Amendments made in 1972 to the Federal Water Pollution Control Act, which "establishes the basic structure for regulating discharges of pollutants into the waters of the United States and regulating quality standards for surface waters."

A public water supply that serves or is intended to serve at least 15 service connections used by residents or regularly serves at least 25 residents

Waters that are too polluted or otherwise degraded to meet the water quality standards set by states, territories, or authorized tribes. Under section 303(d) of the Clean Water Act, states, territories, and authorized tribes are required to develop lists of impaired waters.

The 303d List is a report on the water quality of Illinois lakes, rivers, and streams produced biennially by the Illinois Environmental Protection Agency (IEPA). It is named for section 303d of the Clean Water Act, and addresses requirements set forth in that section of the act in Appendix A. In addition, Appendix B of the report addresses water quality in relation to possible uses, corresponding to section 305(b) of the Clean Water Act.

A public water supply that is not a community water supply

"...all mains, pipes and structures through which water is obtained and distributed to the public, including wells and well structures, intakes and cribs, pumping stations, treatment plants, reservoirs, storage tanks and appurtenances, collectively or severally, actually used or intended for use for the purpose of furnishing water for drinking or general domestic use and which serve at least 15 service connections or which regularly serve at least 25 persons at least 60 days per year."