silt Confining unit 

60 Sand Cheswold aquifer 

100 Silt Confining unit

 35 Glauconitic sand Historically included

 with Piney Point

 aquifer 

Middle Eocene Piney Point 215 Sand and sandy silt Piney Point aquifer 

Middle to 

Lower Eocene Deal 310 Silts and clays Confining unit 

Upper Paleocene 

Upper Paleocene Vincenttown 55 Glauconitic sandy to

 clayey silt 

Confining unit 

Lower Paleocene Hornerstown 30 Glauconitic silt Confining unit 

Cretaceous Upper Cretaceous Navesink 20 Glauconitic silt Confining unit 

Mt. Laurel 70 Silt-clay matrix Confining unit 

 with glauconite

 and shell calcite 

Marshalltown 35 Very fine sand and silt Confining unit 

Englishtown 60 Fine to very fine sand Confining unit 

Merchantville 100 Coarse silt and very fine 

sand 

Confining unit 

Magothy 100 Sands and silts Magothy aquifer 

Potomac >50 Clays and sands Confining units and

 aquifers in other

 parts of Delaware 

A Quaternary deposits in Delaware Geological Survey stratigraphy. 

Hydrogeology 9 
Frederica aquifer, which consists of the upper sand of the 

Calvert Formation, subcrops under the overlying Columbia 

Formation (fig. 4) (Pickett and Benson, 1983). South of 

DAFB, the Choptank aquifer subcrops under the Columbia 

Formation and overlies the upper confining unit of the 

Calvert Formation (Benson and Pickett, 1986). Numerous 

publications include cross sections showing the geology of 

this area (see Pickett and Benson, 1983; Benson and Pickett, 

1986; Benson and Spoljaric, 1996). 

The Calvert Formation consists of a gray-to-blue to 

greenish-gray silt, with subordinate sand and shell beds 

(Leahy, 1982; Benson and others, 1985; Spoljaric, 1988; 

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

1997a). It ranges in thickness from about 290 ft beneath 

DAFB to over 600 ft in southern Delaware (U.S. Army 

Corps of Engineers and Dames & Moore, Inc., 1997a). In 

the DAFB area, it is divided into five units; two sandy layers 

that separate three silty layers (Marine and Rasmussen, 

1955; Benson and others, 1985; U.S. Army Corps of 

Engineers and Dames & Moore, Inc., 1997a). This 

investigation focused on the upper parts of the Calvert 

Formation; the sand layer known as the Choptank aquifer, a 

clay and silt layer that forms the confining bed between the 

surficial aquifer and the underlying sand, and the Frederica 

aquifer (table 1). 

At DAFB, the Columbia Formation 1 consists of fluvial 

deposits of fine-to-coarse sand with silt and clay lenses and 

less common lenses of gravel. The sediments generally fine 

upward (U.S. Army Corps of Engineers and Dames & 

Moore, Inc., 1997a), but the amount of fining varies. The 

top of the Columbia Formation throughout the investigation 

area is defined as the land surface, which was derived from 

USGS 7.5-minute quadrangle topographic maps with a 

contour interval of 5 ft. The base of the Columbia Formation 

(fig. 5) was defined using two different data sets. Within 

DAFB boundaries, the base of the surficial aquifer was 

determined from well logs. Outside of DAFB, the 

thicknesses of the Columbia Formation and recent marsh 

sediments (Benson and Pickett, 1986; Pickett and Benson, 

1983) were subtracted from land-surface altitude to obtain 

the base of the surficial aquifer. The thickness of the 

formation ranges from about 35 ft in the northwestern corner 

of the Base to about 85 ft in the eastern part of the Base. 

In some areas of DAFB, the upper Columbia Formation 

contains fine-grained sediments such as silts and clays, and 

the lower surficial aquifer consists of cleaner sands, silts and 

gravels. In some of these areas, a clay and silt sequence 

separates the upper and lower Columbia Formation. This 

sequence was seen at DAFB in split-spoon samples and 

direct-push cores. In areas outside the Base boundary, the 

sequence was not mapped. 

Gamma logs were used to extend the clay and silt 

sequence to areas with no split-spoon samples or direct-push 

cores. Figure 6 shows the locations of the gamma logs, 

which show the gamma radiation of the sediments. In 

general, higher gamma counts are correlated with finergrained materials, which was confirmed by sediments in 

samples collected from wells. The gamma logs collected for 

this investigation were not calibrated; consequently, the 

gamma counts are relative counts. A count of 40 cps (counts 

per second) was used as a dividing line between sediments 

classified as fine-grained and coarser sediments. At each 

well, the total thickness of these finer-grained sediments was 

noted, as was the top and base of the finer-grained 

sediments. The top of the fine-grained sediments was 

subsequently compared to the average water level in the 

well. The average water levels are from regional data that 

includes the DAFB area. The thickness of the finer-grained 

sediments below the average water table was plotted and 

contoured (fig. 6). The rationale for mapping only that part 

of the section below the water table will be addressed 

later. 

Hydrologic Framework 

This investigation focused on ground-water flow in and 

through the shallow hydrogeologic system in the DAFB 

area. The units of interest included the upper and lower 

surficial aquifer, the confining unit in the upper part of the 

Calvert Formation, and the Frederica aquifer (fig. 3 and table 

1). These units are of primary importance to the 

environmental work being conducted at DAFB for the 

following two reasons: The sources of contamination are all 

at or just below the ground surface, and most of the 

contamination discovered to date is in the surficial aquifer, 

with only minor amounts in the Frederica aquifer. 

Ground-water recharge to the shallow hydrogeologic 

system in the DAFB area comes from precipitation that (a) 

does not run off directly into surface drainage ways, (b) is 

not evaporated, or (c) is not transpired by plants. Estimates 

of recharge rates range from a low of 4 in/yr (inches per 

year) (Applied Research Associates, Inc., 1996) to a high of 

22 in/yr (Cushing, Kantrowitz, and Taylor, 1973), with most 

estimates of yearly recharge ranging from 8.5 in/yr to 

16 in/yr (Marine and Rasmussen, 1955; Woodruff, 1967; 

Johnston, 1973, 1977; Talley, 1988). Discharge from the 

shallow ground-water system flows to local surface-water 

bodies, to pumped wells, or into the deeper, regional aquifers 

in the area. Four major surface-water bodies surround the 

Base: Little River, Delaware Bay, St. Jones River, and 

several water-filled sand-mining pits adjacent to the southwestern part of the Base at a quarry operated by Tilcon 

Delaware, Inc. (referred to in this report as the Tilcon ponds) 

(figs. 1b and 1c). 

1. The Delaware Geological Survey (DGS) is redefining the extent of the Columbia Formation in Delaware. When DGS remaps the Dover Area, the 

Columbia Formation may not extend to DAFB. It is beyond the scope of this investigation to formally rename the geologic units at DAFB. For 

consistency with other hydrogeologic investigations at the Base, the term “Columbia Formation” is used in this report as the equivalent of the 

unconfined, surficial aquifer. 

10 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Hydrogeology 11 
12 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Hydrogeology 13 
Table 2. Base flow at surface-water sites at Dover Air Force Base, Delaware 

Discharge at selected surface-water sites (in cubic feet per day) 

Date SW1-near exercise area Pipe Elm Branch A Upper Golf Course Ditch Sand Ditch Morgan Branch 

July 16, 1997 15,552 51,840 6,048 864 864 

Sept. 16-17, 1997 2,592 39,744 5,184 0 0 

Nov. 20, 1997 20,736 81,216 2,592 1,728 8,640 

July 27, 1998 21,600 25,920 2,592 0 0 

A Pipe Elm Branch is affected by tides and by a manmade structure. Twice a day the tides cause a rise in stage of about one foot. A surfacewater pollution-control structure, located downstream, forms a small pond that controls the low water stage of Pipe Elm Branch. 

The surface-water drainage at DAFB is controlled 

primarily by overland flow to a storm-water drainage system 

consisting of underground pipes and open ditches. Surface 

runoff from the southwestern part of the Base flows through 

the drainage system and eventually discharges to the 

St. Jones River. Surface runoff from the eastern part of the 

Base flows toward the Morgan and Pipe Elm Branches of the 

Little River. Most of the Base drainage is collected in open 

or covered ditches and directed towards Pipe Elm Branch, 

which then enters the Little River. The lower reaches of 

Pipe Elm Branch are tide affected and this tidal zone extends 

to DAFB. There are gaining reaches upstream from the 

tidally influenced reaches. The golf course is drained by a 

tributary to the St. Jones River that gains water in the area of 

the golf course, and empties into a series of ponds that are 

connected to the St. Jones River. The St. Jones River and the 

Little River empty into the Delaware Bay, which is about 

2.5 mi east of DAFB. 

Surface-water base-flow measurements were made at 

several surface-water bodies 5 days after the most recent 

rainfall event, when it was assumed that no surface runoff 

was still occurring and all the base flow was ground-water 

discharge. These measurements could be made only in the 

branches and streams entering the St. Jones River, the Little 

River, and the Delaware Bay (fig. 1). Measurements were 

not possible within the rivers or the Bay because of the tidal 

effects. These measurements show that Pipe Elm Branch 

receives most of the ground-water discharge at DAFB (table 

2). 

The surface drainage system is necessary to keep the 

runways dry, and the system has all but eliminated wetlands 

over most of the Base. Some wetlands remain, however, 

along sections of Morgan Branch and Pipe Elm Branch and 

along some drainage ditches. On the Base, these wetlands 

are only a few feet wide. The most extensive wetlands are 

along the banks of the St. Jones River on the southwestern 

boundary of the Base. Outside the Base, the lower reaches 

of the St. Jones River and the shore of the Delaware Bay are 

extensive wetlands (fig. 2). The wetlands shown on figure 2 

are compiled from topographic maps, infrared aerial 

photographs, and field surveys. 

The only pond on DAFB is on the Base golf course 

southwest of Delaware Route 113 (fig. 1c). This pond is not 

part of the stormwater drainage system, but is used to hold 

irrigation water for the golf course. Water enters this pond 

from ground-water discharge and surface runoff, and is 

occasionally replenished from the Base water system (D. 

Phelan, U.S. Geological Survey, oral commun., 1995). The 

water levels in the pond vary depending on recharge and 

withdrawal from the pond. 

Surficial Aquifer The surficial aquifer extends across 

the entire DAFB area. It consists of the sediments of the 

Columbia Formation, which were described earlier in this 

report. 

Hydraulic-Head Distribution and Fluctuations—In 

general, the configuration of the water table appears to be 

controlled by the locations of surface-water bodies such as 

the St. Jones River, Little River, and Delaware Bay, and by 

the amount of recharge. The shape of the water table varies 

according to the amount of precipitation. If there is no 

recharge and no pumping from the surficial aquifer, the 

water table would be a gently sloping surface (fig. 7a). The 

altitude of the water table would be determined by the stages 

of the Delaware Bay, the St. Jones River, the Little River, 

and Pipe Elm Branch. The slope of the water table depends 

on the tidal range and the damping effect of the aquifer 

material, but it would be nearly horizontal (flat), with an 

altitude of about one foot. 

How recharge changes the shape of the water table would 

depend on the hydraulic conductivity of the aquifer and the 

distance from the recharge areas to the discharge areas. 

