When Laidlaw Environmental Services, Inc. bought a wastewater treatment plant, it was granted a National Pollutant Discharge Elimination System (NPDES) permit that granted it permission to discharge treated water and "limited" pollutants. However, Laidlaw repeatedly released amounts of mercury into South Carolina's North Tyger River that exceeded those limits, and eventually, plaintiff-petitioners, Friends of the Earth (FOE) and others, filed a citizen suit under the Clean Water Act (which regulates the discharge of pollutants).
After the lawsuit began, Laidlaw began to comply with the permit and argued that the case was now moot (meaning "resolved") because the company had corrected its wrongdoing [sources: Oyez, EPA, U.S. Legal].
The Supreme Court wasn't having it. In the 7-2 opinion delivered by Ginsburg, the Court held that a case from a citizen for civil penalties doesn't have to be dismissed as "moot" just because the defendant begins complying with regulations after litigation has already begun.
In part, Ginsburg wrote, "a defendant's voluntary cessation of allegedly unlawful conduct ordinarily does not suffice to moot a case. Congress has found that civil penalties in the Clean Water Act cases do more than promote immediate compliance ... they also deter future violations." The Court also ruled that it was fair for FOE to sue on behalf of its members [sources: Justia, Oyez].