U.S. Supreme Court Reverses Ninth Circuit Ruling on Municipal Storm Water Discharges

Good news for owners and operators of municipal separate storm sewer systems, also known as “MS4s.”  In Los Angeles County Flood Control District v. Natural Resources Defense Council, the U.S. Supreme Court held that storm water flowing from an improved, channelized portion of a navigable waterway into an unimproved, natural portion of the same waterway does not qualify as a “discharge of a pollutant” under the Clean Water Act.  The Ninth Circuit had ruled to the contrary, that the flow of pollutants from channelized portions of two rivers into what it termed “naturally occurring portions of those rivers” was the point where a “discharge of a pollutant” occurred.  In a five-page opinion authored by Justice Ginsburg, the Supreme Court unanimously reversed the Ninth Circuit’s decision as wholly inconsistent with the rule announced in South Florida Water Management District v. Miccosukee Tribe of Indians, 541 U.S. 95 (2004), which held that the flow of polluted water between two parts of the same waterway does not constitute a “discharge of a pollutant” under the Clean Water Act.

The Los Angeles County Flood Control District (District) operates a massive MS4 that collects, channelizes, and transports storm water runoff through various rivers into the Pacific Ocean.  The runoff originates from 84 municipalities, which together with the District, are subject to a single Clean Water Act permit that governs municipal storm water discharges in Los Angeles County.  The Natural Resources Defense Council sued the District for allegedly violating its permit, because receiving water monitoring stations located within the Los Angeles and San Gabriel rivers demonstrated that pollutant levels exceeded regulatory levels. The monitoring data measured aggregate pollution originating from hundreds of sources located upstream of the MS4.  A central issue in the courts below was whether the data was sufficient to establish the District as solely liable for the aggregate pollution flowing down the rivers. The Ninth Circuit  held the District liable, and based its holding on the mistaken premise that the channelized portions of the rivers were distinct systems from the naturally occurring sections of the rivers themselves.

The Supreme Court declared that the Ninth Circuit got it wrong; under the Miccossukee rule, storm water flowing out of a concrete-lined channel into an unlined portion of the same river does not add any new pollutants to the river, and thus does not constitute a “discharge of a pollutant” under the Clean Water Act.  However, this narrow ruling left unanswered the larger question of how MS4 permits should be clarified to determine and apportion liability among multiple municipal permittees, especially when a single permit regulates hundreds of storm water outfalls that feed into the same MS4.