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MOUNTAIN HOUSE NEIGHBORHOODS K AND L INITIAL STUDY 5.ENVIRONMENTAL CHECKLIST 16.UTILITIES AND SERVICE SYSTEMS L� i As discussed in the SPII Initial Study,the wastewater treatment plant(WWTP) will need to be expanded before it can serve all of SPI and SPIT. The SPII Initial Study noted, however,that a substantial portion of SPIT development can pro- ceed before the WWTP must be expanded. The WWTP must also be designed to accommodate peak wet-weather flows that include infiltration and inflow caused by rainfall. The MHCSD must issue a"will serve" letter indicating that sufficient capacity will be available for each neighborhood before each neighborhood Tentative Subdivision Map is approved. The Neighborhoods K and L applicant, Shea Mountain House, LLC,would contribute its fair share of the cost to build additional WWTP capacity. Environmental impacts of full buildout of the WWTP, including construction impacts,were evaluated in a separate Initial Study(SJCCDD, 1998). As noted in the Setting section above, rf the WWTP was expanded and improved to provide treatment for 3.0-mgd average dry weather flow. The expansion is likely to be adequate to serve development proposed for Neighborhoods K and L should this occur prior to development of Neighborhoods I and J. `—' Demand for Wastewater Service. As already noted,the currently proposed development would allow a similar number of dwelling units as that identified in SPII, and only about 190 more jobs than projected under SPIT. The proposal P� would not create substantial new demand for wastewater service beyond that i evaluated in the SPII Initial Study. Planned land uses for Neighborhoods K and L would generate an estimated 740,000 gallons per day(gpd)of wastewater. tk Conclusion. The above discussion reveals that construction of new water or wastewater treatment facilities or expansion of existing facilities would not cause potential significant impacts that were not adequately addressed in the 1994 —' MEIR or the SPII Initial Study. r c) Would the project require or result in the construction of new storm wafer 4 drainage facilities or expansion of existing facilities, the construction of which could cause significant environmental effects? Construction of new storm drains with catch basins(a.k.a. drop inlets)would be required for Neighborhoods K and L. The storm drain system would consist of 3 gravity-flow pipelines, constructed under neighborhood streets and easements, with lateral pipes to catch basins installed along the curbs and gutters. These storm drains would collect surface runoff and ultimately convey it to Mountain House Creek. Local storm drains along most side streets would convey storm 1, runoff to large storm drains(trunks)along major arterial streets. The storm drain system would be designed to accommodate runoff flows from { the 10-year storm event unless a 100-year storm event would cause excessive street flooding that could prevent access for emergency vehicles. if this situation would occur,that portion of the system would be designed to accommodate a 100-year storm event. Mountain House creek has already been constructed to accommodate 100-year storm flows and no further modifications are needed.A 10-year storm has a 10-percent statistical chance of occurring every year, and a r � sari 1) 5-201