The Illinois management plan relies on the IDA’s groundwater monitoring well network and the Illinois EPA’s public water supply well pesticide-monitoring sub-network to determine the occurrence of pesticides in groundwater and whether there are significant, spatial or temporal trends in pesticide concentrations. The management plan requires action by the IDA when pesticides are reported at concentrations greater than 10 percent of the groundwater reference value (or the minimum reporting level (MRL) if 10 percent of the reference value is less than the MRL). If pesticides are present at concentrations greater than the ―action level‖ the IDA will conduct, with assistance from the Interagency Committee on Pesticides, the ICCG, the registrant, and other state and federal agencies, an evaluation to determine the appropriate course of action. At the very least, the presence of a pesticide in groundwater in concentrations greater than the action level would initiate a cause investigation. The components of the response plan in the Illinois Generic Management Plan for Pesticides in Groundwater that apply to the groundwater monitoring network are: Notify pesticide registrant; Identify cause; Perform vulnerability assessment and define response areas; Expand monitoring; Encourage adoption of voluntary best management practices; Impose use restrictions; and Prohibit use.
The Illinois Generic Management Plan for Pesticides in Groundwater targets areas where aquifer materials occur within 50 feet of land surface (Figure 11). These aquifers have been demonstrated to be vulnerable to contamination by pesticides as a result of labeled uses (Goetsch, Bicki and McKenna 1992; Schock and others 1992). As described by McKenna and Keefer (1991), the distinction between aquifer materials and aquifers is that aquifer materials have the hydrogeologic characteristics to be classified as aquifers, but the materials may not be saturated. Aquifers, as defined in the IGPA, are saturated. In Illinois, the water table generally occurs within 20 feet from ground surface. Below this depth, aquifer materials are generally saturated and capable of yielding water to a well. Sand and gravel greater than five feet thick, sandstone greater than 10 feet thick and fractured carbonates (limestone and dolomite) greater than 20 feet thick are considered to be aquifer materials. Loess, glacial till, shale, and non-fractured carbonate rocks have relatively low hydraulic conductivities and generally will not provide a sufficient volume of water to a drilled well and are not considered aquifer materials.
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