U.S.
GEOLOGICAL SURVEY
Water-Resources Investigations Report 03-4258
Prepared
in cooperation with
The City of Virginia Beach
Department of Public Utilities
By Barry S. Smith
Population and tourism continues to
grow in Virginia Beach, Virginia, but the supply of freshwater is limited.
A pipeline from Lake Gaston supplies water for northern Virginia Beach, but
ground water is widely used to
water lawns in the north, and most southern areas of the city rely solely on
ground water. Water from
depths greater than 60 meters generally is too saline to drink. Concentrations
of chloride, iron, and manganese
exceed drinking-water standards in some areas. The U.S. Geological Survey, in
cooperation with the city of
Virginia Beach, Department of Public Utilities, investigated the shallow aquifer
system of the southern
watersheds to determine the distribution of fresh ground water, its potential
uses, and its susceptibility to
contamination.
Aquifers and confining units of the
southern watersheds were delineated and chloride concentrations in the
aquifers and confining units were contoured. A ground-water-flow and
solute-transport model of the shallow
aquifer system reached steady state with regard to measured chloride
concentrations after 31,550 years of
freshwater recharge. Model simulations indicate that if freshwater is found in
permeable sediments of the
Yorktown-Eastover aquifer, such a well field could supply freshwater, possibly
for decades, but eventually the
water would become more saline. The rate of saline-water intrusion toward the
well field would depend on the
rate of pumping, aquifer properties, and on the proximity of the well field to
saline water sources. The
steady-state, ground-water-flow model also was used to simulate drawdowns around
two hypothetical well
fields and drawdowns around two hypothetical open-pit mines. The chloride
concentrations simulated in the
model did not approximate the measured concentrations for some wells, indicating
sites where local
hydrogeologic units or unit properties do not conform to the simple hydrogeology
of the model.
The Columbia aquifer, the Yorktown
confining unit, and the Yorktown-Eastover aquifer compose the
hydrogeologic units of the shallow aquifer system of Virginia Beach. The
Columbia and Yorktown-Eastover
aquifers are poorly confined throughout most of the southern watersheds of
Virginia Beach. The
freshwater-to-saline-water distribution probably is in a dynamic equilibrium
throughout most of the shallow
aquifer system. Freshwater flows continually down and away from the center of
the higher altitudes to mix with
saline water from the tidal rivers, bays, salt marshes, and the Atlantic Ocean.
Fresh ground water from the
Columbia aquifer also leaks down through the Yorktown confining unit into the
upper half of the Yorktown-Eastover
aquifer and flows within the Yorktown-Eastover above saline water in the lower
half of the aquifer. Ground-water
recharge is minimal in much of the southern watersheds because the land surface
generally is low and flat.
CONTENTS
Abstract
Introduction
Shallow Aquifer System of the Southern Watersheds
Hydrogeologic framework
Hydraulic properties
Ground-water levels
Saline Water in the Shallow Aquifer System
Simulation of Ground-Water Flow and Solute Transport
Analyses of Ground-Water Flow and Saline-Water
Intrusion
Summary
and conclusions
References Cited
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Send questions or comments about this report to the Virginia District Office (804) 261-2600.
For more information about USGS activities in Virginia, visit the USGS Virginia District home page.
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