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Trends in surface-water quality for the Contentnea Creek Basin, North Carolina, 1980-1996

Project Abstract
By Gerard McMahon and Doug A. Harned

Abstract

Temporal trends in riverine water quality in the Contentnea Creek Basin of North Carolina were examined as part of the U.S. Geological Survey's National Water-Quality Assessment (NAWQA) Program. The Contentnea Creek Basin is a 737-square-mile tributary to the Neuse River in the Albemarle-Pamlico (ALBE) drainage area. Approximately 50 percent of the basin land use is agricultural, resulting in the highest nitrogen and phosphorus inputs and the highest instream loads of any of the major rivers in the ALBE drainage.

Water-quality data for the period 1980-96 from six sites in the Contentnea Creek Basin were evaluated for trends by using the seasonal Kendall test, with adjustments for streamflow data where available. The Hookerton site on Contentnea Creek had the most extensive water-quality data. Water-quality trends for this site indicate that profound changes have occurred over the past 15 years. Nutrient concentrations are declining along with concentrations of sulfate, turbidity, total solids, and dissolved oxygen. Total nitrogen, nitrite plus nitrate, and nitrate concentrations declined significantly, with the greatest reductions occurring from 1980 to 1992. Total ammonia and organic nitrogen concentrations, which increased during the 1980's, have declined since around 1990. Total phosphorus, dissolved phosphorus, and orthophosphorus concentrations, which increased during the 1980's, have shown a significant decline since 1988--coinciding with the first year of the legislated phosphate-detergent ban in North Carolina. Improvements in wastewater treatment also have probably caused lower nutrient concentrations. However, concentrations of these nutrients are still high enough to indicate the potential for nuisance algal growth. In particular, phosphorus concentrations are high at Hookerton (median = 0.7 mg/L total phosphorus), possibly resulting from a geologic source. Concentrations of magnesium, calcium, and potassium in Contentnea Creek at Hookerton have increased since 1980. These increases may reflect the leaching of base cations from the soil by acid precipitation. The pH of the stream has increased slightly since 1985 following a period of decline from 1980-85. The acidity of precipitation has decreased, suggesting that acidification of soil could decrease in the future.

Many of the trends observed at the Hookerton site are also apparent at the other sites. Solids concentrations have declined significantly at all six sites. Turbidity has declined at Contentnea Creek near Lucama (1986-96), at Turner Swamp (1986-96), and Contentnea Creek at Grifton (1986-96). Dissolved-oxygen concentrations have declined at Turner Swamp (1980-96) and at Contentnea Creek at Grifton (1980-96). Declines in turbidity and solids, which may reflect improved wastewater treatment and improvements in land-management practices, are particularly important in relation to the condition of the downstream estuary. Improved clarity of the water allows greater light penetration, which may promote algal blooms and yet have beneficial effects on other organisms by decreasing silt.