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

Proceedings of the Conference on Managing the Albemarle-Pamlico Sounds
By Douglas A. Harned and Gerard McMahon

Abstract

Temporal trends in riverine water quality for the Contentnea Creek Basin in the Albemarle-Pamlico drainage area were examined as part of the U.S. Geological Survey's National Water-Quality Assessment (NAWQA) Program. The Contentnea Creek Basin was a focus of the NAWQA study because the basin has the highest nitrogen and phosphorus instream loads of all of the large river basins in the Albemarle-Pamlico region. Available data for the study basins was evaluated for monotonically increasing or decreasing trends for selected water properties and constituents, including suspended sediment, solids, and nutrients.

The statistical test used for trend analysis was the seasonal Kendall test. The test compensates for seasonal water-quality variation, and only stations with sufficient seasonal data coverage were evaluated. Variation in water quality as a result of variation in streamflow also was accounted for in cases where streamflow data were available. The method used for streamflow adjustment involved using residual values about a smoothed data curve of the water-quality constituent related to streamflow. The method of curve smoothing used in streamflow adjustment and in presentation of data scatter-plot smoothing was Locally Weighted Scatter-Plot Smoothing (LOWESS). Although adjustments for streamflow were not possible for a few of the stations tested for trends, variation in streamflow in the Contentnea Creek is strongly seasonal; therefore, the seasonal compensation used in the seasonal Kendall trend test for non-flow adjusted concentration at least partially accounts for the seasonal variation of streamflow. A significance level of 0.05 was considered to show statistical significance of the trend test.

Water-quality data from six stations in the Contentnea Creek Basin were evaluated for trends; the stations were Contentnea Creek near Lucama, Turner Swamp near Eureka, Nahunta Swamp near Shine, Contentnea Creek at Hookerton, Little Contentnea Creek near Farmville, and Contentnea Creek at Grifton. Adjustment for streamflow was made for all stations except for Turner Swamp and Contentnea Creek at Grifton. The most extensive water-quality data were available for the Contentnea Creek at Hookerton station.

Nutrient concentrations for Contentnea Creek at Hookerton have generally declined since 1980. Total nitrogen, nitrate plus nitrite, and nitrate concentrations declined significantly, with the greatest reductions occurring from 1980 to 1992. Total ammonia and organic nitrogen concentrations, which were increasing during the 1980's, have declined since around 1990. Total phosphorus, dissolved phosphorus and orthophosphorus, which increased during the 1980's have shown a significant decline since 1988-- the first year of the legislated phosphate detergent ban. However, concentrations of these nutrients are still high enough to indicate potential for nuisance algal growth. Phosphorus concentrations, in particular, are high (median at Hookerton=0.7 mg/L total phosphorus), probably due to a geologic source.

Concentrations of magnesium, calcium, and potassium in Contentnea Creek at Hookerton have increased since 1980. These increases probably reflect the leaching of these base cations from the soil by acid precipitation. However, the pH of the stream has increased slightly since 1985 following a period of decline from 1980-85, suggesting that acidification of soil could decrease in the future.

Sodium, sulfate, turbidity, solids and dissolved-oxygen concentrations have declined since 1980 at Hookerton. Many of the trends observed for the station at Hookerton are apparent at the other stations. Solids concentrations have declined (1982-94) significantly at all of the sites. Turbidity has declined at Contentnea Creek near Lucama (1980-96) , 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). The declines in turbidity and solids, which probably reflect improved waste-water treatment, are particularly important to the condition of the downstream estuary. Improved clarity of the water allows greater light penetration, which may promote algal blooms, yet may have beneficial effects on other organisms with decreased silting of habitats.

These changes in the stream chemistry of Contentnea Creek and its tributaries during the past 15 years reflect changes in agricultural land management, basin development, and atmospheric inputs. Corn and tobacco acreage in the basin and the use of agricultural fertilizer and lime has declined during the last 20 years, while soybean and cotton acreage, and hog and chicken production have increased. Urban development has grown. The acidity of precipitation has decreased.

Harned, D.A., and McMahon, Gerard, 1997, Trends in surface-water quality for the Contentnea Creek Basin, 1980-1996 [abs]: Proceedings of the Conference on Managing the Albemarle-Pamlico Sounds, New Bern N.C., June 5-6, 1997