The U.S. Geological Survey and the Pennsylvania Department of Environmental Protection conducted a study from 1984 to 1990 to determine theeffects of the implementation and practice of nutrient management [an agricultural best-management practice (BMP)] on the quality of surface runoff and ground water at a 55-acre crop and livestock farm in carbonate terrain nearEphrata, Pa. Implementation of nutrient management at Field-Site 2 resulted in application decreases of 33 percent for nitrogen and 29 percent for phosphorus. There wereno significant changes in nitrogen or phosphorusloads for a given amount of runoff from the pre-BMP to the post-BMP periods. However, less than 2 percent of the applied nutrients weredischarged with runoff throughout the study period.After the implementation of nutrient management, statistically significant decreases in concentrations of nitrate in ground-water samples occurred at threeof the four wells monitored throughout the pre- and post-BMP periods. The largest decreases in nitrate concentrations occurred at wells where samples hadthe largest nitrate concentrations prior to nutrient management. Changes in nitrogen applications to the contributing areas of five wells were correlated with nitrate concentrations of the well water. The correlations between the timing and amount of applied nitrogen and changes in ground-water quality met the four conditions that are characteristic of a cause-effect relation: an association, consistency, responsiveness, and a mechanism. Changes in ground-water nitrate concentrations lagged behind changes in loading of nitrogen fertilizers (primarily manure) by approximately 4 to 19 months.