It is well known that stormwater gathers a variety of pollutants as it flows across the landscape and even through stormwater drainage systems. See, for example, http://cfpub.epa.gov/npdes/home.cfm?program_id=6 and http://www.epa.gov/owow_keep/NPS/index.html.
In NRDC v. County of Los Angeles, Ninth Circuit, Docket 10-56017 (3/10/11), the Ninth Circuit held that the CWA is indifferent to the entity that originates pollutants that enter stormwater. Instead, the CWA focuses on entities responsible for discharges.
In this case urban stormwater flowed across the landscape and through the stormwater collection system. In the course of its travels, the stormwater picked up sediments, suspended metals, nitrogen, phosphorus, trash, used motor oil, pesticides, raw sewage, and "other toxic contaminants". The County operated a number of monitoring stations per MS4 that detected violations of relevant standards, but did not treat the stormwater. Instead, the County allowed the water to flow into rivers and thence to the ocean. The County argued that they were not responsible for the contaminants.
The Court held that the CWA holds liable any entity that channels and causes contaminated stormwater to be released into, for example, rivers. The County was found liable because it controlled the "point source" of the standards-exceeding polluted water that was released into rivers, and thence continued on to the ocean.
Because the plaintiff did not offer evidence as to what entity, if any, controlled certain rivers, the District Court ruling in favor of County stands as to those bodies of water. However, the 9th Cir. opened the door for the plaintiffs to show responsibility for these conveyances also.
If this opinion stands, then local governmental entities will have to consider how to treat contaminated stormwater before it is released.