Quality of surface-water supplies in the Triangle area of North Carolina, water year 2008

Abstract

Surface-water supplies are important sources of drinking water for residents in the Triangle area of North Carolina, which is located within the upper Cape Fear and Neuse River Basins. Since 1988, the U.S. Geological Survey and a consortium of governments have tracked water-quality conditions and trends in several of the area's water-supply lakes and streams. This report summarizes data collected through this cooperative effort, known as the Triangle Area Water Supply Monitoring Project, during October 2007 through September 2008. Major findings for this period include: •Antecedent drought conditions during 2007 contributed to below-average flows at streams throughout the study area during 2008. Continuous records from 9 of the 10 project stream gages documented below-average streamflow during most of the year. •More than 8,000 individual measurements of water quality were made at a total of 27 sites—15 in the Neuse River Basin and 12 in the Cape Fear River Basin. •North Carolina water-quality standards were exceeded one or more times for nine constituents, including dissolved oxygen, dissolved oxygen percent saturation, pH, chlorophyll a, mercury, copper, iron, manganese, and zinc. Exceedances occurred at 26 sites, 14 of which were in the Neuse River Basin, and 12 of which were in the Cape Fear River Basin. •Stream samples collected during storm events contained elevated concentrations of iron, copper, and total phosphorus relative to non-storm samples. •The first full year of sampling was completed for a new project site at Lake Butner in Granville County. Among all lakes sampled during 2008, Lake Butner had the lowest concentrations of total ammonia plus organic nitrogen, total phosphorus, chlorophyll a, and specific conductance and the highest water clarity. •Concentrations of nitrogen and phosphorus were within ranges observed during previous years; however, Falls Lake at U.S. Interstate 85 had elevated levels of nitrate plus nitrite and total phosphorus relative to other sites. •Five lakes had chlorophyll a concentrations in excess of 40 micrograms per liter at least once during 2008, including Little River Reservoir, Falls Lake, Lake Benson, University Lake, and Jordan Lake.