samples (7.9 percent). Atrazine was detected in five samples, metolachlor was detected in three samples, and acetochlor and simazine were each detected in one sample. Three of those samples had concentrations above levels of concern. One or more of the atrazine degradation products was present above the MRL in 19.0 percent of the samples. One or more of the metabolites of the chloroacetanlide herbicides was detected in 53.8 percent of the samples. None of those samples had concentrations above levels of concern.
Pesticides are more likely to be present in monitoring wells in areas where aquifer materials occur within 20 feet of the land surface than in areas where aquifer materials are more than 20 feet below land surface (Figure 13).
Figure 13. Comparison of the Effect of Depth to Aquifer Materials on the Frequency of Occurrence of Parent Pesticides and Metabolites (2008-2010)
The Illinois Generic Management Plan for Pesticides in Groundwater requires the IDA to conduct an investigation of the cause if pesticides are detected at concentrations greater than the action level. If selected, pesticides for which preventive notification is required under the IGPA are detected in groundwater, (e.g., atrazine) the IDA is required to resample the well within 30 days of receipt of laboratory results. Since U.S. EPA concurrence with the Pesticide Management Plan in February 2001, four monitoring well samples have contained a pesticide at a concentration greater than the action level (atrazine parent compound and simazine). The IDA re-sampled the wells and found the concentrations to be less than the action level. The IDA also notified the registrant and conferred with the IDPH. IDA intends to continue to follow the sampling and analysis plan laid out in the generic management plan and the quality assurance project plan for the foreseeable future.
FREQUENCY OF OCCURRENCE
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