DAFB lies on a peninsula-like feature that is surrounded by 

the Delaware Bay, the St. Jones River, and the Little River 

(fig. 7a). If the Delaware Bay, the St. Jones River, and the 

Little River were the only surface-water features 

surrounding DAFB, then the water-table contours would 

reflect the outline of those features (fig. 7b), culminating in 

one elongated water-table high centered at about DAFB. 

Several streams intersect this area—Morgan Branch, 

14 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Hydrogeology 15 
Pipe Elm Branch (fig. 7c). These streams would intersect 

any water-table high and the result is a series of water-table 

highs located roughly along the axis of this area (fig. 7d). 

The exact location of the highs is controlled by the distance 

from the recharge area to the discharge area, and by the 

hydraulic conductivity of the aquifer. 

The relations discussed above can be seen in regional 

maps of the water table, the depth of which varies seasonally 

and spatially, and can range from ground surface to about 

30 ft below ground surface. A general picture of the average 

configuration of the water table over the entire DAFB area 

was compiled from historical data (Adams and others, 1964; 

Boggess and Adams, 1965; Boggess and others, 1965; Davis 

and others, 1965) and is shown in figure 8. The Northwest 

Runway Divide trends from Route 8 east of Route 113 

through the Northwestern part of the Base. The Southeast 

Runway High is a closed elliptical water-table high that is 

over the eastern part of the Base. Ground-water troughs 

form along river tributaries: Pipe Elm Branch and the 

Golf Course Tributary. Within DAFB boundaries these 

highs and lows roughly divide the Base into quarters 

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

1997a) (fig. 9). 

Variation in recharge causes these highs and divides to 

increase and decrease in amplitude. During periods of low 

recharge (fig. 10), the Southeast Runway High becomes very 

broad while the amplitude of the Northwest Runway Divide 

decreases. During periods of higher recharge (fig. 11), 

the Southeast Runway High becomes more pronounced 

and the amplitude of the Northwest Runway Divide 

increases. 

Superimposed upon this recharge-controlled water table 

are local features seen in the upper surficial aquifer, which 

are referred to as "local ground-water highs" in this report. 

Continuous-recorder data of water levels, gamma logs, and 

lithologic samples were collected to study these local watertable highs. Water levels were recorded at well pairs 

consisting of shallow and deep wells screened in the surficial 

aquifer. The shallow wells have 10-ft screens placed near 

the water table. The deep wells have 10-ft screens placed at 

the base of the surficial aquifer. In well pairs, water-level 

hydrographs show a significant head difference of up to 

about 6 ft, with the higher head in the shallow well. This 

difference is sustained through normal wet and dry periods. 

During abnormally long dry periods, the difference in head 

decreases, and at one well pair, the difference decreased to 

about 0.05 ft (see DM348 S and D, fig. 12). The surficial 

aquifer is saturated to the level of the screens in the shallow 

wells, because the water levels in the deep wells are as high 

as the screens in the shallow wells; therefore, the water in the 

shallow wells is not perched (fig. 13). Gamma logs show 

layers with higher gamma counts towards the top of the log 

(fig. 13). Lithologic samples indicate that the layers with 

higher gamma counts are finer-grained sediments such as 

clays and silts, while the layers with the lower gamma counts 

are mainly sands. Figure 14 summarizes the relations 

between the local ground-water highs and the depth of well 

screens relative to clays in the surficial aquifer. If both wells 

are screened above the clay layer, then the water levels in 

both wells are about the same (fig. 14a). If the shallow well 

is screened above the clay layer and the deep well is 

screened below the clay layer, during times of recharge the 

water levels in the shallow well are higher than the water 

levels in the deeper well (fig. 14b). If the shallow well is 

screened in the clay layer and the screen does not penetrate 

the layer, there can be a local ground-water high (fig. 14c). 

If both wells are screened below the clay layer, there is not a 

large difference in heads from the two wells and a local 

ground-water high can can go unrecognized (fig. 14d). In 

summary, two conditions must be met to form a local 

ground-water high that can be recognized: (1) the finegrained sediments must be continuous over an area that can 

support a local ground-water high, and (2) the screen in the 

shallow well must be in or above the fine-grained sediments. 

If these conditions are met, local ground-water highs can 

form and be recognized under average recharge conditions 

(fig. 15). These local ground-water highs show a similar 

response to recharge as do regional highs. For example, the 

amplitude of the high that is near the Southeast Runway 

increases as recharge goes from drier (fig. 16) to wetter 

(fig. 17) conditions. 

Comparison of hydrographs from on-Base wells to a 

hydrograph from a long-record well, Jd42-03, approximately 

2 mi. northwest of DAFB, shows that the dry and wet 

periods seen in the mid–1990's are common (fig. 12). The 

hydrographs from wells DM110S and DM110D at DAFB 

show the rise in the water table from a relatively dry time, 

October 1995, to a relatively wet time, July 1996, and back 

to an average summer of 1997 (fig. 12). The long-record 

hydrograph from the area of DAFB shows that the synoptic 

water-level measurements mentioned earlier were obtained 

during extremes in the water table. The December 1993 (fig. 

16) synoptic was collected during a period when the water 

table was low. The May 1994 synoptic (fig. 17) was 

collected during a period when the water table was high. 

Transient water-level conditions should be considered in the 

design of any long-term remedial systems. If a remedial 

system was built during a dry period and it was necessary to 

collect samples at the water table, then the sampling points 

placed at the water table during the dry period could be 

under the water table during the wetter periods. 

Two of the hydrographs (DM412D and MW33D) in 

figure 12 show water-level declines that may be related to 

operations in the Tilcon Ponds, which are used in surface 

mining of sands and gravels. The water levels in these ponds 

are affected by the gravel mining operations. Sand and 

gravel are removed with a dredge system, mixed with water 

from the pond that is being actively mined, and transported 

in a slurry through a pipe to the cleaning and separating 

plant. Water from the slurry is discharged into a pond near 

the plant, and returns to the active pond through a series of 

canals. The water levels in the pond that is being actively 

mined may be lowered due to the removal of water from this 

pond, especially if the return canals are blocked for some 

16 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Hydrogeology 17 
18 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Hydrogeology 19 
20 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Hydrogeology 21 
reason. During the summer of 1998, Tilcon Delaware, Inc., 

began dredging operations in a new mining pond 

approximately 2,000 ft southwest of monitoring well 

DM412D, on the southwestern side of DAFB. The water 

level in DM412D shows a marked drop for this period (fig. 

12). A possible explanation is that pumping in the actively 

mined pond lowered the water level in this pond, thus 

depressing the water table, which is reflected in the 

hydrograph. A similar decline in water level is seen in the 

spring of 1997 for well MW33D, which is also adjacent to 

the Tilcon Delaware, Inc., property (fig. 12). The mining 

operation was not monitored at this time, however, and it is 

unknown if this decline in water level at MW33D was due to 

mining in the pond adjacent to MW33D, or to some other 

cause. 

Hydraulic Conductivity—Hydraulic conductivity values 

for the surficial aquifer were obtained from various 

investigations in the DAFB area (table 3), and range from 

about 0.1 ft/d (feet per day) (Eng, 1995) to about 250 ft/d 

(CH2M Hill Southeast, Inc., 1988b). Such hydraulic 

conductivity values are typical of the sediments that make up 

the surficial aquifer at DAFB, which range from clays and 

silts to clean sands. 

For the upper part of the surficial aquifer, hydraulic 

conductivity was assumed to be relatively constant within 

each of two regions in the investigation area—the finegrained sediments adjacent to major surface-water bodies, 

and the coarse-grained sediments in the upland areas farther 

away. Silts and mud deposited in wetlands adjacent to the 

Delaware Bay and smaller estuaries give these areas a finer 

sediment texture than the sands and gravels that predominate 

in the uplands. The extent of the wetlands and marshes is 

shown in figure 2. Fine-grained sediments in the upper 

surficial aquifer are seen at the Wildcat Landfill 

(CH2M Hill Southeast, Inc., 1988a, 1988b). Slug tests of 

10 wells screened in the upper surficial aquifer yield a mean 

value of 1.86 ft/d (U.S. Army Corps of Engineers and 

Dames & Moore, Inc., 1997a). 

No hydraulic conductivities have been determined for the 

clays that separate part of the upper surficial aquifer from the 

lower surficial aquifer. Values from previously published 

studies were used as a basis for estimated values. Fetter 

(1988) suggests 2.8 x 10-3 ft/d to 2.83 x 10-6 ft/d as the range 

of hydraulic conductivities of clay, although it is not noted if 

this is a horizontal or vertical hydraulic conductivity. 

22 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Hydrogeology 23 
24 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Hydrogeology 25 
26 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Table 3. Hydraulic properties for selected aquifers and confining units in the Dover Air Force Base area, Delaware–Continued

.0

0

Table 3. Hydraulic properties for selected aquifers and confining units in the Dover Air Force Base area, Delaware

[The table is arranged in alphabetical order by author and year in the reference column; in/yr, inches per year; ft/d, feet per day; ft, feet; ft2/d, feet squared per day; –, no data available;

K, hydraulic conductivity; GRFL, Ground-Water Remediation Field Laboratory] 

Horizontal hydraulic Horizontal hydraulic Frederica aquiferconductivity of the conductivity of the Confining unit horizontal

Reference Comments	 Recharge to upper surficial lower surficial hydraulic hydraulic

surficial aquifer aquifer aquifer conductivity conductivity(in/yr) (ft/d) (ft/d) (ft/d) (ft/d) 

This investigation	 12 0.1 to 5 65 0.002 25 Applied Research Associates, A This value used for a ground-water-flow 4 A – 8.5 B 0.0084 C – Inc. (1996) model for the area of the GRFL.

B This value is an estimated value, range


 of two pumping tests and slug test, 1.76
 to 45.52.


C This value is an average value, range of


 six cone-penetrometer tests, 2.2 x10-3


 to 4.2 x 10-2.


CH2M Hill Southeast, Inc. D Aquifer test, Table G-4, Volume II, – – 77 to 200 D 0.05 to 5.1 F 11 to 36 G (1988a, 1988b) CH2M Hill Southeast, Inc. (1988b). 7.1 to 94 E 

E Slug test, p. 4-29. CH2M Hill


Southeast, Inc. (1988a). 


F Slug test in wetland silts.G CH2M Hill Southeast, Inc.


(1988a, p. 4-29), do not consider the
Frederica aquifer values to be
representative because of recharge
from the St. Jones River.


Hydrogeology 27 
H

ydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 

Table 3. Hydraulic properties for selected aquifers and confining units in the Dover Air Force Base area, Delaware–Continued

Horizontal hydraulic Horizontal hydraulic Frederica aquiferconductivity of the conductivity of the Confining unit horizontal

Reference Comments	 Recharge to upper surficial lower surficial hydraulic hydraulic

surficial aquifer aquifer aquifer conductivity conductivity(in/yr) (ft/d) (ft/d) (ft/d) (ft/d) 

Cushing, Kantrowitz, and H From analysis of base flow of 33 streamsTaylor (1973) on the Delmarva Peninsula.

I Represents total range of values from

 extreme years. 

EA Engineering, Science, and J Average of two slug tests on wellsTechnology, Inc. (1991)	 completed only in the lower 10 ft of

 the surficial aquifer. This report also had two transmissivities determined by

a pump test (approximately 

 18,500 ft2/d and approximately

 10,900 ft2/d) with an average of 14,800 ft2/d, which gives a K of

 537 ft/d when the transmissivity is divided by a reported saturated thickness of 27.5 ft. 

