(CN) - Los Angeles County should not have been held liable for polluted water that flows down concrete-lined portions of the same rivers, the Supreme Court ruled Tuesday.
MS4 is the name given to the 2,800 miles of storm drains and 500 miles of open channels that carry storm water runoff in Los Angeles.
The system runs from the county's urban streets and parking lots to the rivers and the Pacific Ocean, picking up trash, metals, used oil, raw sewage and other contaminants along the way. It passes through numerous separate municipalities and receives runoff and discharges from thousands of unmonitored entities.
Studies have found a clear excess level of pollution in the Los Angeles, San Gabriel, Santa Clara and Malibu Creek Rivers, which drain into the Pacific Ocean at San Monica Bay, Los Angeles Harbor and Long Beach Harbor.
In 2008, the Natural Resources Defense Council and Santa Monica Baykeeper sued Los Angles County and the Los Angeles County Flood Control District for violating a National Pollutant Discharge Elimination System permit.
Since there is no comprehensive map of the Byzantine storm-drain system, however, courts have called it a "Sisyphean" task to assign blame for specific pollutants.
U.S. District Judge Howard Matz granted summary judgment to the county and the district, ruling that the plaintiffs had not provided enough evidence that the pollutants had been under the defendants' control.
The case went to the Supreme Court after a three-judge panel of the 9th Circuit found clear evidence that the county and district had polluted the Los Angeles and San Gabriel Rivers.
They agreed that the evidence related to the two other rivers was wanting.
In reversing Tuesday, the mostly unanimous high court noted that there is no debate that the flow of water out of a concrete channel within a river does not qualify as a "discharge of a pollutant," under the Clean Water Act, or CWA.
This finding aligns with precedent struck in 2004 with South Florida Water Management District v. Miccosukee Tribe.
Agreement between the parties today "is hardly surprising, for we held in Miccosukee that the transfer of polluted water between 'two parts of the same water body' does not constitute a discharge of pollutants under the CWA," according to the opinion Tuesday authored by Judge Ruth Bader Ginsburg.
Though the NRDC and Baykeeper agree with this holding, they argued that the 9th Circuit reached the right result for the wrong reason.
But the groups failed to show the lower court that "the exceedances detected at the instream monitoring stations are by themselves sufficient to establish the district's liability under the CWA for its upstream discharges," Ginsburg wrote.
"It is not embraced within, or even touched by, the narrow question on which we granted certiorari," she added. "We therefore do not address, and indicate no opinion on, the issue the NRDC and Baykeeper seek to substitute for the question we took up for review."
Judge Samuel Alito concurred in the judgment, while the rest of the court joined in Ginsburg's opinion.
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