Hydrogeologic Framework and Simulation of Ground-Water Flow and Travel
Time in the Shallow Aquifer System in the Area of Naval Support Activity
Memphis, Millington, Tennessee
INTRODUCTION
Naval Support Activity
(NSA) Memphis, formerly Naval Air Station Memphis, is a Department of the
Navy (Navy) facility located at Millington, Tennessee (
fig.
1). NSA Memphis encompasses about 3,490 acres (Kingsbury and Carmichael,
1995) and is divided into northern and southern complexes by Navy Road
(
fig. 2). Major operational areas
include an airfield, former training facilities, and a hospital in the
"Northside" area, and former housing and training facilities in the "Southside"
area. The Northside area is undergoing transfer to the city of Millington,
and the Southside is being realigned to become the site of the Navy's Bureau
of Personnel under the Base Closure and Realignment Act (BRAC) of 1990.
Past operations at NSA Memphis have contaminated
the soil, shallow ground water, and surface water locally. Sixty-seven
Solid Waste Management Units (SWMU's) and one Area of Concern (AOC) have
been identified at the facility. The SWMU's and AOC are under investigation
as part of the Resource Conservation and Recovery Act (RCRA) Corrective
Action Program. The objective of the Corrective Action Program is to obtain
information to fully characterize the nature and extent of the contaminants
and determine appropriate corrective measures. As part of a cooperative
investigation with the Navy at NSA Memphis, the U.S. Geological Survey
(USGS) and EnSafe (formerly EnSafe/Allen and Hoshall), Memphis, Tennessee,
have collected environmental data at many of the SWMU's and the AOC, including
13 SWMU's (fig. 2), requiring RCRA
Facility Investigations under the Corrective Action Program (Carmichael
and others, 1997).
The Navy seeks to determine if contaminants
in the shallow ground-water system may move through the subsurface or into
nearby creeks, reaching other parts of NSA Memphis or to off-base property.
As part of the U.S. Department of Defense Installation Restoration Program,
the Navy is considering remedial-action options to control the movement
of contaminants at NSA Memphis. Numerical simulation of ground-water-flow
systems is a quick and cost effective way to evaluate the potential for
migration of contaminants through the subsurface, into surface-water drainages,
or into sources of public water supply. Thus, in 1995, the USGS began constructing
a numerical ground-water-flow model of the shallow aquifer system beneath
NSA Memphis and the surrounding area as part of the cooperative hydrogeological
investigation with the Navy.
This report presents the
results of a study conducted from 1995 through 1997 to simulate ground-water
flow over an area of about 30 square miles (mi
2) that includes
all of the Southside and most of the Northside areas of NSA Memphis (
fig.
3). Data presented in this report include ground-water level measurements,
the results of test drilling, borehole geophysical surveys, sediment-core
analyses, and an aquifer and well specific-capacity test. A description
of the development and calibration of a numerical model used to simulate
the flow of ground water at NSA Memphis is presented. The calibrated numerical
model and an advective-flow particle-tracking program were used to estimate
ground-water-flow direction, time-of-travel, and to evaluate the potential
for migration of contaminants.
The study was organized
into three phases: (1) development of a conceptual model of the ground-water
system, (2) development and calibration of a numerical model of flow in
the shallow ground-water system, and (3) particle-tracking analyses. Borehole
geophysical surveys, lithologic logs, and sediment cores were used to correlate
hydrogeologic units across the study area. Two synoptic water-level measurement
surveys were used to determine seasonal high, low, and mean water levels
in the shallow aquifers. The results of sediment-core analyses, the aquifer
and well specific-capacity tests, and a parameter-estimation program were
used to estimate the hydraulic characteristics of the hydrogeologic units.
A steady-state, numerical model of the shallow ground-water-flow system
was calibrated to mean ground-water levels based on the data collected
during the two synoptic water-level measurement surveys. The calibrated
numerical model and an advective-flow particle-tracking program were used
to simulate ground-water-flow direction and time-of-travel. The potential
for contaminants in the ground-water system to migrate to sources of public
water supply or to surface-water drainages was evaluated with the calibrated
numerical model and the particle-tracking program.
The geology and hydrology
of the study area and surroundings have been described in numerous reports,
including those by Graham and Parks (1986), Brahana and others (1987),
Parks (1990), Parks and Carmichael (1990a, b), Kingsbury and Carmichael
(1995), and Carmichael and others (1997). Extensive lists of other selected
references are given by Graham and Parks (1986) and Brahana and others
(1987). Carmichael and others (1997) present the hydrogeology and ground-water
quality at NSA Memphis and summarize the post-Midway Group geologic units
underlying the facility and their hydrologic significance. Maps showing
the potentiometric surfaces of the Memphis and Fort Pillow aquifers in
1995 were published by Kingsbury (1996). A constant-withdrawal aquifer
test referenced in this study was analyzed using the computer model VS2DT
(Lappala and others, 1987; Healy, 1990). The U.S. Army Corps of Engineers
(1989a, b) published the results of a study to alleviate urban flooding
in the Millington area. McDonald and Harbaugh (1988) document the USGS
modular ground-water-flow model (MODFLOW) used to simulate the shallow
ground-water-flow system at NSA Memphis. Halford (1992) documented the
parameter-estimation program used to facilitate model calibration. The
USGS particle-tracking program used to delineate ground-water-flow paths
is described by Pollock (1989, 1994).
The NSA Memphis study
area is located in northern Shelby County, Tennessee (
fig.
1). The major surface-water drainages in the NSA Memphis area, Big
Creek Drainage Canal and its tributaries, Royster, North Fork, and Casper
Creeks (
fig. 3), have been channelized.
Most soils in the Big Creek Drainage Canal basin are silt, clay, and sand.
Land-surface altitudes in the area range from about 250 to 370 feet above
sea level. Topographic relief varies from relatively flat alluvial plains
to gently undulating upland areas. Most of the area has been cleared for
agricultural, institutional, or recreational use.
The climate of Shelby County is temperate
to subtropical. Average precipitation over the study area is about 50 inches
per year (in/yr), and is uniformly distributed throughout the year (Owenby
and Ezell, 1992). The long-term potential evaporation rate in the study
area has been estimated to be 43 in/yr (Farnsworth and others, 1982, map
3). The mean annual temperature is about 62 degrees Fahrenheit (oF).
Summer temperatures typically range from 75 oF to 95 oF,
and winter temperatures range from 35 oF to 60 oF
(Owenby and Ezell, 1992).