Eng (1995) K Determined by analysis of grain size.

 Maximum range of values from fig. 8 of Eng (1995). 

Johnston (1973) Johnston (1976) L Values given are discharge to streams,

 which is added to evapotranspiration to

 obtain the ground-water recharge cited in most studies; the four values are from four small basins that Johnston studied. 

8.5 H, – –	 – – 

2.2 to 22 I 

– – 32 J	 – – 

– –	 0.3 to 160 K – – 

14 – –	 – – 

13, 12, 14, 15 L – –	 – – 

28 
Table 3. Hydraulic properties for selected aquifers and confining units in the Dover Air Force Base area, Delaware–Continued

Hydrogeology 

Horizontal hydraulic Horizontal hydraulic Frederica aquiferconductivity of the conductivity of the Confining unit horizontal

Reference Comments	 Recharge to upper surficial lower surficial hydraulic hydraulic

surficial aquifer aquifer aquifer conductivity conductivity(in/yr) (ft/d) (ft/d) (ft/d) (ft/d) 

Johnston (1977)	 14 – 50 to 250 – – Marine and Rasmussen 10 – – – – (1955) Science Applications M Science Applications International	 – – 110 M – – International Corporation 	 Corporation (1989) determined (1989)	 transmissivity at two wells then

applied this transmissivity to other

parts of the Base and divided by an

aquifer thickness at other parts of the

Base to obtain hydraulic conductivityvalues. 

Talley (1988)	 13 to 16 – – – – U.S. Army Corps of N Average of slug tests on 10 shallow wells – 1.9 N 12 O .0015 to .0068 6.9 P Engineers and Dames & in the surficial aquifer.Moore, Inc. 	 (1997a) 

O Average of slug tests on 10 deep wells in


 the surficial aquifer.


P Average of slug tests on three wells


 completed in the Frederica aquifer.


Woodruff (1967)	 12 – – – – 

29 
Freeze and Cherry (1979) indicate a similar range of 

hydraulic conductivities—2.83 x 10-4 ft/d to 

2.83 x 10-7 ft/d—for clay, but again do not specify whether 

this is a horizontal or a vertical hydraulic conductivity. 

The lower part of the surficial aquifer has a higher 

hydraulic conductivity than the upper part of the aquifer 

(table 3) (M. Noll, Applied Research Associates, Inc., oral 

commun.; 1995; R. Lyon, Dames & Moore, Inc., oral 

commun., 1995). Slug tests of ten wells screened in the 

lower surficial aquifer yield a mean value of 11.70 ft/d (U.S. 

Army Corps of Engineers and Dames & Moore, Inc., 1997a). 

Other investigators also used slug tests to determine 

hydraulic conductivities from 7.09 ft/d to 93.54 ft/d, and 

aquifer tests to determine hydraulic conductivities from 

76.82 ft/d to 198.43 ft/d (CH2M Hill Southeast, Inc., 1988a, 

1988b). Eng (1995) used a grain-size analysis to determine 

that hydraulic conductivities ranged from 0.3 ft/d to 

155.9 ft/d. Applied Research Associates, Inc. (1996) used 

slug tests and aquifer tests to determine that hydraulic 

conductivities ranged from 1.76 ft/d to 45.52 ft/d. 

Upper Confining Unit of Calvert Formation The 

upper confining unit of the Calvert Formation underlies the 

surficial aquifer. Its top and bottom surfaces are equivalent 

to the base of the surficial aquifer and the top of the 

Frederica aquifer, respectively. As discussed in earlier 

sections of this report, the altitudes of these surfaces were 

obtained from various data sources. Within DAFB, the top 

and bottom of the confining unit were determined from 

lithologic logs. Gamma logs also were collected in most of 

the wells that penetrated this unit. Outside DAFB, the 

altitude of the bottom of the surficial aquifer is somewhat 

less reliable, and the altitude of the top of the Frederica 

aquifer was determined from well-completion reports in the 

DNREC water-supply files. A GIS was used to combine the 

altitude information for the top of the Frederica aquifer. 

These values were then contoured manually. By use of a 

GIS, the thickness of the confining unit (fig. 18) was 

calculated by subtracting the altitude of the top of the 

Frederica aquifer from the altitude of the base of the surficial 

aquifer. Only a few of the wells that penetrate the Frederica 

aquifer outside DAFB have reliable lithologic logs; thus the 

map outside of DAFB is generalized. 

The clays and silts of the upper Calvert Formation act as 

a confining unit for the underlying Frederica aquifer. The 

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

(1997a) estimated that ground-water flow from the surficial 

aquifer to the Frederica aquifer through the confining unit 

would take approximately 100 years. To obtain this number, 

they assumed an average thickness of 19.6 ft, a vertical 

hydraulic conductivity (given as permeability in the RI) of 

1.98 x 10-3 ft/d, a head gradient of 0.11 ft (for December 

1993 conditions) and a porosity of 40 percent (U.S. Army 

Corps of Engineers and Dames & Moore, Inc., 1997a, 

p. 3–67). In the center of DAFB, U.S. Army Corps of 

Engineers and Dames & Moore, Inc., (1997a) discovered a 

small area, just to the west of where the runways cross, 

where the confining unit is missing and the surficial aquifer 

is in direct contact with the Frederica aquifer. A gamma log 

for well MW85P (on file at the USGS office, Dover, 

Delaware), however, shows about 5 ft of fine-grained 

material at this location. Thus, it is likely that a thin 

confining unit is present between the surficial aquifer and the 

Frederica aquifer in the area.

 No wells are completed in this section of the 

Calvert Formation so there are no maps of hydraulic-head 

distribution nor any data on hydraulic-head fluctuations. 

Falling-head permeameter measurements were used to 

determine that the vertical hydraulic conductivity of the 

confining unit ranges from 0.00153 ft/d to 0.00683 ft/d 

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

1997a). 

Frederica Aquifer Beneath the upper confining unit of 

the Calvert Formation is the Frederica aquifer, a sand about 

20 ft thick in the DAFB area (U.S. Army Corps of Engineers 

and Dames & Moore, Inc., 1997a). On the Base, 14 

monitoring wells are screened in this aquifer. The 

approximate altitude of the top of the Frederica aquifer is 

shown in figure 19. Information about the altitude of the 

bottom of the Frederica aquifer is limited, so for this 

investigation, the thickness is assumed to be constant. 

Hydraulic-Head Distribution, Fluctuation, and 

Gradient—The Frederica aquifer is recharged from the 

subcrop area (fig. 4) and from leakage through the confining 

unit. Figure 20 shows an interpretation of its potentiometric 

surface. In general, the fluctuation of head in the Frederica 

aquifer is similar to that of the head in the surficial aquifer. 

An exception to this pattern occurs during periods of 

irrigation in the growing season. Figure 21 shows the water 

level in wells DM102F and DM421F declining to below sea 

level during the growing seasons of 1996 to 1999. These 

declines were not seen in another well (DM378F) screened 

in the Frederica aquifer, which is located in the northwestern 

part of the Base. The cause of these declines may be 

irrigation pumping from the Frederica aquifer south and east 

of the Base. Although the wells can be seen from the road, 

the State of Delaware does not have records on the wells; 

thus, it is not known which specific wells pump from the 

Frederica aquifer, nor is the amount of pumpage known. 

For most of DAFB, the vertical head gradient is from 

the lower surficial aquifer towards the Frederica aquifer. 

For a well group located on the golf course and near the 

St. Jones River (fig. 1c) however, this gradient can reverse. 

Table 4 shows this reversal. For most of the synoptic 

water-level measurements, the vertical head gradient is 

from the Frederica aquifer towards the lower surficial 

aquifer. The month of July 1998 fell within an extended dry 

period when the gradient reversed, perhaps due to pumping 

in the Frederica aquifer (see fig. 21). During this time, the 

vertical head gradient was from the lower surficial aquifer 

towards the Frederica aquifer. December 1993 also fell 

during a dry period, and the gradient was from the 

surficial aquifer towards the Frederica aquifer. During 

this time, it is highly unlikely that there was any 

irrigation that would lower the head in the 

30 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Hydrogeology 31 
32 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Hydrogeology 33 
 Table 4. Hydraulic gradient between the Frederica aquifer and the surficial aquifer

 at well pair DM376D and DM376F (location shown on figure 1c)

 at Dover Air Force Base, Delaware 

Date Water levels, in feet above mean sea level Hydraulic gradient, in feet per foot A 

Well DM376D Well DM376F 

July 1998 3.06 2.20 -0.041


November 1997 3.30 4.10 0.038


 September 1997 3.02 3.38 0.017


May 1994 3.27 5.50 0.105


 December 1993 2.99 2.97 -0.001


A. Negative value indicates flow is from the surficial aquifer towards the Frederica aquifer. 

34 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Frederica aquifer, although its low value may have been the 

result of summer irrigation from which the aquifer had not 

recovered. With no records of water levels for the summer 

however, this idea is just speculation. 

Hydraulic Conductivity—The hydraulic conductivity of 

the Frederica aquifer ranges from about 7 ft/d to 36 ft/d 

(table 3). Results of slug tests in the Frederica aquifer at 

DAFB indicate an average hydraulic conductivity of 

6.92 ft/d, with a range of 2.84 ft/d to 14.5 ft/d (U.S. Army 

Corps of Engineers and Dames & Moore, Inc., 1997a). At 

well MW16, the hydraulic conductivities ranged from 11 ft/d 

to 36 ft/d. These values were determined from a pumping 

test; however, the investigators thought that the latter values 

may be too high, due to leakage of water from the 

St. Jones River (CH2M Hill Southeast, Inc., 1988b, Table 

G–4). 

Ground-Water Age

 Information about the age of ground water can be used 

in several ways. First, the age can be used to test a 

conceptual model of the ground-water-flow system. Second, 

the age of ground water can give an idea of when the water 

will reach a particular point or boundary. Third, the age can 

be used to help calibrate a numerical model of ground-water 

flow. The age of ground water at DAFB was determined by 

use of two dating methods—one of which is based on 

concentration of chlorofluorocarbons in the water, and the 

other on the concentration of tritium (Plummer and others, 

1993; Reilly and others, 1994; Szabo and others, 1996). 

These two methods were used because the results from a 

single method are not always unique or easily interpreted. If 

one method produces ambiguous results, the second method 

can be used to corroborate, or discount the accuracy of those 

results. 

Chlorofluorocarbon Dates Chlorofluorocarbons 

(CFCs) are stable volatile organic compounds that were first 

produced in the 1930's, and have been found useful in dating 

young ground water (Plummer and others, 1993). There are 

three main types of CFCs: dichlorodifluoromethane, 

CCl2F2, or CFC–12, which was first produced in the 1930's; 

trichlorofluoromethane, CCl3F or CFC–11, first produced in 

the 1940's; and trichlorotrifluoromethane, C2Cl3F3 or 

CFC–113. 

Ground water can be dated to within a few years by use 

of CFCs under the following optimal conditions: the 

temporal variations of atmospheric CFC concentrations are 

known, there are no local sources of CFCs, there are no local 

chlorinated solvents in ground water, and there are no anoxic 

zones, which can support bacteria that use CFCs in their 

metabolism (Plummer and others, 1993). Chlorinated 

solvents and/or anoxic conditions are present in some parts 

of DAFB. The wells sampled in these areas had to be 

carefully selected to avoid these problems. Another factor 

that can affect the accuracy of the results is the recharge 

temperature. Noble gases were collected to obtain data on 

the recharge temperature. Because of these difficulties 

associated with the interpretation of CFC data, the ages 

presented in this report are not considered exact, and the 

ages are referred to as "apparent ages." 

