Sources. All urban and developing areas contribute bacteria to stormwater, with older and more intensively developed areas contributing higher numbers. Urban areas have large populations of pets, generate much garbage and other wastes, and have significant numbers of animals such as pigeons. squirrels and mice, all of which contribute bacteria to stormwater runoff. Leaking sewers and sewer overflows, which occur more frequently in older urban areas, are especially strong sources of bacteria, as is leachate from malfunctioning septic tanks.

Impacts. Receiving waters can be contaminated with bacteria to the point where recreation and public water supplies are impaired. Swimming, fishing and shell fishing areas can be closed if high concentrations of bacteria are present. Operators of public water systems may be required to install more expensive treatments or even obtain other sources of water to meet public health requirements.

Bacteria tend to associate with soil particles and settle out of the water, but can accumulate and concentrate in deposited sediments, presenting a threat to health and safety if the sediments are stirred up.

Indicator parameters. Foul odors and slimy growths in channels, pipes, streams and in quiet backwaters and bays of lakes may indicate the presence of bacteria. If bacteria are present in very high concentrations, the water will be gray and cloudy.

Source reduction. Leaking sanitary sewers and overflowing combined sewers should be repaired or rebuilt to prevent sewage from entering stormwater. Animal wastes should be removed from street gutters to prevent their being carried into receiving waters with the next rain. Garbage and other refuse should be covered to prevent exposure to rain, and contaminated water from garbage cans and dumpsters should be discharged to a sanitary sewer.

Removal. Bacteria can be removed by settling in dry detention basins and wet retention ponds. Sand filters are effective in reducing bacterial concentrations in runoff by their filtering action. Infiltration techniques also remove bacteria, as the bacteria are captured on soil particles as water moves through the soil.

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