Nonpoint-source contamination of the principal aquifers in a 7800 km2 area of central Nebraska was evaluated utilizing aquifer condition, well depth, soil type, and physiographical and land use settings. A two-dimensional geographical information system linked with a three-dimensional geological visualization and analytical program was used in the random selection of acceptable wells for the monitoring of nitrate concentrations in groundwater. Locations of existing wells were super-imposed on the three-dimensional geological block diagram and more than 200 wells randomly were selected for monitoring. The three-dimensional system also was used to show three-dimensional contours of nitrate concentrations that can be used interactively to determine the volumetric percentage of an aquifer that contains nitrate concentrations exceeding a specified threshold. The two-dimensional geographical information system was used in comparing nitrate concentrations in differing physiographical, soil, and land use settings. Preliminary results suggest that approximately 6% (volumetric) of water in the High Plains aquifer has nitrate concentrations above the US Environmental Protection Agency Maximum Contaminant Level of 10 mg 1-1 as N.