Between August 21 and 28, 1996, water samples were 

collected from 15 wells for CFC and tritium dating (fig. 22). 

At DAFB, for most of the wells in the surficial aquifer, the 

CFC ages of ground water are consistent with the hydrogeologic settings of the wells. For well pairs, the ages of 

water samples from the upper surficial aquifer are younger 

than the ages of samples from the lower surficial aquifer. 

This relationship is expected Basewide except in groundwater-discharge areas, but no samples were collected from 

wells in discharge areas, such as near streams or drains. The 

ages of samples collected farther along the flow paths are 

older than the ages of samples collected along shorter flow 

paths. An example is the apparent age of water in DM206D 

compared to the younger apparent age of water in DM342D 

(fig. 22). 

The ages given in column A of table 5 were estimated 

with recharge temperatures that were determined by analysis 

of noble gases (for a discussion of this method see the 

following papers: Dunkle and others, 1993; Heaton, 1981; 

Heaton and Vogel, 1981). Because of concerns about the 

recharge temperature varying over the Base, this 

temperature was changed to an average temperature of 

8.8 ο

C and the ages recalculated (column B). This change 

resulted in recharge ages that are slightly different than the 

recharge ages calculated by use of the recharge temperatures. 

The following are the mean differences between these two 

recharge ages: 4.2 yr (years) for CFC–11, 4.0 yr for CFC– 

12, and 2.8 yr for CFC–113. 

Tritium Dates Tritium, the radioactive isotope of 

hydrogen, can be used to age-date ground water relative to 

the atmospheric testing of hydrogen bombs in the 1950's and 

1960's (Plummer and others, 1993). Several approaches can 

be used for dating ground water using tritium; the simplest 

approach is to determine whether tritium is present in 

ground-water samples. If it is present, at least some of the 

water in the sample entered the system as recharge since 

1953 (Plummer and others, 1993). If it is not detectable, 

then significant amounts of post–1953 water are not present 

(Plummer and others, 1993). 

The simplest tritium-dating approach (pre– or post–1953 

dates, described above) was used. The concentrations of 

tritium in samples in which it was detected was low (8 to 25 

tritium units). At this concentration range, it is difficult to 

determine accurate post–1953 dates because a large range of 

years is possible. For this reason, tritium dates were used 

only to augment the dates determined by CFC analysis. 

The concentration of tritium in water samples from the 

surficial aquifer (about 8 to 25 tritium units) indicates that 

the ages of all the samples are post–1953. These ages are 

consistent with the CFC ages for the surficial aquifer. 

Tritium was not detected in samples from the Frederica 

aquifer, which indicates that the ground water was recharged 

before atmospheric testing of hydrogren bombs began in 

1953. This age is consistent with the hydrogeologic setting 

and with the ages determined by CFC analysis. 

Hydrogeology 35 
36 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Table 5. Age of ground water from analysis of chlorofluorocarbons (CFCs) in selected wells at Dover Air Force Base, Delaware

[Recharge elevation is 30 ft; well no., well number; T, recharge temperature in degrees Celsius; Modern age, 1994 and younger; A, CFC age indicated by use of the recharge temperature in the“T” column; B, CFC age calculated by use of an average recharge temperature of 8.8 ο C; 1980.5, 1980 + 6 months; Interference, interference by unknown halocarbon at the CFC range on the

chromatograph; the concentration of these unknown halocarbons are at the nanogram/liter level or higher]

Hydrogeology 

 Estimated CFC recharge dates for each CFC 

Well Sampling T CFC–11	 CFC–12 CFC–113 Interpreted CFC age for no. date ( ο C) each well or comments 

A B A B A B 

UPPER SURFICIAL AQUIFER

MW40S 08/21/1996 8.2 1980.0 1980.5 Interference Interference 1983.0 1983.5 MW40S 08/21/1996 8.2 1980.0 1981.0 Interference Interference 1983.5 1984.0 Early 1980’sMW40S 08/21/1996 8.2 1980.0 1980.5 Interference Interference 1983.5 1984.0 DM102S 08/23/1996 11.1 1986.0 1983.5 1985.5 1983.0 1986.0 1984.5 DM102S 08/23/1996 11.1 1985.5 1983.0 1984.5 1982.0 1986.5 1985.5 Mid–1980'sDM102S 08/23/1996 11.1 1986.5 1983.5 1985.5 1982.5 1986.5 1985.5 DM342S 08/23/1996 13.3 1987.0 1982.0 1989.5 1984.5 1988.0 1986.0 DM342S 08/23/1996 13.3 1986.5 1981.5 1990.0 1985.0 1988.5 1986.0 Mid– to Late–1980'sDM342S 08/23/1996 13.3 1987.0 1982.5 1988.0 1983.5 1986.5 1984.5 DM367S 08/21/1996 12.4 1988.0 1984.5 1991.5 1986.5 1985.5 1984.0 

Mid–1980's to Modern

DM367S 08/21/1996 12.4	 1991.5 A 1986.0 Interference Interference 1987.0 1985.5 

 1997.0 

MWD4B 08/27/1996 8.9 1986.5 1986.5 Interference Interference 1986.5 1986.5 MWD4B 08/27/1996 8.9 1986.5 1986.5 Interference Interference 1987.0 1987.0 Mid–1980'sMWD4B 08/27/1996 8.9 1986.5 1986.5 Interference Interference 1987.0 1987.0 

37 
Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 

Ta

ble 5. Age of ground water from analysis of c

hlorofluorocarbons (

CFCs) in selecte

d w

ells at Dov

er Air Force 

B

ase, Delaware-­

Continued

 Estimated CFC recharge dates for each CFC 

Well Sampling T CFC–11 CF

C–12 CFC–113 Interpreted 

CFC age for 

no. date ( ο C) each well or comments 

A B A B A B 

LOWER SURFICIAL AQUIFER

M

W40D 08/21/1996 8.2 1975.5 1975.5 1972.5 1972.5 1954.5 1954.5 

M

W40D 08/21/1996 8.2 1975.0 1975.5 1972.5 1972.5 1954.5 1954.5 Mid–1970's

M

W40D 08/21/1996 8.2 1975.0 1975.5 1973.0 1973.0 1963.0 1963.0 

DM102D 08/23/1996 11.6 1974.0 1973.0 1974.5 1973.0 1974.5 1973.0 DM102D 08/23/1996 11.6 1974.0 1972.5 1975.0 1973.5 1974.0 1972.5 Mid–1970'sDM102D 08/23/1996 11.6 1974.0 1972.5 1975.5 1974.0 1973.0 1972.0 DM206D 08/28/1996 14.1 1972.0 1970.0 1977.0 1974.0 1975.5 1973.5 DM206D 08/28/1996 14.1 1972.0 1970.0 1977.5 1974.5 1971.5 1969.5 Early– to Mid–1970'sDM206D 08/28/1996 14.1 1972.0 1970.0 1978.5 1975.0 1978.0 1975.5 DM342D 08/23/1996 8.4 1985.0 1985.0 1990.0 1991.0 1986.5 1986.5 DM342D 08/23/1996 8.4 1984.5 1985.0 1990.0 1991.0 1986.5 1987.0 Mid– to Late–1980'sDM342D 08/23/1996 8.4 1983.5 1984.0 1988.0 1988.5 1986.0 1986.0 DM367D 08/21/1996 13.4 Interference Interference Interference Interference 1977.0 1975.0 No reliable date 

38 
Table 5. Age of ground water from analysis of chlorofluorocarbons (CFCs) in selected wells at Dover Air Force Base, Delaware-­

Continued

 Estimated CFC recharge dates for each CFC 

Hydrogeology 

Well Sampling T CFC–11 CFC–12 CFC–113 Interpreted CFC age for no. date ( ο C) each well or comments 

A B A B A B 

LOWER SURFICIAL AQUIFER--Continued 

DM405D 08/28/1996 12.1 1982.5 1978.5 1978.0 1975.5 1971.5 1970.0 DM405D 08/28/1996 12.1 1981.0 1977.5 1976.5 1974.5 1977.0 1975.5 Mid– to Late–1970'sDM405D 08/28/1996 12.1 1981.5 1978.0 1976.5 1974.5 1978.0 1976.5 DM412D 08/28/1996 13.9 Interference Interference Interference Interference Interference InterferenceDM412D 08/28/1996 13.9 Interference Interference Interference Interference 1954.5 1954.5 No reliable dateDM412D 08/28/1996 13.9 Interference Interference Interference Interference 1954.5 1954.5 MW206D 08/27/1996 8.4 1964.5 1964.5 1970.0 1970.5 1954.5 1954.5 MW206D 08/27/1996 8.4 1964.5 1964.5 1969.0 1969.5 1954.5 1954.5 Mid–1960's to early 1970'sMW206D 08/27/1996 8.4 1964.0 1964.5 1970.5 1971.0 1957.0 1957.0 

FREDERICA AQUIFER

DM378F 08/23/1996 12.6 1975.0 1973.5 1983.0 1979.5 1992.0 1988.5 

1997. 0 

Data are typical of a well that

DM378F 08/23/1996 12.6 1971.0 1969.5 1974.5 1973.0 1987.0 1985.5 was not completely purged.DM378F 08/23/1996 12.6 1964.5 1963.5 1966.5 1965.5 1987.0 1985.0 DM421F 08/27/1996 8.4 1964.5 1964.5 1967.0 1967.0 1982.0 1982.0DM421F 08/21/1996 8.4 1956.0 1956.0 1959.5 1960.0 1954.5 1954.5 --do-­DM421F 08/21/1996 8.4 1956.5 1956.5 1957.0 1957.0 1955.0 1955.0 

A Two dates are given because the CFC curve has peaked and the concentrations of CFC–11 correspond to two dates, which bound the peak in the curve. 

39 
0 0 0

Table 6. Water budget for the Dover Air Force Base area, Delaware 

[ft2, square feet; in/yr, inches per year; ft/d, feet per day; ft3/d, cubic feet per day] 

Recharge 

area 

(ft

2

) 

Average 

precipitation 

(in/yr) 

Average 

recharge 

(in/yr) 

Average 

overland 

flow and 

evapotranspiration 

(in/yr) 

Recharge 

(ft/d) 

Recharge 

into the 

investigation 

area 

(ft

3

/d) 

Discharge from 

the ground-water 

system to 

surface-water 

bodies 

(ft

3

/d) 

1.5 x 10 9 46 12 34 2.74 x 10-3 4.1 x 10 6 4.1 x 10 6 

Water Budget 

In an estimate of the water budget for the ground-water 

system in the DAFB area, recharge constituted flow into the 

ground-water system and ground-water discharge to surfacewater bodies constituted flow out of the system. In this 

water budget, the source of water is precipitation. Not all of 

this precipitation enters the ground-water system—some of 

the precipitation is lost to surface-water bodies by overland 

runoff, and another part of the precipitation is used by plants 

as transpiration. What is left of precipitation is available for 

recharge. Cushing, Kantrowitz, and Taylor (1973) studied 

these relations extensively for the Delmarva Peninsula and 

the analysis in this report follows their interpretation. About 

4.1 million cubic feet of water flow through the system in a 

day (table 6). The recharge area was estimated from a map 

of the area. No recharge was allowed into the major surfacewater bodies, such as the Delaware Bay, the St. Jones River, 

or the Little River, or for runways, shopping centers, and 

other paved areas. Seasonal effects of irrigation pumpage 

from the Frederica aquifer or from the surficial aquifer were 

ignored. Finally, the amount of recharge is assumed to equal 

the amount of discharge into the surface-water bodies. 

