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IMPACTS: Surface Water Residential developments typically change runoff characteristics in various ways, two of which are to increase the volume of runoff and to increase the magnitude of peak flows. Generally, the volume of runoff is related to infiltration characteristics of the soil , slope of the land, and the type and amount of vegetative cover. The amount of runoff is increased when project improvements decrease infiltration or increase runoff efficiency. As hydrologic efficiency of runoff is increased, so are peak flows from storms due to decreased duration of flows. Typically, residential projects involve the placement of impervious cover such as roads, parking areas, buildings, patios and other pavement. This unavoidably decreases infiltration of runoff into temporary storage in the soil or into long-time storage as aquifer recharge. The removal of vegetation, compaction of soil , and installation of pavement increase runoff efficiency and decrease the amount of water removed from the soil by evapotranspiration. Implementation of the project would result in the installation of impervious cover over approximately 85 percent of the site. This includes buildings, roads, driveways, parking areas, walkways, patios, and compacted soil . For an average storm and until the soil becomes saturated, it is estimated that• the amount of runoff from the site would increase by 90-100 percent if the project is implemented. More intense storms and storms of greater duration would yield even more runoff (see discussion of public services, storm water disposal ) . The volume of runoff will be estimated during the drainage study so that the storm water conveyance system can be sized properly. Surface Water Quality Development of the site as proposed would not involve direct discharges of substances which would degrade the water quality of existing waterways. However, it is reasonable to assume that future residents of the site would make use of household substances as typically used in residential areas. This would include garden pesticides and fertilizers, detergents, soaps, paints, and water- proofing substances. In addition, roadways and driveways would collect rubber compounds, oils, greases, and other chemicals from automobiles. It is also reasonable to assume that some unknown amount could eventually be washed down by surface runoff into downstream receiving waters. Minor amounts of silt which do not settle out in the stormwater detention basin(s) may also be washed downstream. Ground Water Since the site is in an area considered to be of "none to slight recharge" [6, p.26], the project would not significantly reduce ground water recharge on an average annual basis. 33