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Estimating a nutrient mass balance for major drainage areas of the Albemarle-Pamlico Drainage Basin, North Carolina and Virginia

Albemarle-Pamlico Estuarine Study Comprehensive Conservation and Management Plan Implementation Forum, June 5-6.
By Gerard McMahon and M.D. Woodside

Abstract

The source and fate of nutrients in the Albemarle-Pamlico estuary system are important water-quality concerns in North Carolina and Virginia. A calculation of a 1990 nitrogen and phosphorus mass balance for the drainage areas of eight National Stream Quality Accounting Network stations in the Albemarle-Pamlico Drainage Basin indicates the importance of atmospheric and agricultural nonpoint nutrient sources and watershed nutrient retention and processing capabilities.

Estimated nutrient contributions were calculated for atmospheric deposition (which averaged 27% of total nitrogen inputs and 22% of total phosphorus inputs); crop fertilizer (27% and 25%); animal-waste (22% and 50%); point sources (3% each); and biological nitrogen fixation (21% of total nitrogen inputs). Nutrient output estimates were made for instream fluxes and crop harvest. The difference between the sum of nutrient contributions and the sum of the instream nutrient flux and crop-harvest nutrient removal was assigned to a residual basin storage and retention category. Nutrient removal by crop harvest, as a percent of the total basin nutrient contributions, averaged 34% of total nitrogen inputs and 36% of total phosphorus inputs. Nutrients exported by instream flux averaged 15% and 10%. The residual category averaged 51% of total nitrogen inputs and 54% of total phosphorus inputs.

The highest instream nutrient load was measured in the Contentnea Creek Basin, a predominantly agricultural drainage area. Intermediate loads were observed in mixed agricultural-urban drainage areas (Dan River at Paces, Va.; Neuse River at Kinston, N.C.; and Tar River at Tarboro, N.C.); the lowest loads were measured in mixed agricultural-forested drainage areas (Blackwater River near Franklin, Va.; Meherrin River at Emporia, Va.; Nottoway River near Sebrell, Va.; and Roanoke River at Roanoke Rapids, N.C.).

The magnitude of the residual category indicates the importance of factors such as the role of wetlands and aquifers in denitrification, the high reported rates of nitrogen retention in forests, as well as uncertainty and error in estimating nutrient inputs and outputs. Additional research is needed to reduce this uncertainty, particularly regarding basin-level estimates of atmospheric and animal-waste contributions to instream nutrient loads and the natural nutrient processing capabilities of terrestrial and surface and ground-water systems. Effective and equitable management of the Albemarle-Pamlico estuary system depends on the availability of information about nutrient inputs and outputs that all involved in water quality management can agree is scientifically sound, comprehensible, and non-partisan.

McMahon, Gerard and Woodside, M.D., 1997, Estimating a nutrient mass balance for major drainage areas of the Albemarle-Pamlico Drainage Basin, North Carolina and Virginia: Albemarle-Pamlico Estuarine Study Comprehensive Conservation and Management Plan Implementation Forum, June 5-6.