Simulation of Ground-Water Flow 

A ground-water-flow model was constructed to simulate 

the hydrogeology of the DAFB area. The design of the 

model was developed on established principles, which have 

been summarized in numerous publications (van der Heijde, 

1992; American Society for Testing and Materials 

Committee E 978–92, 1992; American Society for Testing 

and Materials Committee D 5447–93, 1993; American 

Society for Testing and Materials Committee D 5490–93, 

1994a; American Society for Testing and Materials 

Committee D 5609–94, 1994b; American Society for Testing 

and Materials Committee D 5611–94, 1994c; American 

Society for Testing and Materials Committee D 5610–94, 

1995; Anderson and Woessner, 1992). The simulation used 

the USGS modular three-dimensional ground-water-flow 

model known as MODFLOW (McDonald and Harbaugh, 

1988; Harbaugh and McDonald, 1996). 

Conceptual Model 

The numerical model is based on a conceptual model of 

ground-water flow that was developed on the basis of the 

hydrogeologic information presented earlier in this report. 

The major features of this conceptual model are shown in 

figure 23, which is a schematic diagram rather than a crosssection. The main source of water into the model is 

recharge. This water flows through the ground-water system 

to surface-water bodies. 

The conceptual model has four layers (fig. 23). The top 

two layers represent the surficial aquifer, with the uppermost 

layer representing the upper surficial aquifer. The hydraulic 

conductivity of the upper surficial aquifer is constant at 

5 ft/d except near large surface-water bodies, where there 

can be extensive areas of marsh sediments that have a lower 

hydraulic conductivity (0.1 ft/d). The surface-water bodies 

in this top layer are represented by the St. Jones River, the 

Delaware Bay, and an ephemeral drain. The river and the 

drain have a thin layer (1 ft thick) of streambed sediments 

with a horizontal hydraulic conductivity of 1 ft/d. This layer 

does not impede ground-water discharge to these surfacewater bodies. The marsh sediments can impede the groundwater discharge to a greater degree, while the flow of water 

to the drains is not impeded by sediment type. The Delaware 

Bay is represented in the model by a constant-head 

boundary, and there is no sediment layer in this 

representation. Below layer 1 is layer 2, the lower surficial 

aquifer, except in two areas where a semi-confining clay unit 

is present. These clay units are shown between the St. Jones 

River and the Delaware Bay. 

The second layer represents the lower surficial aquifer. 

The hydraulic conductivity of this aquifer is uniform at 

65 ft/d. The thickness of the lower surficial aquifer can vary. 

One area of exceptional thickness is under the upper reaches 

of the St. Jones River in the model area, where the river 

channel has eroded part of the upper Calvert confining unit. 

The question marks in figure 23 indicate that the exact nature 

of these sediments is unclear, but show that there is a 

thinning of the confining unit. Another area of thinning of 

the confining unit is shown to the right of the St. Jones River. 

This thinning represents the thinning seen locally under 

DAFB. 

40 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Simulation of Ground-Water Flow 41 
The third model layer represents the upper Calvert 

confining unit, which is between the lower surficial aquifer 

and the Frederica aquifer. There are two places where this 

layer does not represent this unit. These areas include the 

subcrop of the Frederica aquifer, and in the model area, the 

upper reaches of the St. Jones River. In the subcrop area of 

the Frederica aquifer, the upper Calvert confining unit is 

missing and in the model, it is given the properties of a thin 

layer (approximately 1 ft) of sand (K, or hydraulic 

conductivity = 50 ft/d). This subcrop area is shown on the 

left side of the diagram. The subcrop area extends down the 

upper reaches of St. Jones River Valley in the model area to 

the Wildcat Landfill area, where the upper Calvert confining 

unit is missing in the subsurface (CH2M Hill Southeast, Inc., 

1988b). The fourth layer of the conceptual model represents 

the Frederica aquifer, with a horizontal hydraulic 

conductivity of 25 ft/d. 

In recharge areas, the gradient is downward from the 

water table to the potentiometric surface for the lower 

surficial aquifer. In discharge areas, such as the St. Jones 

River or the Delaware Bay, this slight gradient is reversed. 

In areas where semi-confining clay units are present at the 

base of the upper surficial aquifer, there is an increase in the 

vertical gradient between the water table and the 

potentiometric surface for the lower surfical aqufier. 

Figure 23 also shows the potentiometric surface of the 

Frederica aquifer. In the left part of the figure, the head 

gradient is downward from the water table towards the 

potentiometric surface of the Frederica aquifer. This surface 

slopes down towards the Delaware Bay, where it is slightly 

above the average water level in the Bay. For most of this 

distance, the potentiometric surface of the Frederica is lower 

than the water table with one exception. This area is on the 

golf course, near the St. Jones River, where the water table 

drops to meet the average stage of the St. Jones River. As 

was discussed earlier in this report, near the St. Jones River 

the head in the lower surficial aquifer is lower than the head 

in the Frederica aquifer, except during dry periods, when 

water is pumped from the Frederica aquifer. A non-pumping 

condition is shown in figure 23. 

The assumed boundary conditions of the ground-water 

system are also shown in figure 23. For layer 1, the lateral 

boundaries are no-flow boundaries except for the Delaware 

Bay. For layers 2 and 3, all of the lateral boundaries are noflow boundaries. For layer 4, the Frederica aquifer, all of the 

lateral boundaries are constant-head, except for the subcrop 

area where it is a no-flow boundary. The Frederica aquifer is 

underlain by about 20 ft of silt that represents a no-flow 

boundary and constitutes the bottom of the model. 

Model Design and Boundary Conditions 

MODFLOW was designed to simulate ground-water 

flow in a multi-layer system using one of two approaches— 

the three-dimensional approach or the quasi-threedimensional approach. In the three-dimensional approach, 

aquifers and confining units are simulated as active layers, 

with hydraulic-head solutions for each layer. In the quasithree-dimensional approach, the aquifers are simulated as 

active layers, and the confining units are simulated as a 

conductance term between the layers. These two approaches 

were used in this model for the following reason. When the 

top of the clay was used as the base of the unconfined layer, 

many dry cells formed. Use of the "wet-dry" package of 

MODFLOW made the model unstable. Therefore, the upper 

and lower surficial aquifer were represented by two separate 

model layers, with the semi-confining unit between them 

represented by a vertical conductance term. The upper 

confining unit of the Calvert Formation was represented by a 

model layer, and the Frederica aquifer was represented by a 

layer. The top of the confining unit below the Frederica was 

simulated as the base of the model, and was represented in 

MODFLOW as a no-flow boundary. 

A finite-difference simulation of ground-water flow 

requires that the model area be discretized into a grid. The 

grid constructed for the DAFB area has 4 layers, 190 rows, 

and 194 columns (fig. 24). The model grid rows are oriented 

parallel to the northwest-southeast runway, which is the 

approximate location of one of the ground-water divides at 

DAFB, and roughly parallel to the reach of the St. Jones 

River that is closest to DAFB. 

Within the DAFB boundary, the model cells are 100 ft by 

100 ft (fig. 24). Outside of DAFB, the model cells increase 

in size because there is less need for detailed information and 

there are less data available. This increase in model-cell size 

is no greater than 1.5 times the adjacent model-cell size. 

Expansion of the grid spacing was necessary to reduce the 

computation time for the simulations, and an expansion ratio 

of 1.5:1 or less was required for a stable solution of the 

finite-difference equations. The largest model cells (those 

far outside the area of interest) are 5,280 ft by 6,320 ft. 

Initial values for the model grid were obtained by use of 

GIS techniques, many of which were described in Hinaman 

(1993). Values were assigned to grid nodes by assuming the 

value of the parameter at the center of the grid cell was 

uniform throughout the cell. 

As the MODFLOW model was assembled, the guiding 

premise was to avoid making the model any more complicated than necessary. Hydraulic conductivities of the 

layers and recharge were assumed uniform. After some 

initial simulations, further development of the model was 

guided by the simulation results. For example, the model 

was modified by adding recharge zones. It was assumed that 

no recharge takes place under paved areas (such as runways, 

tarmacs, parking lots, and large shopping areas in southern 

Dover, shown in fig. 2). In all other areas of the model a 

uniform recharge of 12 in/yr was simulated. Approximately 

8 in/yr is an estimate of the average recharge to land surfaces 

in the area of the simulation. An additional modification 

was made for the upper surficial aquifer, where the wetlands 

(fig. 2) along the St. Jones River and the Delaware Bay were 

assigned a horizontal hydraulic conductivity of 0.1 ft/d. This 

low conductivity zone was based on observations at Wildcat 

Landfill (CH2M Hill Southeast, Inc., 1988a, 1988b). In 

model layer 3, which represents the upper Calvert confining 

unit, a horizontal hydraulic conductivity of 50 ft/d was 

42 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Simulation of Ground-Water Flow 43 
assigned to the Frederica subcrop area and to the upper 

reaches of the St. Jones River in the model. In this area, the 

upper Calvert confining unit has eroded away and the lower 

surficial aquifer is lying directly on the Frederica aquifer. In 

the model, there is still a layer for the upper Calvert 

confining unit, and to simulate the lower surficial aquifer 

lying on the Frederica aquifer in the subcrop area the 

following was done: The model layer representing the upper 

Calvert confining unit was assigned a small thickness (1 ft), 

and was also assigned a hydraulic conductivity of 50 ft/d, 

which is between the hydraulic conductivity of the overlying 

lower surficial aquifer (65 ft/d) and the hydraulic 

conductivity of the Frederica aquifer (25 ft/d). 

Vertical hydraulic conductances were calculated by use 

of formulas from McDonald and Harbaugh (1988, p. 5–11 to 

p. 5–18). Vertical hydraulic conductances were calculated 

by use of arrays of the tops and bottoms of the hydrogeologic 

layers (aquifer or confining layer) and by use of arrays of the 

horizontal hydraulic conductivities between the following 

units: (1) the lower surficial aquifer and the upper Calvert 

confining layer, and (2) the upper Calvert confining layer 

and the Frederica aquifer. These calculations were made by 

use of a computer program, in which it was assumed that the 

vertical hydraulic conductivities were 10 percent of the 

horizontal hydraulic conductivities. The vertical hydraulic 

conductance between the upper and lower surficial aquifer 

was calculated in a different way, which accounted for the 

semi-confining unit between the two layers. This semiconfining unit represents a clay and silt sequence that is 

present in only one part of the surficial aquifer (fig. 5). The 

method used to map this sequence was discussed earlier in 

this report. Where the water table was below the clay and 

silt sequence, the sequence was ignored and an arbitrary 

conductance was assigned based on a thickness of 1 ft and a 

horizontal hydraulic conductivity of 50 ft/d. Where the 

water table intersected this clay and silt sequence, the 

thickness used in the calculations was the amount of clay and 

silt below the water table. In this area, the clay and silt 

sequence was assigned a horizontal hydraulic conductivity 

of 0.0001 ft/d. An array of vertical hydraulic conductance 

between the upper and lower surficial aquifer was calculated 

using arrays of the thicknesses of the upper surficial aquifer, 

the intervening clay and silt sequence, and the lower surficial 

aquifer, along with arrays of hydraulic conductivities of the 

upper surficial aquifer, the intervening clay and silt 

sequence, and the lower surficial aquifer. 

