In accordance with Sections 305(b) and 303(d) of the Federal Clean Water Act (CWA), the Illinois EPA must report to the USEPA on the quality of Illinois surface water (e.g., lakes, streams, Lake Michigan, wetlands) and groundwater resources (Section 305(b) and provide a list of those waters where their designated uses are deemed "impaired" (Section 303(d). In addition, the Illinois EPA must assess the water quality of all publicly-owned lakes in accordance with section 314(a)(1). To aid in making these determinations, the Illinois EPA annually collects chemical, physical, biological, habitat, and toxicity data, depending on the type of water body. Data collected from outside sources may also be considered during this process.
Resource quality conditions are assessed in terms of the degree to which waters attain "beneficial uses", also called "designated uses." Pollution control programs are designed to protect designated individual uses, including aquatic life, indigenous aquatic life, primary contact (swimming), public and food processing water supply (drinking water), secondary contact, aesthetic quality, and fish consumption. Each state has the responsibility to set water quality standards that protects these uses. In Illinois, the Illinois Pollution Control Board is the regulatory body responsible for establishing water quality standards.
Results of more recent statewide assessments can be found in the following "Integrated Water Quality Report and Section 303d List" documents and Illinois Water Quality Mapping Tool. Previous assessments can be found in "Illinois Water Quality Reports" and associated "Condition of Illinois Water Resources" reports.