Groundwater Quality Protection Program

Ambient Groundwater Monitoring

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Ambient Network of Community Water Supply Wells - The Illinois EPA continues to operate an Ambient Network of Community Water Supply Wells (CWS Network) consisting of 362 fixed locations. The CWS Network is designed to:

• Provide an overview of the groundwater conditions in the CWS Wells in Illinois;
• Provide an overview of the groundwater conditions in the major aquifers in Illinois;
• Establish baselines of water quality within the major aquifers in Illinois;
• Identify trends in groundwater quality in the major aquifers in Illinois; and
• Evaluate the long term effectiveness of Clean, and Safe Drinking Water Acts program activities in protecting groundwater in Illinois.

Since assessing the 2,988 active CWS wells was not practical, the CWS Network design was based upon the sampling of existing CWS wells and incorporates a random probability based scheme (95 % percent confidence, plus or minus 5 percent precision and accuracy) which is randomly stratified by aquifer type, geologic susceptibility and well depth. This approach maximizes the ability of the network to characterize changes in groundwater conditions and minimizes the fiscal requirements. The use of existing wells is less costly than installing wells specifically designed for monitoring and also facilitates the detection of long-term groundwater degradation by yielding data which can be compared with historic records of groundwater quality analyses compiled over decades. The "principal aquifers" used in Illinois were classified by O'Hearn and Schock in 1984 into three basic categories: sand and gravel, shallow bedrock, and deep bedrock. A principal aquifer is defined as an aquifer with a potential yield of 100,000 gallons per day per square mile and has an area of at least 50 square miles (O'Hearn and Schock, 1984).

The map below illustrates an example on the ambient conditions relative to CWS using unconfined aquifers:

Pesticide Monitoring Subnetwork of the CWS Network

Since 1993, the Illinois EPA has operated a Pesticide Monitoring Subnetwork of the CWS Network. Initially, Illinois EPA tested all wells in the CWS Network for triazine and alachlor using immunoassay screening methods. However, in the 1998 monitoring cycle Illinois EPA discontinued the use of immunoassay and randomly selected 50 percent of the network wells which were then analyzed for synthetic organic chemicals (SOCs) using standard laboratory test methods. In the year 2000 monitoring cycle, the remainder of the wells in the network will be analyzed for SOCs. The Illinois EPA anticipates that this rotation will be maintained in the future.

Rotating Monitoring Network

The purpose of this monitoring network is to maximize resources and increase groundwater quality monitoring coverage at CWS wells. During the 1997 monitoring cycle, the Illinois EPA initiated a rotating monitoring network program. As a result of funding limitations, the Illinois EPA was forced to evaluate the CWS Network monitoring frequency. Illinois EPA determined that the primary purposes of the CWS Network referred to above, could be realized by reducing the monitoring frequency of the network to a biennial basis.

The Illinois EPA is currently able to concentrate on specialized monitoring at high priority areas during alternate years. In 1997, monitoring was focused on concerns related to highly susceptible CWS wells. These wells were prioritized as a result of the detections of organic contaminants in treated water samples obtained during routine monitoring required by the Safe Drinking Water Act. During the 1999 monitoring cycle, attention focused on "new" CWS wells with little monitoring history.

• Ambient Groundwater Monitoring Networking Fact Sheet

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