Boundary conditions for each of the model layers 

(figures 25, 26, 27, and 28) were set to provide a simulation 

that was sufficiently accurate. Where possible, boundary 

conditions were selected to mimic the natural hydrologic 

boundaries within the modeled area. In some cases, 

however, natural hydrologic boundaries do not exist or are 

difficult to define. In these cases, the boundary conditions 

were set far enough away from the principal areas of interest 

so that they would have minimal influence on simulation 

results for those areas. 

All or part of the top three model layers are represented 

by no-flow boundaries. The model area was designed to be 

large enough so that these distant boundaries would have a 

negligible effect on the flow field at DAFB. For the upper 

surficial aquifer, the no-flow boundaries are either at or very 

close to surface-water divides, which are assumed to 

coincide with ground-water divides for the surficial aquifer. 

In the upper surficial aquifer layer, the model cells that 

represent the Delaware Bay and the ponds at Tilcon also 

were assigned constant heads. For the Frederica aquifer 

layer, cells in the outer edge of the model were assigned 

constant heads, except those in the subcrop area, which were 

assigned variable heads, which represent active model cells 

(fig. 28). 

River nodes were used to simulate the St. Jones River 

and the tidal reaches of the Little River and the 

St. Jones River (figs. 29 and 30). River stages were 

calculated from actual measurements and from topographic 

maps. The streambed hydraulic conductivity was assumed 

to be 1 ft/d. Parts of the streambed are sandy and other parts 

are silty, but the detailed distribution of these sediments is 

not known. The 1 ft/d value for hydraulic conductivity of 

the streambed sediments was used as a compromise value for 

hydraulic conductivity. The sediments were assumed to be 

1 ft thick, and the channel was assumed to be 10 ft wide. As 

discussed earlier in this report, the hydraulic conductivity of 

the upper surficial aquifer, layer 1 of the model, was set to 

0.1 ft/d in marshy areas, which were identified on aerial 

photographs and topographic maps. In areas with marshes, 

the 0.1 ft/d horizontal hydraulic conductivity controls 

ground-water discharge to rivers. 

The “drain package” of MODFLOW was used to 

simulate open drains and ephemeral streams. Actual drains 

at DAFB are classified as open drains or covered drains. 

Covered drains were not simulated in the model. Sediments 

in the streambeds of the open drains and ephemeral streams 

were given a hydraulic conductivity of 1 ft/d. As was the 

case with the rivers, the beds of some of the drains are sand 

while others are silt. The distribution of these sediments was 

not mapped. Sediment thickness was assumed to be 1 ft, and 

the width of the channel was assumed to be 5 ft. In the area 

with marshes, the 0.1 ft/d horizontal hydraulic conductivity 

controls ground-water discharge to drains. 

Calibration of the Model 

The ground-water-flow model of the DAFB area was 

calibrated by systematically adjusting hydrologic parameters 

within the known range of measured values until simulated 

heads and flow were consistent with measured hydraulic 

heads and flow to within an acceptable error range. 

Calibration of the model ensures that it closely represents the 

actual head distribution in order to properly simulate heads 

and flow directions within the natural ground-water-flow 

system. Some details of the flow system will always be 

unknown, however, and the model, when calibrated, will 

represent some average of these unknowns. 

44 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Simulation of Ground-Water Flow 45 
46 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Simulation of Ground-Water Flow 47 
48 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Simulation of Ground-Water Flow 49 
50 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Calibration of the ground-water-flow model was 

considered complete when the following seven criteria were 

met: 

1.	 the root-mean-squared error (RMSE) between 

simulated heads and measured heads in the upper 

surficial aquifer was minimized; 

2.	 the RMSE between simulated heads and measured 

heads in the lower surficial aquifer was minimized; 

3.	 head gradients between the upper and lower surficial 

aquifer at selected well pairs were closely matched; 

4.	 head gradients between the lower surficial aquifer and 

the Frederica aquifer at selected well pairs were 

closely matched; 

5.	 the RMSE between simulated head and actual heads in 

the Frederica aquifer was minimized; 

6.	 simulated ground-water discharge to Pipe Elm Branch 

of the Little River closely matched the measured 

ground-water discharge; and 

7.	 the shape and magnitude of simulated off-Base 

contours of head for the lower surficial aquifer were 

consistent with the shape and magnitude of off-Base 

contours of head for the lower surficial aquifer 

measured in October 1959. 

Details of the calibration methods and the degree to which 

the objectives were met are discussed below. 

Calibration of this model was a highly iterative process 

that used a trial-and-error method with sensitivity analyses 

as a guide. The objective was to construct an initial groundwater-flow model with appropriate boundary conditions, 

layering, node spacing, and estimated values of the 

hydrologic parameters. The following hydrologic 

parameters were adjusted during calibration: horizontal 

hydraulic conductivity of the upper surficial aquifer, 

horizontal hydraulic conductivity of the lower surficial 

aquifer, horizontal hydraulic conductivity of the semiconfining unit in the surficial aquifer, horizontal hydraulic 

conductivity of the Calvert Formation confining unit, 

horizontal hydraulic conductivity of the Frederica aquifer, 

and recharge rate. Reasonable ranges for these parameters 

were determined from available data or from literature 

sources. The hydrologic parameters were then adjusted one 

at a time to determine values that produced the simulation 

that most carefully represented flow and head conditions in 

the ground-water-flow system at DAFB. 

The model had several calibration targets. One target 

was to produce simulated heads that were consistent with 

observed heads of September 1997, with an RMSE of less 

than 10 percent of the range of observed heads for the upper 

and lower surficial aquifer. Another objective was to have 

less than a 10-percent error between the observed 

September 1997 base-flow discharge from the aquifer to 

Pipe Elm Branch and the simulated discharge to Pipe Elm 

Branch. The hydraulic properties were determined from the 

quantitative calibration. This calibration was based on 

observed data from DAFB. To determine if the model was 

valid outside the boundaries of DAFB, the model was 

compared to October 1959 heads, for which there are 

regional data for both head and interpreted contours. 

The horizontal hydraulic conductivity of the upper part 

of the surficial aquifer was adjusted between 1 ft/d and 

50 ft/d. The horizontal hydraulic conductivity of the clay in 

the surficial aquifer was adjusted between 0.001 ft/d and 

0.00001 ft/d. The horizontal hydraulic conductivity of the 

lower part of the surficial aquifer was adjusted over a range 

of 50 ft/d to 200 ft/d. For each of the hydraulic 

conductivities of the lower surficial aquifer, recharge to the 

model was adjusted between 4 in/yr and 20 in/yr. 

The next step was to calibrate the hydraulic conductivity 

of the confining unit in the upper Calvert Formation. The 

calibration target was to closely match the head in the 

Frederica aquifer. During these analyses, the values of 

recharge and horizontal hydraulic conductivity of the 

surficial aquifer were held constant, and the horizontal 

hydraulic conductivity of the confining unit was adjusted 

from values just above to below values reported in the 

previously published literature. The heads in the Frederica 

aquifer were compared to measured values of September 

1997, and an RMSE was calculated. 

Figure 31 shows scatter plots of calculated and observed 

heads for the upper and lower surficial aquifers for 

September 1997. For these layers, the agreement is good 

throughout the range of values. For September 1997 

conditions, the RMSE of the upper and lower surficial 

aquifers were 1.47 ft and 1.10 ft, respectively. The 

percentage of the total range of observed heads was 

9.4 percent of the range of 2.48 ft to 18.12 ft and 8.6 percent 

of the range of 3.02 ft to 14.96 ft, respectively. The RMSE 

for the Frederica aquifer was not as good at the value of 

1.57 ft, which represents an error of 53 percent, however, 

there were only four observation points and the total range in 

head was only 2.95 ft. 

The model was calibrated against observed ground-water 

discharge to Pipe Elm Branch because ground-water 

discharge to this stream is much greater than that to the other 

surface-water bodies and because ground-water discharge in 

Pipe Elm Branch is derived from a larger part of the Base 

than any other stream or drainage ditch (table 7). The 

difference between simulated ground-water discharge and 

observed ground-water discharge for September 1997 for all 

of the reaches is given in table 7. The error difference 

between measured and simulated ground-water discharge to 

Pipe Elm Branch is about 3 percent, which is acceptable 

because it is below the 10-percent criteria for acceptable 

error in ground-water discharge. Some of the other errors 

are quite large, such as the simulated ground-water discharge 

to Morgan Branch, which in actuality had no ground-water 

discharge. But the amount of ground-water discharge is 

small. The error of all of the measured ground-water 

discharge compared to the simulated ground-water discharge 

is 5 percent, which is less than the 10-percent error criteria 

for an acceptable model calibration. 

Simulation of Ground-Water Flow 51 
52 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Table 7. Observed and simulated ground-water discharge to stream reaches in the 

Dover Air Force Base area, Delaware 

[nc, not calculated because of a division by zero] 

Stream or Observed flow Simulated Absolute

drainage reach Sept. 16, 1997 flow difference

(cubic feet per day) (cubic feet per day) (in percent) 

SW1 5,184 8,005 54 

Morgan Branch 0 480 nc 

Pipe Elm Branch 39,744 38,567 3 

Sand Ditch 0 0 nc 

Upper Golf Course Tributary 864 1,128 31 

Total 45,792 48,180 5

 Table 8. Water budget for simulation of ground-water flow in the Dover Air Force Base area, Delaware

 [ft

2

, square feet; in/yr, inches per year; ft/d, feet per day; ft

3

/d, cubic feet per day; %, percent] 

Average Recharge Discharge overland into the from the 

Type Recharge Average Average flow and investigation ground-water system to 

area 

(ft

2

) 

precipitation 

(in/yr) 

recharge 

(in/yr) 

evapotranspiration 

(in/yr) 

Recharge 

(ft/d) 

area 

(ft

3

/d) 

surface-water bodies 

(ft

3

/d) 

-3 Estimated from map 1.5E + 09 46 12 34 2.74 x 10 4,091,292 4,091,292 

-3 Simulated – – 12 – 2.74 x 10 4,004,603 4,148,392 

Error – – – – – 86,689 -57,100 

Percent error – – – – – 2% -1% 

Regionally, for the lower surficial aquifer, contours of 

head for both the average observed head and the simulated 

head show generally good agreement with the general shape 

of the potentiometric surface of the lower surficial aquifer 

(fig. 32) and the upper surficial aquifer (fig. 33). A similar 

relation is also seen between contours of observed head in 

the Frederica aquifer and simulated heads in the Frederica 

aquifer (fig. 34). 

Simulated Water Budget 

The simulated water budget for the DAFB area is 

consistent with the estimated water budget (table 8). 

Simulated recharge into the system, however, does not 

exactly match the discharge. Table 8 does not show two 

other sources of water into the system in the simulation: One 

is a modest flow, 34,295 ft3

/d (cubic feet per day), from river 

cells into the ground-water system. Another source of flow 

is from constant-head cells where there is some minor flow, 

109,485 ft

3

/d, into the system. Overall, these two sources 

contribute a 3-percent error of flow into the system. 

