Contributors: J.A. Arnold, S.W. Coffey, D.E. Line, and J. Spooner, Extension Biological & Agricultural Engineering Specialists, and D.W. Moody, U.S. Geological Survey
Sources. Older, highly developed residential areas often contribute more organic matter especially if the area contains a large pet population. Leaking sewers and malfunctioning septic tanks also add organic matter to stormwater. Low-density developments, on the other hand, contribute lower amounts of organic matter to stormwater. Industrial areas may also contribute significant amounts of organic matter, especially if those industries do not practice "good housekeeping," such as picking up debris on their site and periodic sweeping of parking lots and entrance roads.
Impacts. Most organic matter occurs in the form of large solid pieces such as leaves, sticks and animal wastes. The large pieces quickly settle out of the runoff, but the substances that dissolve from the pieces present a more serious problem. These dissolved substances serve as food for bacteria and other organisms which not only multiply, but consume oxygen, which is vital for other living things. If this consumption reduces the oxygen to very low levels, the water may become foul, producing strong odors, and the number and mixture of aquatic species will change.
Indicator parameters. Slimy growths, brownish or gray color, and strong odors may indicate the presence of dissolved organic matter in the water. Mats of leaves, papers, sticks and general refuse indicate excessive organic matter is getting into the water. A "rotten egg" smell also indicates a high concentration of organic matter in the water.
Source reduction. Leaves, grass clippings and other such yard wastes should be removed from the street gutters and used in compost piles. Litter and household garbage should be disposed of properly and removed from streets and gutters where such matter can enter the stormwater. Good housekeeping will help prevent problems with organic matter.
Removal. Most organic matter, such as street litter and leaves, can be removed by settling in dry detention basins and wet retention ponds. Substances that dissolve from the organic matter may be removed through biological breakdown by bacteria and aquatic plants in wet retention ponds. Infiltration techniques also are effective in removing dissolved organic substances as the biological processes in the soil will consume the substances.
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