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Water-quality trends and basin activities and characteristics for the Albemarle-Pamlico estuarine system, North Carolina and Virginia

Open-File Report 90-398
By D.A. Harned and M.S. Davenport

Abstract

The Albemarle-Pamlico estuarine system has a total basin area of nearly 31,000 square miles and includes the Neuse, Tar, Pamlico, Roanoke, Chowan, and Alligator Rivers, and the Albemarle, Pamlico, Currituck, Croatan, and Roanoke Sounds. Albemarle Sound receives the greatest freshwater inflow of all the sounds in the estuarine system. Inflow to this sound averages about 13,500 cubic feet per second. Inflow to Pamlico Sound from the Pamlico River averages around 5,400 cubic feet per second, and average inflow into the Neuse River estuary is about 6,100 cubic feet per second. Approximately one-half of the inflow into the system is from ground-water discharge.

The Neuse River basin has had the greatest increases in wastewater discharges (650 percent since the 1950's) and had the greatesttotal wastewater discharges of any of the basins in the study area, averaging about 200 million gallons per day in 1988. Wastewater discharges into the Neuse and Tar Rivers were nearly equal to the 7-day, 10-year low flows for these rivers.

Land-use data compiled in 1973 for the lower parts of the Neuse River basin and lower part of the Tar-Pamlico River basin indicate that 25 percent of the area was evergreen forest, 25 percent was forested wetlands, 20 percent was cropland and pasture, 12 percent was mixed forest, 10 percent was nonforested wetland, and 4 percent was urban. The amount of nonforested wetland in the part of the study area along the Outer Banks declined 6.5 percent from 1973 to 1983.

The numbers of farms and acreage in agricultural use in the study area have declined since the 1920's. A decrease of more than 60 percentin the number of farms was shown between the early 1950's and 1982. Fertilizer sales increased through the 1970's, but declined in the 1980's. Manufacturing employment has increased in the last 30 years, while agricultural employment has decreased.

Data from seven stations of the U.S. Geological Survey National Stream Quality Accounting Network were used to evaluate water quality for the major streams flowing into the Albemarle-Pamlico estuarine system. Water-quality data for 296 stations in the estuarine system were examined for the period 1945-88.

The statistical test used for trend analysis was the Seasonal Kendall test (Hirsch and others, 1982). This nonparametric procedure is useful for analyses of water-quality properties that show non-normally distributed frequency distributions. The Seasonal Kendall trend analyses of water-quality data indicate that change has occurred in the water quality of the Albemarle-Pamlico estuarine system from 1945 to 1988. Dissolved-oxygen concentrations increased at a mean rate of 0.1 milligram per liter per year throughout the estuarine system, except in the Chowan River where decreases of approximately 0.06 milligram per liter per year occurred. In general, pH increased in streams throughout the area at a mean rate of 0.04 pH unit per year, except in the Pamlico River where pH decreased by 0.03 pH unit per year. A general increase in pH and dissolved-oxygen concentrations (if daytime measurements) might be indicative of more productive estuary conditions for algal growth. Suspended-solids concentrations decreased throughout the area at a mean rate of 1.1 milligrams per liter per year, probably as a result of a general decrease in suspended inorganic material. Increasing trends of salinity concentrations, as much as 0.1 part per thousand per year, were detected in Albemarle Sound.

Total ammonia plus organic nitrogen concentrations decreased (-0.03 milligram per liter per year) in streams throughout most of the area but increased (0.02 milligram per liter per year) in the Pamlico River. However, ammonia nitrogen concentrations decreased (-0.0035 milligram per liter per year) in the Pamlico River; therefore, increases in organic nitrogen probably caused the observed increase in combined ammonia plus organic nitrogen concentrations. This probably results from increased eutrophication in the system and its associated increased production of plant biomass. Nitrogen concentrations generally increased downstream and were usually sufficient for development of algal blooms.

Total phosphorus concentrations increased (0.003 milligram per liter per year) in the Pamlico River and decreased (-0.004 milligram per liter per year) elsewhere. There was a general pattern of decreasing phosphorus concentrations downstream for the Neuse and Pamlico Rivers; however, phosphorus concentrations in the Pamlico River peaked near Durham Creek.

Soluble nutrient concentrations, including ammonia nitrogen, nitrite plus nitrate, and dissolved phosphorus, are a net result of the effects of biological uptake, solution and dissolution of nutrients available in sediment, and new nutrient inputs. If plant biomass increases over time, this could be reflected in decreases in soluble nutrients such as observed ammonia nitrogen and phosphorus concentrations in the estuary system.

On the basis of annual median concentrations, nitrogen was the limiting nutrient for algal growth in the Neuse and Pamlico Rivers. Phosphorus was the limiting nutrient in most of the rest of the Albemarle-Pamlico system. Direct tests for specific nutrient limitations need to be made to confirm limitations at specific sites in the estuarine system.

Trends in chlorophyll-a concentrations increased in the Neuse River, upper Pamlico River, in the upstream end of Albemarle Sound, and near Bull Bay in Albemarle Sound (maximum rate, 1.0 microgram per liter per year). Chlorophyll-a concentrations decreased in the part of the Chowan River near Mount Gould. A pattern of increases in chlorophyll-a concentrations downstream in the Neuse, Chowan, and Alligator Rivers is apparent. Chlorophyll-a concentrations in the Pamlico River increased downstream, peaked in Durham Creek, and declined farther downstream. Chlorophyll-a concentrations were largest in the Pamlico (interquartile range 3-27 micrograms per liter) and Neuse Rivers (interquartile range 3-17 micrograms per liter) and in Currituck Sound (interquartile range 7-22 micrograms per liter).

Evaluation of water-quality data and more than 50 basin variables indicated 121 significant correlations between 11 basin activities or characteristics and 12 water-quality constituents at 21 estuary zones or locations and 7 National Stream Quality Accounting Network stationsDissolved oxygen, suspended solids, total ammonia nitrogen, total ammonia plus organic nitrogen, and total phosphorus are among the constituents that correlated significantly (alpha = 0.01) with basin activities and characteristics.

Increases of dissolved oxygen with increases in crop acreages and fertilizer use could reflect increased plant biomass in the estuary, probably as a result of agricultural activities. Decreases in suspended solids in the estuarine system probably reflect decreases in corn and tobacco production or improved agricultural soil management.

Decreases in ammonia nitrogen in the Pamlico River correlated with decreases in tobacco acreage and fertilizer use, and increases in total ammonia plus organic nitrogen in the Pamlico River correlated with increases in crops and livestock. A decrease in ammonia nitrogen could occur with an increase in production of plant biomass, which in turn may be reflected in an increase in total ammonia plus organic nitrogen. These correlations might indicate that expanding agricultural operations are associated with nitrogen concentrations in the Pamlico River.

Harned, D.A., and Davenport, M.S., 1990, Water-quality trends and basin activities and characteristics for the Albemarle-Pamlico estuarine system, North Carolina and Virginia: U.S. Geological Survey Open-File Report 90-398, 164 p.