BMP IMPACTS ON SEDIMENT AND NUTRIENT YIELDS FROM AN AGRICULTURAL WATERSHED IN THE COASTAL PLAIN REGION

Abstract

The goal of the Nomini Creek watershed monitoring study was to quantify the effectiveness of BMPs at thewatershed scale and to determine if the improvements in water quality could be sustained over a longterm period.Information on the longterm effectiveness of BMPs is critical since BMPs are being implemented under the state costshareprogram to reduce nonpoint source pollution (NPS) to the Chesapeake Bay. The Nomini Creek project started in 1985 andwas completed in 1997. A pre versus postBMP design was used. A combination of managerial and structural BMPs wasimplemented. Major BMPs implemented in the Nomini Creek watershed included notillage, filter strips, and nutrientmanagement. The data collected at the 1463 ha Nomini Creek watershed consisted of land use, hydrologic, water quality,soils, and geographical information.The BMPs implemented at Nomini Creek reduced average annual loads and flowweighted concentrations of nitrogen (N)by 26% and 41%, respectively. Average annual totalN loads discharged from the watershed were reduced from 9.57 kg/haduring the preBMP period to 7.05 kg/ha for the postBMP period. Largest reductions were observed for dissolvedammoniumN, soluble organicN, and particulateN. In contrast, nitrateN loads increased after BMP implementation.Increase in nitrate exports was likely due to ammonification and nitrification, and subsequent leaching of particulateNspecies that were conserved on the field. In comparison to N, reductions in phosphorus (P) loads and concentrations werenot significant. BMP implementation resulted in a mere 4% reduction for totalP with a corresponding 24% reduction inflowweighted concentration. The average annual totalP loads exported from the watershed were 1.31 and 1.26 kg/ha forthe pre and postBMP periods, respectively. Reductions in totalP loads were due to decreases in particulateP. Exportsof orthoP and dissolved organicP increased after BMP implementation. It is likely that some of this postBMP increasein dissolved P fractions was associated with dissolution and leaching of particulateP, and higher rainfallrunoff activityin the watershed during the postBMP period. In comparison to nutrients, there was no significant change in suspended solidsdischarged from the watershed. Overall, the findings of this study indicate that the BMPs were effective in reducing the lossesof some forms of nutrients, such as ammoniumN and particulateP, from the Nomini Creek watershed, but additional BMPsare necessary to achieve significant reductions in all forms of N and P.