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MOUNTAIN HOUSE NEIGHBORHOOD' 'PROJECT INITIAL STUDY presents data that it could take up to 30 days depending on the number of pumps rented to drain the accumulated water behind the levees after a 100-year design storm. CET estimation of maximum water levels may be erroneous if certain conditions occur. In the CET study, it was assumed that the storage areas were completely dry pending the 100-year, 72-hour storm. This may be erroneous because rarely does a 100-year storm occur alone. It is common that several strong but less severe storms could occur prior to the 100-year storm. Storm water from these previous storms could still be stored behind the levees,taking up the assumed storage capacity. Thus,when the 72-hour storm did hit, water would pond to a higher elevation than previously thought. The maximum water surface elevation in the storage areas is 24 feet, substantially above the Old River levee, about 13 feet. This should be considered the maximum water surface elevation in the east storage area. The elevation of the effluent pond berms are higher then the Old River levee to prevent spillage of flood waters into the pond and possible release of stored effluent. Because water quality standards could be violated in the case of flooding, conditions of approval would be necessary to reduce this potential impact. These are summarized below. Condition of Approval HYDRO-1: The applicant shall provide specific design and function information on the proposed interim water quality basin, shown on the Carlson, Barbee, and Gibson exhibit(Figure 3). The design of the basin shall conform to design parameters set forth in the California State Best Management Practices Manual and shall be reviewed and approved by the Mountain House Community Services District This approval shall occur at the time of review of the Site Improvement Plans and prior to issuance of building permits. Condition of Approval HYDRO-2: A detailed interim storm water pumping plan shall be developed. At a minimum, the plan shall specify the following:the number of pumps;how they would be mobilized,their capacity; location;power sources;the operational and maintenance entity;and othergeneral logistical requirements Yearly maintenance and operation tests shall be completed and documented. The pumps shall be capable of dewatering the storage areas after the 100-year storm event within a timeframe approved by the Mountain House Community Services District This plan shall be reviewed and approved by the Mountain House Community Services District prior to issuance of building permits b) Substantially deplete groundwater supplies or interfere substantially with groundwater recharge such that there would be a net deficit in aquifer volume or lowering of local groundwater table level(e.g. the production rate of pre-existing nearby wells would drop to a level which would not support existing land uses or planned uses for which permits have been granted)? The water supply of the project would be provided by the Byron Bethany Irrigation District(BBID). BBID's water supply is primarily from surface water sources. Little or no groundwater would be utilized for the project's water supply. No new wells would be installed for the project. The project would have a net increase of impervious surface area on the project site. In theory, this could reduce the amount of on- site aquifer recharge. A study prepared by CET on the farm drainage aspects of the project indicates that there are extensive rile or subsurface drains on the site. These drains lower the groundwater surface elevations so that farming can be accomplished on the fields. It is likely that, given the site's position in the watershed, an insignificant amount of on-site groundwater recharge is occurring. Lateral flow moving within the alluvial fan deposits likely accounts for the high groundwater levels on the site. The project should have a slight impact on these groundwater surface elevations but it is likely to impact the site less than the current file drain network on the site which would be abandoned. c) Substantially alter the existing drainage pattern of the site or area, including 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? C:VOBS\SJCVNfML=Y\FINALINRIAL=Y•PARTI.DOC(7122/00) 96 V.J. HYDROLOGY AND WATER QUALITY