The project investigated regional impacts of mining on the ground water system of the Powder River Basin by (1) establishment of a regional ground water monitoring network and (2) development of a set of digital computer models to simulate ground water flow and quality, ground water contributions to surface water flow and quality (i.e., baseflow), and watershed runoff and recharge. Over 100 wells were monitored for physical and chemical parameters. Data were used as input to the models and for the development of a comprehensive data base. The basin was modeled on a regional and subregional scale. Two watershed models were evaluated for their ability to provide runoff and recharge inputs to the basic ground water model. Application of the regional and subregional models using conservative mining scenarios showed that drawdown may extend west from the mines as far as 10 miles but that impacts would be insignificant on a regional scale. The watershed model could be effectively applied to small watersheds only.