Milwaukee v. Illinois (Milwaukee II)

Illinois (plaintiff) sought to prevent Milwaukee (defendant) from allegedly discharging some 200 million gallons of raw sewage daily into Lake Michigan. Congress had enacted the Federal Water Pollution Control Act of 1948 over 20 years earlier, but the act proved inadequate. In Milwaukee I, the Supreme Court ruled that the 1948 act could not provide relief but Illinois could pursue a federal common-law nuisance action. Illinois promptly sued in federal district court. Five months later, Congress passed the Federal Water Pollution Control Act Amendments of 1972, now known as the Clean Water Act (CWA). The CWA established a comprehensive system of regulation that prohibits discharging pollutants into water without a permit. Milwaukee obtained permits without bringing its treatment plants into compliance. An enforcement action required Milwaukee to comply with effluent limits and improve its plants to control overflows. Meanwhile, Illinois’s federal nuisance action proceeded to trial. The federal district court found Illinois had established a nuisance action under federal common law and ordered Milwaukee to eliminate overflows and comply with more stringent effluent limits. The Seventh Circuit affirmed that the CWA had not preempted Illinois’s federal common-law nuisance claim but reversed to the extent that the effluent limits the court imposed were more stringent than the permits and Environmental Protection Agency (EPA) regulations. The Supreme Court granted review.