We present a rapid online decision support tool capable of predicting long term, high resolution spatio-temporally distributed well contamination from spatio-temporally distributed nonpoint sources. The online tool efficiently computes several hundred years of concentration breakthrough at receiving wells and streams. The tool has been validated for the Central Valley, California, with over 20,000 production wells and 150,000 domestic wells. The tool is able to efficiently evaluate historic and alternative land management scenarios and their effect on nitrate in well water.