The Illinois Generic Management Plan for Pesticides in Groundwater addressed the management of pesticides for the protection of groundwater quality and relies heavily on the use of a statewide map of aquifer sensitivity to contamination by pesticide leaching. This map combined select soil properties (hydraulic conductivity, the amount of organic matter of individual soil layers, and drainage class) with information on the depth to uppermost aquifer material. Prior to the implementation of this plan, a dedicated monitoring well network was installed to test the aquifer sensitivity map. A stratified random sampling plan was used to distribute the monitoring wells. All monitoring wells were located near agricultural production fields (primarily corn and soybeans) where the only known source of pesticides were those used in normal agricultural production.

Sampling of this monitoring well network included a one-time sampling program (159 wells sampled from September 1998 through February 2001), to assess the distribution of pesticide occurrence over the various units of aquifer sensitivity, and a time-series sampling program (215 samples were collected from 21 wells from October 1997 through July 2000), to provide data on the temporal variability of pesticides in shallow groundwater.

Results from both sampling programs suggested that pesticide occurrence was not dependent on the aquifer sensitivity mapping units, but was dependent on just the depth to uppermost aquifer material. Pesticide occurrence was three times more common in samples from shallow (<20 ft) aquifers than those from deeper (20 to 50 ft) aquifers. The combined occurrence of pesticides and nitrate-nitrogen above 10 mg/L was dependent on the aquifer sensitivity mapping units. Pesticide occurrence was generally dependent on sampling time. Post-application (June through October) versus other time (November through April) frames was the strongest temporal relationship. Occurrence during post-application months were 2.5 to 3 times higher than occurrence during other months. One possible explanation for higher occurrence in the post-application period is that precipitation flushes the newly applied pesticides into the shallow aquifer.