Watershed-scale response to agricultural diffuse pollution control programs in Vermont, USA

Abstract

From 1979 to 1990, the LaPlatte River Watershed and the St. Albans Bay Watershed Rural Clean Water Program projects in Vermont (USA) sought to reduce sediment, nutrient, and bacteria loads to parts of Lake Champlain impaired by eutrophication. Best Management Practices (BMPs) to control diffuse sources of pollution from dairy agriculture were widely implemented through a voluntary program of technical assistance and cost-sharing by agencies of the U.S. Department of Agriculture. Intensive water quality monitoring was undertaken to document water quality changes resulting from the land treatment programs, including studies of BMP effectiveness and long-term watershed-scale trend monitoring. Some BMPs significantly reduced edge-of-field pollutant delivery to surface waters. Phosphorus export from corn fields was up to 1500% higher where manure was winter spread and up to 15% of the phosphorus applied in winter-spread manure was lost in runoff. A vegetated filter strip retained more than 90% of sediment and nutrients in milking center waste and functioned effectively year-round. Watershed-level response, however, was not simply the sum of edge-of-field changes. Sediment concentration and export decreased in both project areas, but anticipated decreases in nutrient concentrations and loads did not occur. The most significant water quality trends observed were 50–75% reductions in indicator bacteria counts in all study watersheds. Factors contributing to the lack of general response in nutrient levels, recommendations for future agricultural pollution control projects, and implications for planning of diffuse source pollution control programs are discussed.