Where downward vertical hydraulic gradients exist, the travel time and the zones of contribution to various monitoring wells was predicted, indicating that the zones of contribution to many wells are very small. Small zones of contribution were also indicated by water chemistry similarities between groundwater and nearby standing surface water and a septic field. Time of travel predictions and tritium analyses indicate that water at many contaminated wells may be a decade or more old. Concentrations of atrazine and desethylated atrazine decrease with time spent in the aquifer. Although some evidence of point-source pollution exists, most groundwater residues detected probably result from normal field applications.
Contaminant distribution reflects pesticide application practices by the three farmers in the area.
If the distribution and extent of contamination at this site is typical of other areas of Wisconsin, revising groundwater standards for total atrazine to include metabolite concentrations will lead to a dramatic increase in the number of groundwater standard violations due to the widespread occurrence of desethylated atrazine in groundwater at concentrations most often exceeding those of the parent compound. Regulatory agencies should be prepared to take appropriate action. It may be many years before the effect of reduced application rates or atrazine bans will be noticeable. The efficacy of atrazine use restrictions may not be measurable in the first few years after policy implementation. The domestic well at the center of this site was probably contaminated in part from pesticide handling near the well.