herbicides was detected in 58.5 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 7).
Figure 7. Comparison of the Effect of Depth to Aquifer Materials on the Frequency of Occurrence of Parent Pesticides and Metabolites
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, only one monitoring well sample contained a pesticide at a concentration greater than the action level (atrazine parent compound). The IDA immediately re-sampled the well and found the atrazine concentration to be less than the action level. The IDA also notified the registrant and conferred with the IDPH, which agreed that no further action was necessary.
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. If current trends in the occurrence of pesticides continue, some adjustments to the sampling plan may be considered.
FREQUENCY OF OCCURRENCE
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