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Groundwater Monitoring Well Network

​​​The Illinois Department of Agriculture (IDOA), under authority of the Illinois Pesticide Act (415 ILCS 60/1 et seq.) and a performance partnership grant agreement with US EPA regarding the Federal Insecticide, Fungicide and Rodenticide Act, is the state lead agency for the regulation of pesticide use in Illinois. The IDOA is responsible for managing pesticide use to prevent adverse effects to human health and the environment.

US EPA's approach for addressing concerns about pesticides in groundwater is the nationwide regulation of pesticide use, supported by strong state and tribal roles in the local management of pesticide use to protect groundwater. Illinois, like many states, is voluntarily implementing the US EPA-recommended provisions of pesticide management plans to protect groundwater. In June 2000, under the leadership of the IDOA, the Pesticide Subcommittee of the ICCG approved the Illinois Generic Management Plan for Pesticides in Groundwater. The management plan, which was revised in 2006 (IDOA 2006), describes the framework to be used by the state of Illinois for addressing the risks of groundwater contamination by pesticides.

The Illinois management plan relies on the IDOA'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 IDOA 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 IDOA 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 an investigation of cause. 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 1). 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 5 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.

In 1995, the IDOA contracted with the ISGS and the ISWS to construct a statewide dedicated groundwater monitoring well network for use with future pesticide management plans. The monitoring well network is designed to provide statistically reliable estimates on the occurrence of selected pesticides in groundwater within shallow aquifers (depth to the top of aquifer material less than 50 feet below land surface) in areas of corn and soybean production. Occurrence is defined as the presence of a specific pesticide at a concentration above the MRL.

The network was designed to determine the regional impacts of pesticide leaching from non-point sources, not the impacts of site-specific point sources. The network is not a research program, but a tool for the management of pesticides in Illinois. Consequently, the pesticides selected as analytes are those with high use in Illinois that were previously detected in groundwater in Illinois or other Midwestern states. Also reflecting the management tool approach is the decision to set MRLs at a maximum of 5 percent of the groundwater reference value when possible, but not to expend limited laboratory resources on determining the presence of pesticides at very low concentrations. The monitoring well network and the IDOA's pesticide laboratory operate in compliance with U.S. EPA-approved quality assurance project plans.

The network currently consists of 141 shallow groundwater-monitoring wells located throughout the state (Figure 2) at well depths varying from 10 to 81.5 feet. Wells are constructed of 2-inch inside diameter polyvinyl chloride well casing. Most wells have a 5-foot long slotted well screen. All wells are located in public rights-of-way adjacent to row-crop fields, and are installed in areas where aquifer materials occur within 50 feet of land surface.

​Each well in the network is sampled once during a two-year period. The ISGS and ISWS conducted a one-time sampling of the network beginning in the fall of 1998 and sampled the network from September 2000 through June 2001. IDOA assumed responsibility for all sampling in July 2001.

Figure 2. Location of Illinois Dept. of Agriculture Dedicated Pesticide Monitoring Wells.


Sampling Results:

"M" Sampling Cycle (M1-M10, June 2023 - April 2024 with 4 events remaining)

Analyte MRL1 (ug/L)6 Frequency of Occurrence MCL2 (ug/L) Groundwater Reference Value (ug/L)3
Acetochlor 0.40 0% --- 1404
Atrazine 0.15 0% --- 3
Desethylatrazine (DEA) 0.15 3.8% 0.48 ---
Desisopropylatrazine (DIA) 0.15 0% --- ---
Desethyldesisopropylatrazine(DEDIA) 0.15 7.6% 0.48 ---
Metolachlor 0.15 3.8% 0.66 (estimated) 7005
Simazine 0.15 0% --- 4

1) Minimum reporting level.
2) Maximum contaminant level.
3) Reference values from Illinois Groundwater Standards 35 IAC 620.410.
4) USEPA lifetime health advisory (HA) level. An HA is an estimate of acceptable drinking water levels for a Chemical substance based on health effects information.
5) Illinois EPA non-TACO (Tiered Approach to Corrective Action Objectives) Class I Groundwater Objective
6) UG/L = micrograms per liter


Appendix I. References Cited by the Illinois Department of Agriculture

Goetsch, W.D., T. J. Bicki and D.P. McKenna. 1992. Statewide Survey for Agricultural Chemicals in Rural, Private Water-Supply Wells in Illinois. Illinois Department of Agriculture, Springfield, IL, 4 p.

Illinois Department of Agriculture. 2006. Illinois Generic Management Plan for Pesticides in Groundwater. Springfield, IL.,39 p.

Keefer, D.A. 1995. Potential for agricultural chemical contamination of aquifers in Illinois: 1995 Revision. Illinois State Geological Survey Environmental Geology 148, 28 p.

McKenna, D.P. and D.A. Keefer. 1991. Potential for Agricultural Chemical Contamination of Aquifers in Illinois. Illinois State Geological Survey Open File Series 1991-7R. 16 p.

Mehnert, E., D.A.Keefer, W.S. Dey, H.A.Wehrmann and S.D. Wilson, C. Ray, University of Hawaii. June 29, 2001. Illinois Statewide Monitoring Well Network for Pesticides in Shallow Groundwater- Network Development and Initial Sampling Results, Draft Final Contract Report. 55p.

Schock, S.C., E. Mehnert, M.E. Caughey, G.B. Dreher, W.S. Dey, S. Wilson, C. Ray, S.F.J. Chou, J. Valkenburg, J.M. Gosar, J.R. Karny, M.L. Barnhardt, W.F. Black, M.R. Brown, and V.J. Garcia. 1992. Pilot Study: Agricultural chemicals in rural, private wells in Illinois. Illinois State Geological Survey and Illinois State Water Survey Cooperative Groundwater Report 14, 80 p.