Ground-water recharge to the regolith-fractured crystalline rock aquifer system, Orange County, North Carolina

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by ground water, planners and managers of suburban development must know the amount of ground water that can be withdrawn without exceeding recharge and(or) overdrafting water in long-term storage.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