Sensitivity of the Model 

A sensitivity analysis was used to help calibrate the 

model. The purpose of this analysis was to determine how 

sensitive the model, or, in other words, the results of the 

simulation, were to changes in each input parameter. If a 

small change in an input parameter resulted in a large change 

in either calculated head or ground-water discharge, then the 

model was considered sensitive to that parameter. 

Conversely, if a large change in the parameter resulted in 

only a small change in either calculated head or calculated 

ground-water discharge, then the model was less sensitive to 

that parameter. The sensitivity analysis was performed on 

the initial model prior to calibration. Thus, in some 

instances, an increase or decrease of the tested parameter 

resulted in an improved fit. 

It should be noted that the sensitivity analysis described 

above assumes no changes in the parameters not being 

tested. If two parameters were tested at the same time, 

however, the sensitivity results could be different. For 

example, if, as recharge was increased, the hydraulic 

conductivity was increased at the same corresponding rate, 

Simulation of Ground-Water Flow 53 
54 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Simulation of Ground-Water Flow 55 
56 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
then the net change in the head distribution would be 

negligible. In this case, although the change in head 

distribution may be negligible, the change in ground-water 

discharge would not be negligible. These linkages between 

model parameters, head distribution, and ground-water 

discharge are the motivation behind measuring ground-water 

discharge. Even with this linkage of parameters and results, 

the sensitivity analysis is important in assessing the 

significance of each of the model-input parameters in order 

to obtain a reasonable calibration. 

In the following discussion, it should be noted that a 

change in the horizontal hydraulic conductivity means that 

the vertical conductance between the two layers was also 

changed. As discussed earlier in this report, it is assumed 

that the vertical hydraulic conductivity is 10 percent of the 

horizontal hydraulic conductivity. Thus, if the horizontal 

hydraulic conductivity changes, the vertical hydraulic 

conductivity changes, and there is a resulting change to the 

calculated conductance between the two layers. In the 

following sensitivity analysis, the vertical hydraulic 

conductivity was not studied separately. Therefore, the use 

of the term "horizontal hydraulic conductivity" includes the 

effects of the change to the conductance between the two 

layers. 

The simulated heads in the upper surficial aquifer are 

most sensitive to recharge and the horizontal hydraulic 

conductivity of the underlying lower surficial aquifer (fig. 

35). Following these two parameters, the simulated heads in 

the upper surficial aquifer are moderately sensitive to the 

horizontal hydraulic conductivity of the upper surficial 

aquifer. Finally, they are least sensitive to the horizontal 

hydraulic conductivity of the clays in the surficial 

aquifer. 

For the lower surficial aquifer, the simulated heads are 

almost equally sensitive to changes in recharge and to the 

horizontal hydraulic conductivity of the lower surficial 

aquifer (fig. 36). Next, the heads are sensitive to the 

horizontal hydraulic conductivity of the upper surficial 

aquifer. Following these parameters, they are almost equally 

sensitive to the horizontal hydraulic conductivity of the 

Frederica aquifer and to the horizontal hydraulic 

conductivity of the upper Calvert Formation. They are least 

sensitive to the horizontal hydraulic conductivity of the clays 

in the surficial aquifer. 

Simulated heads in the Frederica aquifer are most 

sensitive to changes in the horizontal hydraulic conductivity 

of the overlying confining unit, are sensitive to recharge (fig. 

37), and are sensitive to the horizontal hydraulic 

conductivity of the Frederica aquifer. They are moderately 

sensitive to the horizontal hydraulic conductivity of the 

upper surficial aquifer and to the horizontal hydraulic 

conductivity of the lower surficial aquifer. The heads in this 

aquifer are least sensitive to the horizontal hydraulic 

conductivity of the clays in the surficial aquifer. Although 

the error decreases as the horizontal hydraulic conductivity 

of the Frederica aquifer is increased to a very high value, this 

high horizontal hydraulic conductivity was not used because 

horizontal hydraulic conductivities in the range of 250 ft/d 

are not supported by any measured values of the horizontal 

hydraulic conductivity. Similarly, the error decreases as the 

horizontal hydraulic conductivity decreases to 2.5 ft/d, but 

simulated heads for normal recharge conditions are too high. 

The low error on the sensitivity plot for other horizontal 

hydraulic conductivities of the Frederica aquifer may be due 

to under-sampling of the potentiometric field by the four 

measured values of Frederica aquifer head. 

Ground-water discharge in the model is most sensitive to 

recharge and is moderately sensitive to the horizontal 

hydraulic conductivity of the lower surficial aquifer (fig. 38). 

It is least sensitive to the horizontal hydraulic conductivity of 

the upper Calvert Formation, the horizontal hydraulic 

conductivity of the upper surficial aquifer, and the horizontal 

hydraulic conductivity of the Frederica aquifer. 

Comparison of Simulated Flow Paths and Ground-Water 

Recharge Dates 

The MODPATH-PLOT program (Pollock, 1994), which 

takes output from MODFLOW and calculates pathlines and 

traveltimes of ground-water particles, was used to determine 

the recharge age of ground water along a flow path in a given 

simulation. The simulated ages were compared to the 

recharge ages determined from analyses of CFCs and 

tritium. The comparison was done by tracing water particles 

in the model from wells, in which recharge dates had been 

determined, to their recharge locations and then calculating 

the amount of time necessary for a ground-water particle to 

have traveled from the top of the water table to the wells. 

The resulting simulated age should be approximately the 

same as the CFCs and tritium-recharge dates, which are the 

approximate date that the water in the sample became 

isolated from the atmosphere. 

Several factors can affect the ground-water recharge 

dates calculated in particle-tracking simulations. The most 

significant is the rate of ground-water movement, which is a 

function of hydraulic conductivity, hydraulic gradient, and 

porosity. Another factor is the depth and location at which 

ground-water particles are placed within the model cell. For 

the simulations in this investigation, a single particle was 

placed in the geographical center of the cell nearest the well 

from which CFC and tritium samples were collected. Initial 

particle depth within the layer coincided with the screen 

elevation of the well. 

For most wells, the agreement between CFCs and 

particle-tracking recharge dates is considered reasonable 

(table 9). In six wells, recharge dates from the simulations 

are within 10 years of the CFC recharge dates. In other wells 

where agreement is not as good, the assumption of steadystate conditions may have affected the results. Most of the 

wells with the larger errors are in areas where there may be 

transient effects due to expanding and contracting groundwater highs, described in the “Surficial Aquifer” section of 

this report. To show this effect, the model was run with a 

recharge of 6 in/yr rather than the 12 in/yr used for the other 

model runs. At well MWD4B, this change had a dramatic 

effect, where the simulated age changed from 51 years for a 

Simulation of Ground-Water Flow 57 
58 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Simulation of Ground-Water Flow 59 
60 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Simulation of Ground-Water Flow 61 
H

ydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 

Table 9. Comparison of chlorofluorocarbons (CFCs) recharge dates to recharge dates from the

 ground-water particle-tracking simulations at Dover Air Force Base, Delaware

[No wells were screened in the upper Calvert Formation and semi-confining unit. The following porosities were used in the MODPATH steady-state flow model—0.3 for the upper surficial aquifer, 0.3 for the lower surficial aquifer, 0.4 for the upper Calvert Formation confining unit, 0.4 for the semi-confining unit, and 0.3 for the Frederica aquifer.A, simulated recharge age; B, traveltime in years; C, error between simulated recharge age and CFC recharge age]

Wellno. 

CFC rechargedate 

CFC traveltime(years) 

GROUND-WATER PARTICLE-TRACKING SIMULATIONS

 Recharge rate = 12 inches per year Recharge rate = 6 inches per year

 Bottom of screen Level of pump Top of screen Level of pump 

A B C A B C A B C A B C 

UPPER SURFICIAL AQUIFER

DM102S 1983 13.5 1993 4.0 10 1993 3.5 10 1996 0.5 13 1992 4.7 9 DM342S 1985 11.5 1992 5.0 7 1995 1.5 10 1995 2.0 10 1994 3.0 9 DM367S 1989 7.5 1996 0.3 7 1996 0.5 7 1996 0.5 7 1996 1.0 7 MW40S 1982 14.5 1993 4.0 11 1994 3.0 12 1994 2.5 12 1992 4.5 10 MWD4B 1985 11.5 1945 51.5 -40 1945 51.5 -40 1945 51.5 -40 1991 5.5 6 

LOWER SURFICIAL AQUIFER

DM102D 1972 24.5 1951 46.0 -21 1974 22.5 2 1974 22.5 2 1955 42.0 -17 DM206D 1970 26.5 1968 28.5 -2 1986 10.5 16 1986 10.5 16 1966 30.5 -4 DM342D 1987 9.5 1976 20.5 -11 1979 18.0 -8 1990 6.5 3 1977 20.0 -10 DM405D 1975 21.5 1934 63.0 -41 1962 34.5 -13 1969 27.5 -6 1923 74.0 -52 MW206D 1970 26.5 1935 61.5 -35 1944 53.0 -26 1944 53.0 -26 1951 45.5 -19 MW40D 1975 21.5 1967 29.5 -8 1967 30.0 -8 1967 30.0 -8 1959 38.0 -16 

62 
recharge rate of 12 in/yr to about 5 years for a recharge rate 

of 6 in/yr. This example shows that for some parts of the 

Base, a transient model would simulate ground-water-flow 

pathways and traveltimes more accurately than does the 

steady-state model used here. 

Results of Selected Particle-Tracking Analyses 

This model can be used for advective-transport analysis 

at individual sites at DAFB. This section of the report 

presents forward-tracking analyses for three remediation 

areas at DAFB. The main objective of these analyses is to 

determine the ground-water-flow pathlines and traveltimes 

of water particles traveling in specific areas of DAFB. The 

results of the following particle-tracking analyses are based 

on average steady-state conditions and generalized aquifer 

characteristics. Transient conditions and local variations in 

aquifer characteristics at specific sites could significantly 

affect these results. 

Natural Attenuation Project Area The Natural 

Attenuation Project Area (fig. 1c) is an area in which natural 

attenuation was investigated as a remedial option for 

contaminated ground water. Bachman and others (1998) 

used a one-dimensional reactive solute-transport model to 

assess the breakdown of chemical contaminants along a flow 

path. Because the model was one-dimensional, questions 

were raised as to whether the simulated ground-water-flow 

path matched the actual flow path in the investigation area. 

In addition, it was not known whether some of the flow paths 

in the area extended beyond the DAFB boundary. As a 

consequence, it was apparent that there was a need for 

additional monitoring wells in the area, and it was 

anticipated that particle tracking could be used to suggest 

well placement. 

Particle-tracking results show the effects of local groundwater highs (fig. 39) at a landfill, LF13. The water particles 

initially flow radially away from the local high, and then 

enter the lower part of the surficial aquifer. The groundwater particles change flow direction where the hydraulic 

gradient of the lower surficial aquifer is different from that 

of the upper surficial aquifer. All four ground-water 

particles placed within LF13 begin their flow paths in a 

radial pattern away from the ground-water high in the upper 

surficial aquifer. Once they move downward into the lower 

surficial aquifer, however, they all move toward 

Pipe Elm Branch. This suggests that contaminated ground 

water in LF13 does not migrate toward the adjacent Base 

boundary. Under the steady-state conditions of the model, 

water recharging at LF13 can take from 10 to 50 years to 

reach Pipe Elm Branch. 

