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MOuNTAIN HousE PROPOSED MACHADo TENTATIvE MAP 4. ENVIRONMENTAL CnE=sr 4.6 GEOLOGY AND sons The MEIR indicated that the project site is located outside areas of southwest San Joaquin l County identified as susceptible to landsliding. Evidence of slope failures and/or landslides has not been mapped within or immediately adjacent to the project site. The potential for landslides at the project site is remote due to the gently sloping terrain. Mitigation Measures No mitigation measures are required. b) Result in substantial soil erosion or the loss of topsoil? Excessive soil erosion is not expected to occur during construction at the project site because average slopes are only one percent. Cut and fill quantities for the house pads and roadbeds are anticipated to be minimal due to the relatively level terrain. However, the project would require stripping of all vegetation,debris,topsoil, or any other unsuitable material. Project construction would have to comply with the National Pollutant Discharge Elimination System(NPDES)program requirements. The NPDES storm water program, administered by _ the State Water Resources Control Board's(SWRCB)Division of Water Quality,regulates construction sites that disturb five or more acres of land. Under the program, contractors are required to prepare and implement a Storm Water Pollution Prevention Plan that includes Best Management Practices (BMPs)to prevent soil erosion and off-site migration of sediment-laden runoff during construction. Additionally,MHMP Policy 6.8.3(b)and the County Development Title require that adequate efforts be implemented during construction to control or eliminate soil erosion and sedimentation associated with construction activities. - Once construction is completed and project topsoil has become stabilized, soil erosion at the project site would be greatly reduced. Additionally, all urban runoff from the project site would flow through the storm drain system to water quality basins within the Mountain House Creek corridor that would help to remove sediment from site runoff. Sedimentation, soil erosion,and water quality issues are finther addressed in Section 4.8: Hydrology and Water Quality of this Checklist. Project development would increase the amount of impervious surfaces at the project site and increase the quantity of runoff. In addition,runoff would flow into storm drains that would result in increased velocity of water leaving the project site. However,runoff from the project site would be conveyed to Water Quality Basin 1 located north of Byron Road on the east side of Mountain House Creek. This basin will substantially reduce flow velocities prior to discharge to Mountain House Creek. In addition,the Mountain House Master Drainage - Plan Update requires in locations where the velocity of the runoff can be expected to cause erosion that protection measures(e.g.,concrete,riprap, soil-reinforcement products)will be , used to prevent erosion(Pace,2002). With implementation of these existing requirements,a less-than-significant impact would occur. Mitigation Measures No additional mitigation measures are required. 4-30