Once upon a time, a South Carolina wastewater treatment plant repeatedly violated the Clean Water Act by dumping illegal amounts of mercury into a river. Unsurprisingly, several environmental organizations responded by suing. They could do so because the Clean Water Act contains "citizen suit" provisions that allow private citizens to sue for the law's violation. They won, and a trial court ordered the company to pay a fine to the government as a punishment. Pretty simple, right? But the question of whether the environmental groups had the right to sue went all the way to the Supreme Court. Why? The …