Similarly, water particles placed in the WP14/LF15 

areas, which are former locations of a liquid-waste disposal 

pit and landfill, also move toward Pipe Elm Branch and 

away from the Base boundary (fig. 40). Traveltimes are 

generally less than 5 years at these sites. 

Long-Term Monitoring at OT-40 The ground-waterflow simulation can also be used to assist in the 

implementation of long-term monitoring at specific sites. 

One of these sites is OT-40, an oil-water separator, where 

long-term monitoring will be implemented (fig. 1c). Particle 

tracking was used to determine ground-water-flow paths 

from this site. Monitor wells were installed along these 

paths. 

In this analysis, the ground-water particles were added at 

the top of the water table. It takes about 4 years for the 

particles to move through the upper surficial aquifer and 

enter the lower surficial aquifer (fig. 41). Once the particles 

enter the lower surficial aquifer, they move at a faster rate, as 

is seen by the distance between the boxes in figure 41. The 

particles take about 9 years to reach the Base boundary, 

which is not shown on figure 41. The final discharge 

location for these particles is the St. Jones River. 

Contaminant Plume at Area 6 Area 6 (fig. 1c) is the 

location of a large ground-water contaminant plume 

consisting mainly of chlorinated solvents (U.S. Army Corps 

of Engineers and Dames & Moore, 1994). Particle tracking 

was used to define ground-water-flow paths in this area. The 

pathways (fig. 42a) show that the flow diverges as it moves 

under Base housing towards the St. Jones River. At the left 

side of figure 42a, particles take about 10 years to reach the 

St. Jones River, while near the center of the figure, some of 

the particles would take about 25 years to reach the St. Jones 

River, while other particles, which enter into layer 3 and 

layer 4 of the model, take much longer to reach the St. Jones 

River. On the right side of the figure, the particles flow to 

the golf course tributary that crosses under Route 113 and 

then move towards the St. Jones River. 

Figure 42a also shows ground-water particles that enter 

the confining unit and then the Frederica aquifer. Figure 

42 b-e shows the flow in each model layer. Most of the 

particles released at the top of the water table flow vertically 

down through layer 1 and into layer 2 (the lower surficial 

aquifer) (fig. 42b). In layer 2, the particles flow mainly 

horizontally (fig. 42c). Some of the particles reach the 

St. Jones River or the Golf Course Tributary (locations given 

in figure 42a and figure 1c) without entering lower layers in 

the model. Other particles enter layer 3, which is the upper 

Calvert confining unit (fig. 42d). Except for the left most 

part of the figure, the particles travel almost totally 

vertically. What is not shown well on this figure is that for 

this vertical flow, many boxes are superimposed. In the left 

most part of the figure, one of the particles in this layer has a 

large horizontal component in its flow. This area is where 

layer 3 has a high hydraulic conductivity and it is thought 

that in this area the sediments of the upper Calvert confining 

unit are missing. Figure 42e shows the flow in layer 4, 

which represents the Frederica aquifer. This flow has a large 

horizontal component. The distance between boxes is less 

than the distance between boxes for layer 2 (fig. 42c) 

because the horizontal gradient is less. In cross section (not 

shown), the flow in this layer would start to have a downward component then, as it approached the St. Jones River, it 

would have an upward component of flow. Finally, the 

particles would enter layer 3 and go up towards the St. Jones 

River. This flow is in response to the vertical head gradient. 

Simulation of Ground-Water Flow 63 
64 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Near the top of the figure, the vertical head gradient is from 

the surficial aquifer towards the Frederica aquifer. This 

direction of the vertical head gradient is maintained until 

near the St. Jones River, where the vertical head gradient 

reverses with flow going from the Frederica aquifer upward 

towards the St. Jones River. 

Selected Limitations of the Ground-Water-Flow Model 

The numerical model described in this report has 

limitations that could produce misleading results if not taken 

into account. Future users of this model need to recognize 

common limitations of ground-water-flow models in general 

and of this model in particular. 

Model-simulation results are sensitive to stresses on the 

ground-water-flow system. If new, large stresses are added 

in the DAFB area, such as pumping from new wells in the 

surficial aquifer, then the model should be recalibrated. 

Franke and Reilly (1987, p. 11) discuss this issue for groundwater models in general. 

Appropriate applications of the model are constrained by 

the cell size of the model grid. An inappropriate application 

of the model would be to put a small discharge well in a 

large model cell, the effect of which is to create a weak sink, 

which occurs when the well does not discharge at a rate that 

consumes all of the water entering the cell (Pollock, 1994). 

A weak sink can allow particles to flow past an area where 

they should be captured by a pumping well. Weak sinks are 

caused by using a spatial discretization that is too coarse 

(Pollock, 1994). Determining the appropriate discretization 

before the model is assembled can be a difficult decision. 

Once the grid discretization is selected, the ZONEBUDGET 

(Harbaugh, 1990) or MODPATH (Pollock, 1994) codes 

could be used to determine the flow to and from a model 

cell. Flow to or from the model cell could subsequently be 

compared to the discharge of the well within the cell. If the 

well discharge is smaller than the flow to or from the model 

cell, then that model cell is a weak sink and the cell size 

should be reduced. 

The particle-tracking simulations presented in this report 

represent advective transport—that is, they do not take into 

account the decay, retardation, or dispersion of manmade 

chemicals in the ground-water-flow system. These chemical 

and mechanical processes are not simulated by the particletracking routines, so that actual chemical transport may be 

Simulation of Ground-Water Flow 65 
faster or slower than the advective-transport predictions. 

The particle-tracking results, therefore, give only the average 

distance a particle of water would travel in the ground-waterflow system over a given time period, and should be 

considered approximate. Because certain aspects of the 

ground-water-flow system, such as the variable distribution 

of hydraulic conductivity, are not fully characterized or 

represented in the model, the model should not be used to 

provide answers to questions that depend upon the details of 

the simulated heads and pathlines. Simulation results of an 

adequately calibrated model should generally provide a good 

approximation of heads and flow direction in a ground-water 

system, but the complexity of the these systems means that 

complete accuracy is impossible. Some of this uncertainty is 

indicated by discrepancies between calculated and observed 

results. Limitations of the use of ground-water-flow models 

are discussed in Konikow (1988a, 1988b), Konikow and 

Bredehoeft (1992), and Bredehoeft and Konikow 

(1993). 

66 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Simulation of Ground-Water Flow 67 
68 Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 
Summary 

Dover Air Force Base is underlain by unconsolidated 

sediments of the Atlantic Coastal Plain. The uppermost sand 

unit in the Calvert Formation at Dover Air Force Base is the 

Frederica aquifer, which is the deepest unit investigated in 

this report. A confining unit of clayey silt in the upper 

Calvert Formation separates the Frederica aquifer from the 

lower surficial aquifer, which is the basal Columbia 

Formation. North and northwest of Dover Air Force Base, 

the Frederica aquifer subcrops beneath the Columbia 

Formation and the upper Calvert Formation confining unit is 

absent. The Calvert Formation dips to the southeast. The 

Columbia Formation consists predominately of sands, silts, 

and gravels, although in places clay layers separate the 

surficial aquifer into an upper and lower surficial aquifer. 

The shape of the water table at Dover Air Force Base is 

controlled by the amount of recharge and the distance to 

discharge areas. The main ground-water discharge areas are 

the Little River, the St. Jones River, and the Delaware Bay. 

If there were no secondary drainage, then the water table 

would form a broad high centered near the Base; however, 

there are several secondary discharge areas, such as Pipe 

Elm Branch, Morgan Branch, and the Golf Course Tributary. 

These secondary discharge areas create a series of groundwater highs and divides in the area of the Base, two of which 

are termed, in this report, the Northwest Runway Divide and 

the Southeast Runway High. The potentiometric surface of 

the Frederica aquifer also forms a broad high, which is 

centered on Dover Air Force Base. 

Long-term water-level hydrographs show changes in 

both seasonal and annual ground-water recharge at Dover 

Air Force Base. These variations in recharge are related to 

temporal changes in evaporation, plant transpiration, and 

precipitation. Differences in water levels in paired wells 

finished in different depths in the surficial aquifer indicate 

areas where extensive silts and clays are present in the 

surficial aquifer. In these areas, a vertical head difference as 

large as several feet may occur between water levels in wells 

screened above and below the clays, and local ground-water 

highs typically form during normal recharge conditions. 

When drought conditions persist, water drains off these 

highs, and the vertical gradients decrease. At the south end 

of Dover Air Force Base, hydrographs of water levels in 

wells completed in the Frederica aquifer show that off-Base 

pumping for irrigation of crops can cause the potentiometric 

surface to decline below sea level during part of the year. 

During the time of the year with no irrigation (and no 

pumping), flow is from the surficial aquifer towards the 

Frederica aquifer, except near the St. Jones River, where 

heads in the Frederica aquifer are above the heads in the 

lower surficial aquifer. In this area, flow is from the 

Frederica aquifer towards the lower surficial aquifer. 

A 4-layer, steady-state numerical model of ground-water 

flow was assembled for Dover Air Force Base and the 

surrounding area. The upper two model layers represent the 

upper and lower surficial aquifers, which are in the 

Columbia Formation. In some areas of the model, a semiconfining unit is used to represent the intermittent clay layer 

between the upper and lower surficial aquifer. This semiconfining unit causes the local ground-water highs in the 

surficial aquifer. The third model layer represents the upper 

Calvert Formation confining unit. The fourth model layer 

represents the Frederica aquifer. The model was calibrated 

to hydraulic heads and to ground-water discharge in 

Pipe Elm Branch, both measured in September 1997. For 

the calibrated model, the root-mean-squared errors for the 

hydraulic heads and the ground-water discharge in the 

Pipe Elm Branch were 9 percent of the range of head and 

3 percent of discharge, respectively. Heads simulated by use 

of the model were consistent with a map based on field 

measurements that shows average water levels in the region. 

The MODPATH particle-tracking program was used to 

simulate ground-water-flow directions for several areas on 

the Base. At LF13, one of the Base landfills, this analysis 

showed the effects of the local ground-water highs. In this 

area, ground water can flow from the highs and then 

dramatically change flow direction as it enters the lower 

surficial aquifer. At Area 6, the pathlines show a divergence 

of flow, with flow going from the Base towards the St. Jones 

River and towards the Golf Course Tributary. The pathline 

analysis also shows that ground water flows from the lower 

surficial aquifer, through the upper Calvert confining unit, 

through the Frederica aquifer, then back up through the 

upper Calvert confining unit and then to the lower surficial 

aquifer. 

The steady-state model has several limitations. The 

entire ground-water system is under transient conditions, due 

mainly to seasonal and yearly changes in recharge and 

withdrawal from irrigation wells. This steady-state model is 

still an effective tool, however, for understanding the 

ground-water-flow system underlying the Base for average 

conditions. If the ground-water system is subjected to new 

stresses, such as pumping from new wells at or near the Base 

in the surficial aquifer or in the Frederica aquifer, then the 

model should be verified for these conditions and, if 

necessary, recalibrated. Nevertheless, the model can be used 

to determine ground-water-flow pathlines in areas of the 

Base where the flow directions are constant. In addition, the 

steady-state model is a necessary step in the development of 

transient models and solute-transport models, which are 

planned for future ground-water monitoring on the Base. 

Summary 69 
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