HYDROLOGY AND WATER QUALITY
OF THE SHALLOW AQUIFER SYSTEM
COLONIAL NATIONAL HISTORICAL PARK
AT YORKTOWN, VIRGINIA
Gary K. Speiran and Michael
U.S. Geological Survey
1730 East Parham Road
Richmond, VA 23228
National Park Service, Northeast
Stewardship and Partnerships
200 Chestnut Street
Philadelphia, PA 19106
a part of Colonial National Historical Park, was the site of the culminating
battle of the American Revolutionary War. The National Park
Service operates and maintains the battlefield primarily to protect
the historical and cultural resources of the area but also to protect
the streams and wetlands. These
streams and wetlands provide (1) critical habitat for rare, threatened,
and endangered species; (2) nurseries for numerous commercial and recreational
sport fisheries species; and (3) opportunities for observation, education,
and recreational fishing. Because groundwater discharge from the shallow
aquifer system can contribute more than half of the flow in the streams
and a large part of the water to the wetlands, this discharge can substantially
affect the quantity and quality of water in the streams, wetlands, and
near the battlefield. Consequently, knowledge of the hydrology and water
quality of the shallow aquifer system is critical to the protection
of these natural resources.
In 1999, the
U.S. Geological Survey, in cooperation with the National Park Service,
began a preliminary delineation of aquifers and confining units and
a general characterization of the hydrology and water quality of the
shallow aquifer system near Yorktown Battlefield. One of the purposes
of the study was to determine the types and amounts of additional information
needed to evaluate the effects of the
shallow aquifer system on the quantity and quality of water in the streams,
wetlands, and associated habitats.
Yorktown Battlefield is underlain by a system of interlayered aquifers
and confining units. The deep part of the aquifer system (generally
deeper than 150 ft) is poorly connected to the shallow part of the aquifer
system, streams, or wetlands. The shallow aquifer system is well connected
to the streams and wetlands and is the main source of ground-water discharge.
The shallow aquifer system at increasing depth consists of the Columbia
aquifer, the Cornwallis Cave confining unit, the Cornwallis Cave aquifer,
the Yorktown confining unit, and the Yorktown-Eastover aquifer.
The physiography near Yorktown Battlefield substantially affects flow
through the shallow aquifer system through associated effects on the
geology, stream incisement, and locations of recharge and discharge
areas. Because terrace deposits are present at different elevations
and are incised by stream valleys,
sediments that form the Columbia aquifer and Cornwallis Cave confining
unit form discontinuous rather than laterally continuous geohydrologic
units. The Columbia aquifer and the Cornwallis Cave confining unit are
also limited in extent because
sediments that commonly form these units are unsaturated beneath the
uplands near the stream valleys. Stream incisement also reduces the
thickness of the Cornwallis Cave aquifer beneath the valleys and partly
controls ground-water flow. Valleys of streams draining to the York
River Basin are shorter and more deeply incised, have steeper walls,
and are separated from one another by narrower upland terraces than
valleys of streams draining to the James River Basin.
The Cornwallis Cave aquifer is the primary source of discharge from
the shallow aquifer system. This aquifer underlies the entire battlefield
except at the northwest boundary along Ballard Creek and the York River.
Stream incisement into the Cornwallis Cave aquifer creates a good hydraulic
connection between the aquifer and the streams and wetlands throughout
the battlefield. Because streams do not
incise through the Yorktown confining unit into the Yorktown-Eastover
aquifer near the battlefield, the Yorktown-Eastover aquifer is poorly
connected to the streams and wetlands near the battlefield.
Because of the good hydraulic connection between the streams and the
Cornwallis Cave aquifer, shallow ground water generally discharges to
local streams rather than flow under the streams to regional discharge
areas. Therefore, the potential is limited for the transport of contaminants
from recharge areas outside the battlefield through the shallow aquifer
system to the battlefield. Flow under streams to regional discharge
areas most likely occurs through the Yorktown-Eastover aquifer, which
is also the most likely pathway for the transport of contaminants from
sources outside the battlefield to the battlefield. Contaminant transport
through the Yorktown-Eastover aquifer, however, probably is limited
by the low permeability of the overlying Yorktown confining unit.
The quality of ground water (represented by spring discharge) near Yorktown
Battlefield depends on the aquifer from which the water discharges and
location of the spring. Concentrations of major ions were lower in water
discharging from the Columbia aquifer than from the Cornwallis Cave
Concentrations of bicarbonate ions in water from two springs discharging
from the Columbia aquifer, for example, were 0 and 6 milligrams per
concentrations in water from 12 springs discharging from the Cornwallis
Cave aquifer ranged from 133 to 410 mg/L. The primary source of high
of bicarbonate ions in the Cornwallis Cave aquifer likely is the dissolution
of aragonite in shell material in the aquifer. Concentrations of bicarbonate
ions in water discharging from the Cornwallis Cave aquifer in the York
River Basin generally were higher than concentrations in discharge in
the James River Basin.
The higher concentrations in discharge in the York River Basin can result
from less precipitation of calcite from water in the York River Basin
where flow paths are shorter and ground water likely is younger than
in the James River Basin.
Concentrations of bicarbonate ion control the buffering of ground water.
The pH of water from two springs discharging from the Columbia aquifer
(low bicarbonate concentrations) was 4.5 and 5.1, whereas that of water
from springs discharging from the Cornwallis Cave aquifer (high bicarbonate
concentrations) ranged from 7.0 to 7.8. The pH, in turn, affects other
aspects of ground-water quality; for example, aluminum concentrations
of water from two springs discharging from the Columbia aquifer were
410 and 140 micrograms per liter (µg/L), whereas
concentrations in water from springs discharging from the Cornwallis
Cave aquifer were less than the 15 µg/L minimum reporting limit.
The preceding analysis was based primarily on surface information (soils,
outcrops, topography, ansprings) at the battlefield and on subsurface
information from wells and boreholes surrounding the battlefield. Because
of the complex geology and the lack of appropriate wells and other sources
of subsurface information at the battlefield, future research would
necessitate the installation of more wells throughout the battlefield
to provide additional geologic and other information. Future research
on the shallow aquifer system could include (1) continued development
of the geohydrologic framework; (2) monitoring of ground-water levels
and quality, rates of spring discharge, streamflow, and stream-water
quality; and (3) evaluation of the connection between the shallow aquifer
system and the streams, wetlands, and associated habitats. The framework
conceptualization of the physical constraints that control the flow
of ground water and transport of contaminants. Monitoring provides essential
information for evaluating long- and short-term trends in the hydrology
and water quality of the shallow aquifer system, streams, and wetlands.
A better understanding of the connection between the shallow aquifer
system and the streams, wetlands, and
associated habitats is needed to protect and manage the quantity and
quality of water in these systems.
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