On remand from the United States Supreme Court, this case addressed whether pollution exceedances in the LA MS4 stormwater drainage system made the governing entities liable. The defendants, the County of Los Angeles (County) and the Los Angeles County Flood Control District (District), argued proof of individual entities discharging the pollutants was required under the Clean Water Act. Plaintiffs, Natural Resources Defense Council (NRDC) and Santa Monica Baykeeper, argued the defendants violated the National Pollutant Discharge Elimination System (NPDES) permit and were therefore liable, despite lack of data determining the County’s portion of contribution to exceedances. The Ninth Circuit Court held violation of the permit was sufficient to establish liability as a matter of law. Compliance requires each permittee who contributes to exceedances mitigate their own discharges to avoid liability.