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MOUNTAIN HOUSE PROPOSED ALTAMONT HEIGHTS TENTATIVE MAP 4. ENVIRONMENTAL CHECKLIST I 4.8 HYDROLOGY AND WATER QUALITY complete development of Specific Plan III (including the proposed project)would result in covering less than 0.2 percent of the Tracy groundwater subbasin with impervious surfaces. This would result in a negligible reduction in percolation to the subbasin as a whole. Therefore, the project would not interfere substantially with groundwater recharge (EDAW, 200Sb). The farmland surrounding the Mountain House Community is irrigated primarily from agricultural ditches supplied by Byron-Bethany Irrigation District and Westside Irrigation District,not by groundwater wells. Therefore,while project development would result in some increase in impervious surfaces over the recharge area, any slight change in groundwater levels would not affect surrounding agricultural operations. Mitigation Measures The impact of the proposed project on groundwater supplies and recharge would be less than significant. No mitigation measures are required. L! c) 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? Refer to the discussion for Item a)above. Construction of the proposed project would alter the existing drainage pattern from overland sheet flow and flow along the drainage swale to an urban storm drain system. Once the project is completed, the increase in impervious surface area resulting from project development would increase the amount and velocity of runoff leaving the site. Catch basins and underground storm drains(pipelines)would collect and convey runoff from the project site along"F" Street(Siegfried Engineering, 2006b)and - Central Parkway. This runoff would be subsequently treated in WQB No.1 for treatment (i.e., sediment and contaminant removal)prior to discharge to Mountain House Creek and then to Old River. Other than elimination of the on-site drainage swale, construction of the proposed project would not alter the course of any stream,creek or river. Mitigation Measures Compliance with the erosion and sediment-control requirements cited above and included in t the MHMP and the SWMP would result in a less-than-significant impact. No additional mitigation measures are required. d) Substantially alter the existing drainage pattern of the site or area, including through " the alteration of the course of a stream or river, or substantially increase the rate or amount of surface runoff in a manner which would result in flooding on- or off-site? t Development of the project site would alter existing drainage patterns, increase the amount of impervious surface area, and result in an increase in runoff from the developed area. However, the primary storm drainage system for the Mountain House Community has been designed to safely convey flows resulting from the 100-year storm event all the way to Old River. The secondary storm drainage system along the local streets has been designed to j I safely convey flows resulting from the 10-year storm event. A 100-year storm event has a 4-44