Relation of Land Use to Streamflow and Water Quality at Selected Sites in the City of Charlotte and Mecklenburg County, North Carolina, 1993-98
Streamflow and water-quality data were collected at nine sites in the city of Charlotte and Mecklenburg County, North Carolina, during 1993–97. Six of the basins drained areas having relatively homogeneous land use and were less than 0.3 square mile in size; the other three basins had mixed land use. Atmospheric wet-deposition data were collected in three of the basins during 1997–98. Streamflow yield varied by a factor of six among the sites, despite the fact that sites were in close proximity to one another. The lowest yield occurred in a residential basin having no curbs and gutters. The variability in mean flow from these small, relatively homogeneous basins is much greater than is found in streams draining basins that are 10 square miles in size or larger. The ratio of runoff to rainfall in the developing basin appears to have increased during the study period. Low-flow suspended-sediment concentrations in the study basins were about the same magnitude as median stormflow concentrations in Piedmont agricultural basins. Sediment concentrations were higher in the mixed land-use basins and in the developing basin. Median suspended-sediment concentrations in these basins generally were an order of magnitude greater than median concentrations in the other five basins, which had stable land use. Some of the highest total nitrogen concentrations occurred in residential basins. Total nitrogen concentrations detected in this study were about twice as high as concentrations in small Piedmont streams affected by agriculture and urbanization. Most of the total nitrogen consisted of organic nitrogen at all of the sites except in two residential land- use basins. The high ammonia content of lawn fertilizer may explain the higher ammonia concentration in stormflow from residential basins. The two basins with the highest median suspended-sediment concentrations also had the highest total phosphorus concentrations. Median total phosphorus concentrations measured in this study were several times greater than median concentrations in small Piedmont streams but almost an order of magnitude less than total phosphorus concentrations in Charlotte streams during the late 1970's. Bacteria concentrations are not correlated to streamflow. The highest bacteria levels were found in 'first-flush' samples. Higher fecal coliform concentrations were associated with residential land use. Chromium, copper, lead, and zinc occurred at all sites in concentrations that exceeded the North Carolina ambient water-quality standards. The median chromium concentration in the developing basin was more than double the median concentration at any other site. As with chromium, the maximum copper concentration in the developing basin was almost an order of magnitude greater than maximum concentrations at other sites. The highest zinc concentration also occurred in the developing basin. Samples were analyzed for 121 organic compounds and 57 volatile organic compounds. Forty-five organic compounds and seven volatile organic compounds were detected. At least five compounds were detected at all sites, and 15 or more compounds were detected at all sites except two mixed land-use basins. Atrazine, carbaryl, and metolachlor were detected at eight sites, and 90 percent of all samples had measurable amounts of atrazine. About 60 percent of the samples had detectable levels of carbaryl and metolachlor. Diazinon and malathion were measured in samples from seven sites, and methyl parathion, chlorpyrifos, alachlor, and 2,4-D were detected at four or more sites. The fewest compounds were detected in the larger, mixed land-use basins. Residential basins and the developing basin had the greatest number of detections of organic compounds. The pH of wet atmospheric deposition in three Charlotte basins was more variable than the pH measured at a National Atmospheric Deposition Program (NADP)site in Rowan County. Summer pH values were significantly lower than pH measured during the remainder of the year, probably as a result of poorer air quality and different weather patterns during the summer. Concentrations of ammonia and nitrate at the Charlotte sites generally were lower than those measured at the NADP site. Summer concentrations of ammonia and nitrate at both the Charlotte and the NADP sites were significantly greater than concentrations measured during the remainder of the year, again probably reflecting poorer summertime air-quality conditions. Sediment yields at the nine sites ranged from 77 tons per square mile per year in a residential basin to 4,700 tons per square mile per year at the developing basin. Residential areas that have been built-out for several years and industrial areas appear, in general, to have the lowest sediment yields for the Charlotte study sites. Average annual yields of total nitrogen loads ranged from about 1.7 tons per square mile to 6.6 tons per square mile. Average annual total phosphorus yields for all sites except the developing basin were less than 1.4 tons per square mile. Phosphorus yield at the developing basin was 13 .4 tons per square mile per year. Biochemical oxygen demand loading in 1993 from all of the permitted wastewater-treatment facilities in Charlotte and Mecklenburg County was about 1.5 tons per day or 548 tons per year. Converting this point-source loading to an annual yield for the 528 square-mile area of Mecklenburg County is equivalent to 1.03 tons per square mile per year, or a yield much lower than any of the yields measured at the nine study sites. In other words, biochemical oxygen demand loading from nonpoint sources in Mecklenburg County probably exceeds loading from all point sources by a large amount. Loads and average annual yields were computed for five metals-chromium, copper, lead, nickel, and zinc. The highest annual average yields for all five of these metals were in the developing basin, which also had the highest annual average suspended-sediment yield of all the sites. Estimated wet-deposition watershed loadings suggest that atmospheric deposition may be an important source of some metals, including chromium, copper, lead, and zinc, in Charlotte storm water. Storm water from residential land-use basins has higher concentrations of total nitrogen, fecal coliform bacteria, and organic compounds than do other land-use types. Reductions in suspended-sediment concentrations should generally result in reduced export of phosphorus and metals. Stable land uses, such as industrial areas and built-out residential basins, have lower sediment concentrations in stormwater than do mixed land use and developing basins. Finally, atmospheric deposition may be an important source of nitrogen and some metals in Charlotte stormwater.