Previous state and national surveys conducted in the mid-continental USA have produced a wide range in results regarding the occurrence of agricultural chemicals in groundwater. At least some of these differences can be attributed to inconsistencies between the surveys, such as different analytical reporting limits. The US Geological Survey has designed a sampling network that is geographically and hydrogeologically representative of near-surface aquifers in the corn- and soybean-producing region of the midcontinental USA. More than 800 water quality samples have been collected from the network since 1991. Six of the seven most frequently detected compounds from this study were herbicide metabolites. A direct relation was determined between tritium content to herbicide and nitrate contamination. The unconsolidated aquifers sampled were found to be more susceptible to herbicide and nitrate contamination than the bedrock aquifers. Knowledge of the regional occurrence and distribution of agricultural chemicals acquired through the study of data collected at network sites will assist policy makers and planners with decisions regarding the protection of drinking-water supplies.