Los Angeles County is likely to appeal to the Supreme Court after the U.S. Court of Appeals for the 9th Circuit denied a rehearing of Natural Resources Defense Council, et al. v. County of Los Angeles, et al. on July 13. The landmark appellate ruling, made in March, stated that cities with municipal separate storm sewer (MS4) permits are strictly liable for all stormwater runoff pollution at their monitoring stations, regardless of whether the pollutant originated outside the MS4 system. Even if the municipality is working to address known violations—an iterative process—penalties may be incurred.

The lawsuit was brought against the county because the county’s flood control district exceeded the pollutant limits of its MS4 permit. However, the county claims that upstream neighbors are responsible for some of the pollutant loads. There are concerns that this ruling could lead to litigation between municipalities regarding who is responsible for stormwater pollutants.