Evaluation of agricultural best-management practices in the Conestoga River headwaters, Pennsylvania: A summary report, 1982-90

Abstract

The effects of selected agricultural best-management practices (BMP's) on surface-water and ground-water quality in the Conestoga River Headwaters, Pa., were investigated by the U.S. Geological Survey. This 9-year study was part of the Rural Clean Water Program and was done in cooperation with the Pennsylvania Department of Environmental Protection and the U.S. Department of Agriculture, Consolidated Farm Service Agency. Surface water and ground water were characterized at three scales-regional (188 square miles), small watershed (5 square miles), and field (22 and 47.5 acres). At the small watershed and field scales, water-quantity and -quality data collected during the characterization phase were compared with similar data collected after implementation of BMP's. Changes in water quantity and quality were evaluated in conjunction with agricultural-activity and precipitation data to evaluate the effects of implementation of BMP's on water resources in the study area. Water quality in the southern one-third of the Conestoga River Headwaters Basin reflects the carbonate mineralogy. In carbonate parts of the study area, water and associated contaminants move rapidly from the land surface to the ground water through highly permeable soils and fractured bedrock. Changes in ground-water level and chemistry were frequently found within 1 day of the onset of precipitation. Elevated concentrations of sediment, nutrients, and herbicides in surface water and ground water reflect the intensive agricultural land use in the carbonate valleys. In contrast, the northern two-thirds of the basin is underlain by noncarbonate rock, and only a relatively small amount of the land is used for agriculture. Concentrations of nutrients and herbicides were substantially lower in surface water and ground water in the noncarbonate parts of the study area. Pipe-outlet terracing, installed at a 22-acre field site underlain by carbonate rock, was effective in reducing sediment losses from the site, but total nitrogen and phosphorus losses with runoff were not significantly different before and after terracing. Although no measurable overall change in the relative amounts of runoff and recharge resulted from terracing, median concentrations of dissolved nitrate at four of six ground-water sampling locations increased after terrace installation. Nutrient applications were reduced at a 47.5-acre field site and a 1.4-square-mile subbasin of the small watershed after implementation of nutrient-management practices. At the field site, where discharge of dissolved nitrate in ground water was 98 percent of the total nitrogen leaving the site with water, dissolved nitrate concentrations in ground water from most of the sampled wells decreased significantly after implementation of nutrient management. In the 1.4-square-mile subbasin, nutrient management was beneficial in preventing increased concentration of dissolved nitrate plus nitrite in the base flow of streams in the subbasin.