| Summary |
The Castle Hayne Aquifer is the source of potable water for the City of Washington, North Carolina. To protect this resource, the City requested the design of a Wellhead Protection Program based upon hydrogeological analysis of the Castle Hayne Aquifer and the overlying groundwater system. The study emphasized analysis of hydrostratigraphic relationships, local and regional recharge, and groundwater chemistry. The drilling and monitoring of test wells at two sites, one in the west-central portion of the wellfield, and the one near the wellfield's eastern edge, provided a means for assessing geologic and hydrogeologic trends across the wellfield. Water levels in test wells at several depths were compared, and variations were correlated with rainfall events and wellfield pumping cycles. Major element geochemistry of groundwater samples from test wells was evaluated. Results indicate that groundwater leaks into the Castle Hayne Aquifer from overlying formations, and that leakance increases from east to west across the wellfield. Water chemistry comparisons between formations provide additional evidence for vertical mixing. The Yorktown Formation clays and the hard caprock of the Castle Hayne Formation are the most effective aquitards. The Castle Hayne Aquifer is largely recharged by leakance, with contributions of about 230,000 gallons/day/mile² in western portions of the wellfield. At least five years are required for surface water to reach the Castle Hayne Aquifer, and strata above the aquifer provide an effective contaminant buffer. Wellhead Protection Program recommendations are designed to eliminate conduits (such as out-of-use wells) which bypass this buffer. Wellhead protection areas have been delineated around each production well and represent distances from which groundwater, already in the aquifer, will reach production wells in five years or less. The test well location just east of the wellfield is suitable for a future production well, and should be included in the Wellhead Protection Program as an alternative source of potable water. Water quality at the new location meets current treatment capabilities, transmissivity is high (25,000 feet²/day), and a properly designed and constructed production well will meet current production rates (800 gallons/minute) with minimal (<20 feet) drawdown. |
