Ground-water recharge to the regolith-fractured crystalline rock aquifer system, Orange
County, North Carolina
Water-Resources Investigations Report 96-4220
By Charles C Daniel, III
Full Report (PDF, 67 pages, 1.8 Mb)
Abstract
Quantitative information concerning recharge rates to aquifers and ground
water in storage is needed to manage the development of ground-water
resources. The amount of ground water available from the regolith-fractured
crystalline rock aquifer system in Orange County, North Carolina, is
largely unknown. If historical patterns seen throughout the Piedmont
continue into the future, the number of ground-water users in the county
can be expected to increase. In order to determine the maximum population
that can be supplied by ground water, planners and managers of suburban
development must know the amount of ground water that can be withdrawn
without exceeding recharge and(or) overdrafting water in long-term storage.
Results of the study described in this report help provide this information.
Estimates of seasonal and long-term recharge rates were estimated for 12
selected drainage basins and subbasins using streamflow data and an
analytical technique known as hydrograph separation. Methods for determining
the quality of ground water in storage also are described.
Orange County covers approximately 401 square miles in the eastern part
of the Piedmont Province. The population of the county in 1990 was about
93,850; approximately 41 percent of the population depends on ground water
as a source of potable supplies. Ground water is obtained from wells tapping
the regolith-fractured crystalline rock aquifer system that underlies most
of the county. Ground water also is obtained from Triassic age sedimentary
rocks that occur in a small area in southeastern Orange County.
Under natural conditions, recharge to the county's ground-water system is
derived from the infiltration of precipitation. Ground-water recharge
from precipitation cannot be measured directly; however, an estimate of
the amount of precipitation that infiltrates into the ground and ultimately
reaches the streams of the region can be determined by the technique of
hydrograph separation. Data from 17 gaging stations that measure streamflow
within or from Orange County were analyzed to produce daily estimates of
ground-water recharge in 12 drainage basins and subbasins in the county.
The recharge estimates were further analyzed to determine seasonal and
long-term recharge rates, as well as recharge duration statistics.
Mean annual recharge in the 12 basins and subbasins ranges from 4.15 to
6.40 inches per year, with a mean value of 4.90 inches per year for all
basins. In general, recharge rates are highest for basins along a north-
south zone extending down the center of the county, and lowest in the
western and southeastern parts of the county. Median recharge rates in
the 12 basins range from 1.08 inches per year (80.7 gallons per day per
acre) to 4.97 inches per year (370 gallons per day per acre), with a
median value of 3.06 inches per year (228 gallons per day per acre) for
all basins.
Recharge estimates for the Morgan Creek Basin upstream from White Cross
and upstream from Chapel Hill are higher than any other basin or subbasin
in Orange County. Ground water also constitutes a higher percentage of
total streamflow in Morgan Creek (44.4 percent upstream from White Cross;
47.9 percent upstream from Chapel Hill) than in any other stream in the
county. Greater topographic relief and depth of channel incision may
explain the high recharge estimates (base-flow rates) in the Morgan Creek
Basin. The presence of large areas of regolith derived from the metaigneous,
felsic hydrogeologic unit may magnify the effects of topographic relief
and channel incision. Base flow in the New Hope River subbasin, as a
percentage of total streamflow, at 32.2 percent, is the lowest of the 12
basins and subbasins. Much of the New Hope River subbasin is underlain by
the Triassic sedimentary rock hydrogeologic unit that occurs within a rift
basin of Triassic age. These data suggest that in areas underlain by Triassic
sedimentary rock, there is less recharge to the ground-water system, and
that the quantity of ground water retained in storage is lower than in
other hydrogeologic units in the county.
Recharge duration statistics also were determined for the same 12 basins
and subbasins. Recharge duration statistics provide information needed by
planners for evaluating the availability of ground water at different levels
of demand so that overuse, or overdrafting, can be prevented; or other
sources of water can be made available during periods of low recharge. Use
of water from ground-water storage is one option during periods of low
recharge. Methods for determining the amount of ground water available
from storage are described and two examples describing the use of recharge
and storage data for planning and ground-water management are presented.
One example illustrates the use of estimates of mean annual recharge and
the area of impervious cover to arrive at minimum lot sizes for single-
family dwellings that will be supplied by individual wells, and wastewater
treatment will be handled by on-site septic systems. The second example
illustrates the use of recharge duration statistics, test data from wells,
and knowledge of the quantity of ground water in long-term storage to
develop a community water system for a planned cluster development
containing multiple homes with on-site wastewater treatment. The wells
that supply water to the development are to be located in an area that
will be set aside as a recreational area; the houses with their septic
systems will be clustered on another part of the tract. In the second
example, the ground-water based community system has 100-percent backup
against pump or well failure by having two wells.
Citation:
Daniel, C.C., III, 1996, Ground-water recharge to the regolith-fractured crystalline rock aquifer system, Orange County, North Carolina: U.S. Geological Survey Water-Resources Investigations Report 96-4220, 59 p.
For more information, contact |
North Carolina Water Science Center
U.S. Geological Survey
3916 Sunset Ridge Road
Raleigh, North Carolina 27607
(919) 571-4000
E-mail
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