PUBLICATIONS

• Publications by topic
  • Groundwater
  • Surface Water
  • Water Quality
  • Water Use

• Publications by date
  • Recent publications

Annual Water Data Report

USGS Publications Warehouse

USGS IN YOUR STATE

USGS Water Science Centers are located in each state.

Water-Quality Trends in the Neuse River Basin, North Carolina, 1974-2003

American Geophysical Union
By Douglas A. Harned

Abstract

Data from two U.S. Geological Survey (USGS) sites in the Neuse River basin were reviewed for trends in major ions, sediment, nutrients, and pesticides during the period 1974-2003. In 1997, the North Carolina Division of Water Quality implemented management rules to reduce nitrogen loading to the Neuse River by 30 percent by 2003. Therefore, the 1997-2003 period was reviewed for trends associated with the management changes. The Neuse River at Kinston basin (2,695 square miles) includes much of Raleigh, N.C., with 8-percent urban and 30-percent agricultural land use (1992 data). The Contentnea Creek basin (734 square miles), a Neuse River tributary, is 42-percent agricultural and 3-percent urban. Agricultural land uses in the Contentnea Creek basin have changed over the last decade from predominantly corn, soybean, and tobacco row crops to corn, soybeans, and cotton, with reduced tobacco acreages, and development of the hog industry. Data for this analysis were collected by the USGS for the National Stream Quality Accounting Network and National Water-Quality Assessment Program.

Data were examined for trends using the Seasonal Kendall trend test or Tobit regression. The Seasonal Kendall test, which accounts for seasonal variability and adjusts for effects of streamflow on concentration with residuals from LOWESS (LOcally Weighted Sum of Squares) curves, was used to analyze trends in major ions, nutrients, and sediment. The Tobit test, appropriate for examining values with reporting limits, was used for the pesticide analysis. Monotonic trends are considered significant at the alpha < 0.05 probability level.

Long-term (1974-2003) decreasing trends in the Neuse River at Kinston were detected for dissolved oxygen, silica, and sediment concentrations; increasing trends were detected for potassium, alkalinity, and chloride. Decreasing trends in Contentnea Creek were detected for silica, sulfate, and sediment concentrations during 1979-2003; increasing trends were detected for pH, hardness, and alkalinity. A pattern of increase until 1990 followed by little change or decline was observed for specific conductance, dissolved solids, hardness, and sulfate in the Neuse River and for potassium in Contentnea Creek. No significant recent (1997-2003) trends were detected for dissolved oxygen, pH, specific conductance, hardness, dissolved solids, or major ions. The Neuse River data indicated a recent declining trend in sediment concentration.

Nitrogen concentrations in the form of ammonia, total ammonia and organic nitrogen, and nitrite plus nitrate have declined in both the Neuse River and Contentnea Creek. Total nitrogen concentrations increased in the Neuse River until about 1990 and then declined, primarily because of declines in nitrate. Recent declines are evident in nitrite plus nitrate in the Neuse River and in ammonia concentrations in Contentnea Creek. The data also show a reduction in variation of extreme values after 1990 in Contentnea Creek. Both observations suggest that the 1997 Neuse River management rules have had a detectable effect on nitrogen concentrations. Concentrations of dissolved and total phosphorus and orthophosphate reduced in a step trend in 1988 at both locations. This reflects the 1988 phosphate detergent ban in North Carolina. Orthophosphate concentrations have continued a recent decline in Contentnea Creek.

Contentnea Creek has sufficient period of record (1994-2003) of concentrations of atrazine, deethyl atrazine, alachlor, carbaryl, diazinon, and prometon to test for trends. Both alachlor and prometon concentrations showed significant declines. Recent changes in agricultural practices coupled with a 5-year drought probably have affected pesticide use and transport to surface waters.

Harned, D.A, 2003, Water-quality trends in the Neuse River basin, North Carolina, 1974-2003: Transactions of the American Geophysical Union 2003, Eos, Fall Meeting Supplement, Abstract H41F-1048, vol. 84, no. 46, p. F703.