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� 5 <br /> Report Abstract <br /> Approximately one in three Orange County citizens rely on ground water for their potable water <br /> supply. While study of surface water supplies and needs have been undertaken,relatively little <br /> has been done to date to study ground water supplies in this area. As the population of Orange <br /> County grows,there will be an increased demand on the ground water resource that very little is <br /> known about. In response to this population growth, the USGS and Orange County began this <br /> study in 1995 to assess ground water recharge rates to the county's ground water aquifer system, <br /> and to examine ways of determining ground water storage capacities throughout the county. The <br /> goal was to produce accurate quantitative data that would help water resource management <br /> efforts. <br /> In this study,a hydrograph separation technique was used along with streamflow data to estimate <br /> seasonal and long term recharge rates in 12 drainage basins in Orange County.Methods for <br /> estimating ground water storage rates are also discussed in the report. <br /> It is estimated that the Morgan Creek Basin(upstream from White Cross and Chapel Hill)has <br /> the highest recharge rates in the county,and that ground water constitutes a higher percentage of <br /> streamflow in Morgan Creek than in any of the other study areas.Conversely,the Haw River <br /> basin in southwestern Orange has the lowest recharge rate on all basins in the County.In general, <br /> recharge rates are highest along a north-south zone in the center of the county,and lowest in the <br /> west and southeast. Overall,the range of net annual average recharge varies from 311 <br /> gallons/day/acre to 477 gallons/day/acre. <br /> Ground water storage capacities can be expected to vary throughout the county. Most ground <br /> water storage in Orange County is found in the regolith,a zone of soil and weathered bedrock. <br /> Storage capacity in the Piedmont is typically a function of the saturated thickness of the regolith <br /> and of the specific yield of the regolith. <br /> Study Objectives <br /> The objectives for the study as determined by the Orange County Water Resources Committee <br /> and the USGS were to: <br /> • Evaluate long-term ground water recharge rates throughout Orange County based on <br /> available data <br /> • Refine those estimates by comparing differences in recharge rates between basins <br /> • Further refine estimates by determining the effect of seasonal climatic changes on recharge <br /> rates <br /> • Produce a report describing recharge rates, including tables and hydrographs which can be <br /> used to accompany GIS watershed maps <br /> Orange County Hydrogeology <br /> Orange County's hydrogeology is defined by an intricate relationship between streams and rivers <br /> that convey runoff from the county,and the regolith-fractured crystalline rock aquifer system <br /> that stores ground water and functions as a conduit for ground water from recharge to discharge <br /> areas. Ground water discharge to streams and rivers is an important component of total <br /> 2 <br />