Agenda - 06-04-1981 - Laserfiche WebLink

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rzv the most developed of the three water supply watersheds in the county sitting as it does just west of Carrboro-Chapel Hill. Its 19,600 acre watershed and no sewage treatment plants, landfills or indus- :- trial uses are located there. The Cane Creek Upper Eno watershed are likewise rural in nature and free from potential concentrated sources of pollution. The water drawn from these areas is thus of good quality. The sources of pollution which have been discussed so far, like sewage treatment plant outfalls and landfills are called "point sources" of pollution. Point source refers to pollutants from a specific source which would enter the water at an identifiable point. An example would be the outfall pipe fram a sewer treatment plant. Another source of pollutants are called"non-pointm sources. Non-point source refers to pollutants which reach receiving streams in a diffuse manner and cannot be attributed to an identifiable or specific source. An ex- ample would be the pollutants that accumulate on a shopping center parking lot or road surface and are washed off in a storm. Studies around the nation have revealed thattgon-point sources of pollution are equally ,if not more important ,contributors to poor water qual- ity than point sources. These same studies reveal that nonpoint source pollution generation is a function of land use. Areas which are mostly forested have the lowest amounts of pollutants in the water that runs off the land. There are two reasons for this. The forested areas lack urban ac- tivities like concentrated automobile use which can produce pollut- ants. They also have the ability to assimilate pollutants that are present. Only a small percentage of precipitation which falls washes off the land into the nearest stream. The ground cover in forested areas slows the velocity of runoff and allows water to infiltrate into the soil where bacterial and chemical processes can change or trap pollutants. As a watershed becomes increasingly urbanized this situation changes dramatically. All the activities of people which can generate pollut- ants are increasingly present in the watershed. And the natural processes of the land and soil which can act to block pollutants from reaching the water supply are greatly reduced in their ability to trap pollutants. One characteristic common to developing areas is that the percentage of impervious cover is increased as development increases. More precip- itation strikes roads and parking areas, the roofs of homes and other impervious surfaces. The increased impervious area alters the relative magnitudes of runoff and infiltration. A greater volume of water runs off the land at an increased velocity. It is carried quickly via natural drainage ways or storm drainage systems,The process of infil- tration and soil purification does not have an opportunity to function). As a result,studies have shown that the water quality of streams flowing through highly urban areas is very poor even where no point sources of pollution can be identified.