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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by ground water, planners and managers of suburban development must know the amount of ground water that can be withdrawn without exceeding recharge and(or) overdrafting water in long-term storage.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.

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.