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 fora st atewide survey of the agricultural chemical contamination of rural pri vate wells. Wells were sampled according to a stratified random sampli ng plan that included four classes of depth to the uppermost aquifer m aterial and two classes of well type. Depth to uppermost aquifer mater ial was defined as the depth from ground surface to a geologic materia l 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-d iameter bored or dug wells than in small-diameter drilled wells. For s mall-diameter wells, occurrence generally decreased as the depth to th e uppermost aquifer material increased. In addition, depth to the uppe rmost 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 a t concentrations exceeding their respective minimum reporting levels. Nitrate and atrazine were the only compounds found at concentrations e xceeding their respective maximum contaminant levels (MCLs) or U.S. EP A lifetime health advisory limits (HALs). A nonparametric statistical technique, contingency table analysis, identified factors associated w ith the occurrence of agricultural chemicals in three of the five stud y areas. Elevated specific conductance (greater than or equal to 500 m u mhos/cm) of the sampled water was strongly associated with the occur rence of agricultural chemicals. This association was common to all th ree study areas analyzed. Identification of the source of the specific conductance could help identify the dominant pathway for transport of agricultural chemicals to ground water.