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Effect of environmental setting on sediment, nitrogen, and phosphorus concentrations in watersheds of the Albermarle-Pamlico Drainage Basin, North Carolina and Virginia

American Geophysical Union
By Gerard McMahon and Douglas A. Harned

Abstract

Understanding the influence of environmental factors on sediment and nutrient concentrations in surface waters draining from watersheds is required to implement effective water quality management efforts. Spatial distributions of sediment, nitrogen, and phosphorus concentrations varied according to seven distinctive environmental settings associated with 37 watersheds located in the Albemarle-Pamlico Drainage Basin. The environmental settings were characterized as areas that were relatively homogeneous in terms of the dominant land use (agriculture, forest, or urban), physiographic region (Coastal Plain, or Piedmont), and soil drainage characteristics (well, or medium drained). The predominant watershed settings included agriculture/ Coastal Plain/medium drained soils (5 watersheds); agriculture/Coastal Plain/well drained (5); agriculture/Piedmont/medium drained (3); agriculture/Piedmont/well drained (6); urban (9); forested/Coastal Plain/medium drained (3); and forested/Piedmont/well drained (6). Water-quality data were collected by the U.S. Geological Survey (USGS), the North Carolina Department of Health, Environment, and Natural Resources, and the Virginia Department of Environmental Quality between 1968-1992 and analyzed as part of the USGS National Water Quality Assessment Program.

Concentrations of various sediment fractions were generally larger in the urban and Piedmont watersheds. Suspended-sediment concentrations were largest In the higher gradient Piedmont drainage basins, with no statistical difference between forested/well-drained and agricultural/medium-drained sites (median concentrations of 29 and 16 milligrams per liter (mg./L), respectively). Agriculture/Coastal Plain/medium drained and Urban settings had the highest dissolved solids concentrations (median concentrations of 101 and 90mg./L, respectively). Urban sites had the largest total volatile and non-volatile solids concentrations (median concentrations of 49 and 153.5 mg./L, respectively).

Nitrogen concentrations were generally largest in the urban and Coastal Plain agricultural basins. Coastal Plain agricultural sites generally had larger nitrogen concentrations than Piedmont agricultural sites. The agricultural/Coastal Plain/medium-drained soils had concentrations of total nitrogen and total ammonia plus organic nitrogen that were significantly larger than other environmental settings (median concentrations of 2.40 and 1.1 mg./L, respectively). Agricultural/Coastal Plain/well-drained and medium-drained and urban sites had the largest concentrations of total ammonia nitrogen (median concentrations of 0.10, 0.10, and 0.10 mg./L, respectively) and total nitrite plus nitrate nitrogen (median concentrations of 0.51, 0.72, and 0.54 mg./L, respectively).

Agricultural/Coastal Plain/medium-drained watersheds bad significantly larger total phosphorus concentrations than other environmental settings, and urban basins had the next largest concentration (median concentrations of 0.35 and 0.12 mg./L, respectively). The agricultural/Coastal Plain/medium and well-drained basins had significantly larger dissolved phosphorus concentrations than other environmental settings (median concentrations of 0.22 and 0.19 mg./L, respectively)

The data suggest that higher sediment concentrations are generated in higher gradient Piedmont watersheds and in urban areas. Nutrient concentrations are generally higher in urban and Coastal Plain agricultural watersheds, with in-stream nutrient concentrations inversely related to the drainage characteristics of basin soils.

McMahon, Gerard, and Harned, D.A., 1995, Effect of environmental setting on sediment, nitrogen, and phosphorus concentrations in watersheds of the Albermarle-Pamlico Drainage Basin, North Carolina and Virginia, American Geophysical Union 1995 Fall Meeting, Eos, November 7, 1995, p. F260.

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