The U.S. Supreme Court on Monday took action that leaves Los Angeles County responsible for cleaning up stormwater pollution that flows down the Los Angeles and San Gabriel Rivers.

The court decided not to hear an appeal by the county and its Flood Control District in a dispute that had meandered for six years through federal district and appellate courts; the action lets stand a ruling from the Ninth Circuit Court of Appeals that found that county authorities had violated the Clean Water Act by allowing polluted runoff to foul waterways.

In a statement, county officials called the Supreme Court's decision “disturbing" and said it assigns liability without considering the sources of the pollution and without evidence of violations.

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Officials also called the decision costly. “This could force municipalities to redirect limited public funds from other critical services to spend on controlling pollution from private and other sources who are the responsible parties," said Gail Farber, chief engineer for the Flood Control District and director of the Los Angeles County Public Works Department.

County lawyers had argued the county was not responsible for the polluted runoff because it comes from multiple sources and various property owners and cities upstream.

Two plaintiffs in the case, the Natural Resources Defense Council and Los Angeles Waterkeeper, say the resolution of the liability question is a major victory.

“No longer can the county ignore the law,” said Liz Crosson, executive director of Los Angeles Waterkeeper, in a release.

Monday's action is not the end of the legal battle. A federal district judge will now hear evidence on the best way to clean up pollution in the county's storm system. The trial is expected to pit experts against experts on technical subjects like pollution monitoring technology, stormwater captures and how water percolates though and is filtered by topsoil.

“Los Angeles County needs to face the facts and get to work to clean up its mess," said Steve Fleischli, senior attorney with the Natural Resources Defense Council. “With a trove of 21st century green infrastructure solutions, the county has no excuse not to ensure this pollution is addressed immediately.”

For now, current pollution levels are likely to remain high. Funding sources for new pollution controls range from federal funds to state bonds to local grants, but securing that money can be challenging.

Regional water quality officials have updated the permit under which the county operates its stormwater system. Administered by the Regional Water Quality Control Board, the permit sets limits on individual pollutants in stormwater and establishes the locations at which to monitor water quality.

The new permit is under appeal to the State Water Quality Control Board, both by environmental groups and by more than three dozen cities.

Local authorities — including the City of Los Angeles — have also made new efforts to capture stormwater closer to where it falls to filter or treat it and then reuse it. L.A.’s “low impact development” ordinance promotes rain barrels and parking lots that permit water to pass into the ground.

County water managers proposed a stormwater parcel tax late in 2012. In March of last year, the county Board of Supervisors sent the proposal back to the drawing board after fielding public criticism about the size of the levy, the way it would be calculated and the transparency for how it would be spent.