Ground-water recharge to and storage in the regolith-fractured crystalline rock aquifer
system, Guilford County, North Carolina
Water-Resources Investigations Report 97-4140
By Charles C. Daniel III and Douglas A. Harned
Full Report (PDF, 71 pages, 1.85 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 Guilford 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ground water that can be withdrawn without exceeding recharge and(or)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 15 selected
drainage basins and subbasins using streamflow data and an anlytical
technique known as hydrograph separation. Methods for determining the
quantity of ground water in storage also are described.
Guilford County covers approximately 658 square miles in the central
part of the Piedmont Province. The population of the county in 1990
was about 347,420; approximately 21 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 all of the county.
Under natural conditions, recharge to the ground-water system in the
county is derived from 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 19
gaging stations that measure streamflow within or from Guilford County
were analyzed to produce daily estimates of ground-water recharge in
15 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 15 basins and subbasins ranges from 4.03
to 9.69 inches per year, with a mean value of 6.28 inches per year for
all basins. In general, recharge rates are highest for basins in the
northern and northwestern parts of the county and lowest in the
southern and southeastern parts of the county. Median recharge rates
in the 15 basins range from 2.47 inches per year (184 gallons per day
per acre) to 9.15 inches per year (681 gallons per day per acre), with
a median value of 4.65 inches per year (346 gallons per day per acre)
for all basins.
The distribution of recharge rates in the county suggests a correlation
between recharge rates and hydrogeologic units (and derived regolith).
The highest recharge estimates occur in the northwestern part of
Guilford County in basins unlain by felsic igneous intrusive rocks and
lesser areas of metasedimentary rocks. Recharge estimates in this area
range from 6.37 to 9.33 inches per year. Basins in the southwestern,
central, and northeastern parts of the county are underlain primarily
by metaigneous rocks of felsic and intermediate compositions, and
recharge estimates range from 5.32 to 5.51 inches per year. In the
extreme southern and southeastern parts of the county, the lower Deep
River subbasin and the lower Haw River subbasins have the lowest
estimated recharges at 4.15 and 4.03 inches per year, respectively.
Although the areas of these subbasins that lie within Guilford County
are underlain primarily by metaigneous rocks of felsic and
intermediate compositions, the larger part of these subbasins lies
south and southeast of Guilford County in areas underlain by
hydrogeologic units of metavolcanic origin.
The distribution of recharge rates in the study area is almost the
reverse of the distribution of precipitation across the study area.
Average annual precipitation varies across the study area from 43 to
48 inches. The lowest rainfall occurs in the northern and northwestern
part of the study area; the highest rainfall occurs in the southern
and southeastern part of the study area. Within Guilford County,
annual rainfall varies from less than 44 inches in the northwest to
about 46 inches in the southeast. The fact that the highest recharge
rates occur in the areas of lowest rainfall and the lowest recharge
rates occur in the areas of highest rainfall, further supports the
conclusion that recharge rates are highly dependent on hydrogeologic
conditions, particularly differences in the infiltration capacities
of regolith.
Recharge duration statistics also were determined for the same 15
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.
The first example illustrates the use of estimates of average 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 serviced by on-site septic systems for wastewater treatment.
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. In order to have the highest possible
recharge rates in the capture area, the wells that supply water to
the development are to be located in an area of forest and old pasture
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 could have
100 percent backup against pump or well failure by having at least
two wells.
Citation:
Daniel, C.C., III, and Harned, D.A., 1998, Ground-water recharge to and storage in the regolith-fractured crystalline rock aquifer system, Guilford County, North Carolina: U.S. Geological Survey Water-Resources Investigations Report 97-4140, 65 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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