11.5 Sustainable stormwater management  (Page 3/26)

Regulating urban runoff

Water pollution risks within watersheds are managed under the federal Clean Water Act , which requires state environmental protection agencies to regulate the discharge of pollutants into navigable waterways and waterbodies pursuant to federal guidelines ( NRC, 2008 ). The Clean Water Act employs maximum concentration standards for common pollutants that can impair the recreational or ecological functions of a river or stream. One class of polluters regulated under the Clean Water Act consists of those that are directly discharging pollutants into a waterway from an industry or sewage treatment plant through a pipe, ditch, outfall or culvert – these are called point sources .

Point sourcesare managed under the Clean Water Act by the requirement that each direct source have a renewable discharge permit, called a National Pollution Discharge Elimination System (NPDES) permit. NPDES permits set limits for the various pollutants being discharged by that source based on the ambient water quality    of the waterway and its proposed use (e.g. its use as a public water supply source, or for fishing, or recreational use). The other regulated class of polluters managed under the Clean Water Act consists of those sources that introduce contaminants into a waterway through overland or subsurface flow – these are called non-point sources , and include most of the water pollution loads carried by urban stormwater runoff.

Since the 1970s, the principal approach used by state and federal environmental protection agencies to control water pollution is to try to simply reduce the quantity of pollutants being released into our rivers and streams ( NRC, 2008 ). NPDES permits control the direct discharge of contaminants into our waterways, while non-point sources are managed through Best Management Practices (BMPs) that are designed to limit the amount of pollutants released into a watershed, where they could later be carried by stormwater runoff or by groundwater flow to a receiving stream or river. Depending on the pollutant of concern, BMPs could be as simple as requiring pet owners to clean up after their pets or as complex as requiring that industries using toxic materials design, construct and manage loading and storage areas in order to keep spilled materials from being transported off-site by stormwater or groundwater flow. BMPs can even include encouraging some industries to change their production processes in order to reduce the total amount of toxic materials they use, a pollutant reduction strategy known as pollution prevention    (since the fewer toxics used, the lower the risk that they will inadvertently be released into the environment).

The strategy of simply reducing the amount of pollutants entering the environment is complicated by the fact that many of the non-point pollutants are not amenable to management through local BMPs. For example, agricultural activities are expressly exempted from the Clean Water Act, even though stormwater runoff from farms and animal feedlots can carry agricultural chemicals, fertilizers and manure into adjacent waterways, along with topsoil from freshly-plowed fields. Pollutants could also be introduced into an urban watershed by the deposition of air pollutants. Airborn particulate matter, for example, can be transported very long distances by the wind, making most locally administered BMPs (except possibly instituting regular street-sweeping programs) ineffective in reducing the distribution and quantities of these types of urban stormwater pollutants.