The occurrence of agricultural chemicals in Illinois' rural private wells: results from the pilot study

Water samples from 240 private wells in rural Illinois were collected over one year and analyzed for 39 agricultural chemicals. Sampling was conducted to provide preliminary information to refine a plan for a statewide survey of the agricultural chemical contamination of rural private wells. Wells were sampled according to a stratified random sampling plan that included four classes of depth to the uppermost aquifer material and two classes of well type. Depth to uppermost aquifer material was defined as the depth from ground surface to a geologic material that, if saturated, could be used as an aquifer. Occurrence, defined as the presence of one or more target analytes in a well water sample above some specified concentration, was shown to be higher in large-diameter bored or dug wells than in small-diameter drilled wells. For small-diameter wells, occurrence generally decreased as the depth to the uppermost aquifer material increased. In addition, depth to the uppermost aquifer material could be used to predict the occurrence of some individual agricultural chemicals, such as nitrate and atrazine, but could not be used to predict the occurrence of picloram or pesticides in small-diameter wells. Of the 39 target analytes, 10 were detected at concentrations exceeding their respective minimum reporting levels. Nitrate and atrazine were the only compounds found at concentrations exceeding their respective maximum contaminant levels (MCLs) or U.S. EPA lifetime health advisory limits (HALs). A nonparametric statistical technique, contingency table analysis, identified factors associated with the occurrence of agricultural chemicals in three of the five study areas. Elevated specific conductance (greater than or equal to 500 micromhos/cm) of the sampled water was strongly associated with the occurrence of agricultural chemicals. This association was common to all three study areas analyzed