Ambient Monitoring

The Illinois EPA continues to operate an Ambient Network of Community Water Supply Wells (CWS Network) consisting of 357 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 all of the nearly 3,000 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).

As a result of funding limitations, the Illinois EPA was forced to evaluate the CWS Network monitoring frequency. During the 1997 monitoring cycle, the Illinois EPA initiated a rotating monitoring network program. 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 sampling for inorganic chemicals and volatile organic compounds during each visit and synthetic organic compounds during every other visit.

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