Urban Runoff

Urban runoff is stormwater that washes pollutants off impervious city surfaces, roads, roofs, and parking lots, into coastal and inland waters, a leading diffuse pollution source. It carries faecal indicator bacteria, nutrients, oil and grease, heavy metals, sediment, and litter, with the heaviest load in the first-flush of a storm. Impervious cover raises peak flows and short-circuits infiltration, so even small storms deliver pulses that breach bathing-water bacteria standards and feed nutrient loading. In the United States, municipal separate storm sewer systems are permitted as point sources under the NPDES MS4 program. Green infrastructure, detention basins, and bioretention cut volumes and loads at source.