The Iowa Statewide Rural Well Water Survey (SWRL) was conducted between April 1988 and June 1989. SWRL was designed to provide a statistically valid assessment of the proportion of private rural wells and rural Iowa residents affected by various environmental contaminants. The survey was a systematic sample, stratified by rural population density. Approximately 14% of wells had detections of pesticides: 16 pesticide compounds (mostly herbicides) were detected, including 11 parent compounds and 5 environmental metabolites; 16 pesticides (mostly insecticides) were not detected. Atrazine and its metabolites were found in 8% of wells. Multiple residues were detected in all regions of the state. The mean concentrations were generally <1 μg/L. Lifetime Health Advisory Levels (HALs) were exceeded in 1.2% of private, rural wells in Iowa. Detailed evaluations of these sites indicate 25% are caused by point source contamination (spill and back-siphoning), while the majority, 62.5%, appear to be nonpoint sources related to normal agricultural practices. Statistical analyses show significant associations between many water quality parameters tested during SWRL, but the associations are not strong predictors based on state-wide data. By far the most significant factor explaining water quality variations is well depth.