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5.ENVIRONMENTAL CHECKLIST MOUNTAIN HOUSE NEIGHBORHOODS K AND L INITIAL STUDY 8.HYDROLOGY AND WATER QUALITY Temporary Lowering of Groundwater Levels During Construction. Groundwater elevations in Neighborhoods K and L range from 3 to 15 feet below ground surface(bgs). If groundwater were encountered during project construction, dewatering of excavated areas would be necessary to create a dry work area. Groundwater is likely to be encountered near Mountain House Creek and Old River, and in areas where deeper excavation is needed for the construction of project facilities. Dewatering of groundwater during project construction activities could temporarily lower groundwater levels in the vicinity of Neighborhoods K and L. After being treated in accordance with regulatory requirements,the majority of groundwater removed by dewatering would be returned to the subsurface and allowed to infiltrate back into the groundwater table. Due to marginal groundwater quality,there are few groundwater wells in the project area that could be affected by lowered groundwater elevations. Since any lowering of groundwater elevations from construction dewatering would be localized and temporary in nature, impacts on groundwater from construction dewatering would be less than significant. Domestic Water Supplies. The Byron Bethany Irrigation District(BBID),through contractual relations with the MHCSD, provides domestic water to Mountain House. The water is Delta water that is diverted at the Clifton Court Forebay. Domestic water supply for Neighborhoods K and L would be the same as for the rest of the Mountain House community.The installation of new wells is not a part of the proposed project, and thus groundwater would not be used for the project's water supply. Implementation of Neighborhoods K and L therefore would not deplete groundwater supplies. Interference with Groundwater Rechar e. The construction of new impervious surfaces areas associated with roadways, new residences, and other proposed land uses in Neighborhoods K and L would reduce the ability of surface water to infiltrate the ground surface,thereby reducing groundwater recharge. However, the permanent lake features,which would contain water year-round,would help to induce groundwater recharge.Thus,the lake features would offset any decrease in groundwater recharge associated with new impervious surfaces. No mitigation measures would be necessary. c) Would the project substantially alter the existing drainage pattern of the site or area, including through the alteration of the course of a stream or river, in a manner which would result in substantial erosion or siltation on- or off-site? Currently, on-site runoff occurs primarily as sheet flow and concentrated shallow flow north across the former agricultural fields to Old River. The removal of the existing irrigation ditches and associated drainage infrastructure, changes in site topography,creation of new impervious surfaces, and construction of the lake features and storm water drainage infrastructure associated with the proposed Isnn i I 5-114