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Water and bed-material quality of selected streams and reservoirs in the Research Triangle area of North Carolina, 1988-94

Water-Resources Investigations Report 95-4282
By C.J.O. Childress and M.W. Treece, Jr.

Full Report (PDF, 84 pages, 9 Mb)

Abstract

The Triangle Area Water Supply Monitoring Project was formed by a consortium of local governments and governmental agencies in cooperation with the U.S. Geological Survey to supplement existing data on conventional pollutants, nutrients, and metals to enable eventual determination of long-term trends; to examine spatial differences among water supplies within the region, especially differences between smaller upland sources, large multipurpose reservoirs, and run-of-river supplies; to provide tributary loading inlake data for predictive modeling of Falls of the Neuse and B. Everett Jordan reservoirs; and to establish a database for synthetic organic compounds.

Water-quality sampling began in October 1988 at 35 sites located on area run-of-river and reservoir water supplies and their tributaries. Sampling has continued through 1994. Samples were analyzed for major ions, nutrients, trace metals, pesticides, and semivolatile and volatile organic compounds. Monthly concentration data, high-flow concentration data, and data on daily mean streamflow at most stream sites were used to calculate loadings of nitrogen, phosphorus, suspended sediment, and trace metals to reservoirs.

Stream and lake sites were assigned to one of five site categories-- (1) rivers, (2) large multipurpose reservoirs, (3) small water-supply reservoirs, (4) streams below urban areas and wastewater-treatment plants, and (5) headwater streams--according to general site characteristics. Concentrations of nitrogen species, phosphorus species, and selected trace metals were compared by site category using nonparametric analysis of variance techniques and qualitatively (trace metals). Wastewater-treatment plant effluents and urban runoff had a significant impact on water quality compared to reservoirs and headwater streams. Streams draining these areas had more mineralized water than streams draining undeveloped areas. Moreover, median nitrogen and nitrite plus nitrate concentrations were significantly greater than all other site categories. Phosphorus was significantly greater than for reservoir sites or headwater streams. Few concentrations of trace metals were greater than the minimum reporting limit, and U.S. Environmental Protection Agency drinking-water standards were rarely exceeded. Detections, when they occurred, were most frequent for sites below urban areas and wastewater-treatment plant effluents.

A small number of samples for analysis of acetanilide, triazine, carbamate, and chlorophenoxy acid pesticides indicate that some of these compounds are generally present in area waters in small concentrations. Organochlorine and organophosphorus pesticides are ubiquitous in the study area in very small concentrations. Trihalomethanes were detected at sites below urban areas and wastewater-treatment plants. Otherwise, volatile organic compounds and semivolatile compounds were generally not detected.

Suspended-sediment, nitrogen, phosphorus, lead, and zinc loads into Falls Lake, Jordan Lake, University Lake, Cane Creek Reservoir, Little River Reservoir, and Lake Michie were calculated. In general, reservoirs act as traps for suspended sediment and constituents associated with suspended sediments.

During 1989-94, annual suspended-sediment load to Falls Lake ranged from 29,500 to 88,200 tons. Because Lake Michie trapped from 83 to 93 percent of the suspended sediment delivered by Flat River, Flat River is a minor contributor of suspended sediment to Falls Lake. Yields of suspended sediment from Little River, Little Lick Creek, and Flat River Basins were between 184 and 223 tons per square mile and appear to have increased increased slightly from yields reported in a study for the period 1970-79. Annual suspended-sediment load to Jordan Lake ranged from 271,000 to 622,000 tons from 1989 through 1994 water years. The Haw River contributed more than 75 percent of the tota load to Jordan Lake. The suspended-sediment yields for Haw River and Northeast Creek were 252 and 284 tons per square mile, respectively. This is more than twice the yield reported for Haw River for the period 1970-79. University Lake received an estimated 1,560 tons per year and Cane Creek Reservoir an average of 2,420 tons per year. Yields in these basins were the lowest in the study area.

Nitrogen loads to Falls Lake ranged from 780 to 1,650 tons per year, and loads out of Falls Lake accounted for 20 to 56 percent of the inflow load. Ellerbe Creek had the greatest average annual nitrogen yield of the Falls Lake tributaries--12 tons per square mile. Nitrogen loads to Jordan Lake ranged from 2,710 to 4,410 tons per year, and loads out of Jordan Lake accounted for about 57 percent of the inflow load. Haw River contributed about 70 percent of the inflow load to Jordan Lake; however, New Hope and Northeast Creeks produced nitrogen yields that were almost double that of Haw River. Nitrogen loads to University Lake and Cane Creek Reservoir averaged less than 50 tons per year.

Annual phosphorus loads to Falls Lake averaged 103 tons, and loads out of the lake accounted for 28 percent of the inflow load. Among the tributaries to Falls Lake, the greatest load was from Knap of Reeds Creek (23 tons per year). Phosphorus yields declined in Little Lick and Ellerbe Creeks and Eno River for the study period compared to the period 1983-87. Annual phosphorus loads to Jordan Lake ranged from 418 to 701 tons per year, and loads out of the lake accounted for 40 percent of the inflow load. Average annual phosphorus yields declined at every site monitored for the study period compared to the period 1983-87.

Average annual lead loads to Falls Lake from tributaries ranged from 363 pounds from Little Lick Creek to 2,300 pounds from Ellerbe Creek. Average annual lead loads to Jordan Lake from tributaries ranged from 667 pounds from Northeast Creek to 40,000 pounds from Haw River. Average annual zinc loads to Falls Lake from monitored tributaries ranged from 1,150 pounds from Little Lick Creek to 10,600 pounds from Eno River. Average annual zinc loads to Jordan Lake from monitored tributaries averaged 135,000 pounds, with Haw River contributing about 75 percent of the load.

Childress, C.J.O., and Treece, M.W., Jr., 1996, Water and bed-material quality of selected streams and reservoirs in the Research Triangle area of North Carolina, 1988-94: U.S. Geological Survey Water-Resources Investigations Report 95-4282, 79 p.