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6.23.3 STORMWATER INFILTRATION AND DETENTION <br />As a watershed becomes more developed, the amount of impervious surface <br />increases, causing a decrease in the rate at which stormwater runoff which can <br />be absorbed into the soil. This results in more stormwater flowing directly into <br />streams and other water bodies. Because this direct runoff has not been <br />filtered through the soil, pollutants from the air and land surface enter streams <br />and increase the potential for pollution of drinking water supplies. <br />The effects of stormwater pollution on drinking water supplies can be minimized <br />by one of two general approaches. First, the amount of stormwater runoff <br />which reaches drinking water supplies can be controlled by assuring that there <br />is adequate ground area into which water can be absorbed. This type of non- <br />structural control is achieved through limitations on impervious surface. A <br />second approach is to collect stormwater run -off in engineered ponds so that <br />pollutants m ay s ettle. The water is then slowly released and contains fewer <br />impurities when it reaches the water supply reservoir. The watershed <br />protection standards adopted by Orange County utilize a combination of these <br />two approaches. <br />aj Non - Structural Stormwater Control <br />The first inch of stormwater run -off shall be controlled to the extent <br />possible through on -site infiltration and through the use of methods <br />which rely on natural soil properties for absorption and treatment. <br />In order to promote infiltration of stormwater runoff into the soil and <br />minimize direct and immediate runoff into streams and water supply <br />impoundments, the maximum percentage of the total lot area which <br />may be covered with an impervious surface shall be specified. This limit <br />is referred to as the "impervious surface ratio ". <br />Impervious surface calculations for an individual development shall be <br />cumulative for original construction and any subsequent additions. One - <br />half of the width of any of any existing or proposed road adjacent to an <br />individual lot shall be included as impervious surface for that lot, except <br />in the case where an existing road was contained within a dedicated <br />public right -of -way at the time that the watershed regulations were first <br />applied to the watershed within which the development is located. <br />Infiltration techniques not only remove both suspended and dissolved <br />pollutants, but they require less maintenance, reduce flooding, promote <br />groundwater recharge and help maintain stream flow during dry periods. <br />Where on -site infiltration methods are utilized, areas for such purposes <br />shall be designated on the plat and shall remain undisturbed both during <br />and after construction. Undisturbed areas for infiltration of run -off shall <br />also be located downslope from impervious surfaces and shall not <br />include areas characterized by floodplains, highly erodible or impervious <br />soils, steep slopes or previously disturbed areas. Areas designated as <br />suitable for septic tank nitrification fields may not be used for stormwater <br />infiltration purposes. <br />6 -28 <br />