Sustainable Building Technical Manual - Etn-presco.net

CHAPTER 6Water Is s u e sWatershed Protection★ S I G N I F I CA N C E .AuthorsLucia Athens andBruce K. FergusonEvery building site is in a watershed, and everything people do on a site has animpact on the watershed’s condition. Sediment from soil disturbance, oil leaks,and fertilizers pollute streams; excessive runoff aggravates flooding and erosion;and deflection of rainwater from its natural paths dries out streams and wetlandsin summer.Watershed protection must occur both during and after construction. Clearing andearthmoving increase erosion by as much as 40,000 times the rate occurring inundisturbed sites.1 Many states and regions have legal requirements for erosion andsediment control. These laws have been supplemented by national standards forstormwater discharges that regulate all non-point-source pollution—water pollutionresulting from urban sources including, for example, nutrients from lawn fertilizersand hydrocarbons from highways and parking lots.After construction, any building development is physically a mosaic of roofs, pavements,and pervious soil areas. Every impervious surface deflects rainwater away from its naturalcourse—soil pores, native plants, and groundwater reservoirs—and into surface channels.Rainwater then concentrates into downstream floods, eroding as it goes. Carried with itare oils from cars, parking lots, maintenance yards, and storage areas; de-icing salts fromroads; metals from construction and industrial materials; and herbicides, pesticides, andnutrients from overmaintained landscapes. These substances can destroy aquatic life andpollute water supplies.In a protected watershed, soils absorb rain and make it part of the ecosystem. Pollutantsare transformed as they filter through porous, humus-rich soil. Soil moisture percolatesto the groundwater, which drains slowly out to streams long after the rain has fallen.