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MOUNTAIN HOUSE PRoPOSEb ALTAMONT HEIGHTS TENTATIVE MAP 4. ENVIRONMENTAL CHECKLIST 4.8 HYDROLOGY AND WATER QUALITY Update, Addendum I(PACE, 2004). Drainage infrastructure includes primary and secondary 3 storm drain systems,improvements to Water Quality Basin(WQB)No. 1, and improvements to Mountain House Creek. New drainage infrastructure would be constructed to convey runoff from the Specific Plan III area, in which the project site is located. Discussion of storm drainage facilities for the proposed project is included in Section 4.16: Utilities and Service Systems. 1!` SIGNIFICANT IMPACTS IDENTIFIED IN 2005 SPIIIEIR The SPIIIEIR identified five drainage and water quality impacts. Three, involving increased erosion and sedimentation, dewatering during construction, and long-term water quality effects of urban runoff,were determined to be less than significant. The following two impacts were determined to be potentially significant: Impact 4.7-2: Result in Flooding or Exceed the Capacity of the Storm-drain System. [Specific Plan III development] would increase the rate and amount of Fif surface runoff and both increase the demand for capacity in the existing downstream MHMP storm-drain system and require the development of new storm-drain facilities to serve the [development]. However, this would not result in flooding or exceeding the capacity of the MHMP storm-drain system because the existing downstream ! MHMP storm-drain system has already been sized to accommodate runoff from the [development]. [New] projects would provide adequate capacity in the new storm- drain facilities required to serve [the new projects]. A less-than-significant impact would occur. [Specific Plan III development] would require expansion of WQB No. 1 and, if not completed before development, a significant impact would occur. - Impact 4.8-1: Water Quality—Potential Temporary Construction-Related Water Quality Effects. Temporary construction-related disturbances within the [Specific Plan III area] could result in the discharge of contaminated stormwater and non-stormwater discharges to drainage systems and ultimately the Mountain House Creek channel and Old River. Apotentially significant impact would occur. MITIGATION MEASURES IDENTIFIED IN 2005 SPIIIEIR Mitigation Measure 4.7-2 requires developers of Specific Plan III to expand WQB No i as required to accommodate runoff from their proposed projects or provide on-site retention storage in lieu of the WQB No. I expansion. Mitigation Measure 4.8-1 requires developers of Specific Plan III to prepare Storm Water Pollution Prevention Plans (SWPPPs)that include effective Best Management Practices (BMPs)to control polluted water discharges during construction. Implementation of these mitigation measures would reduce potential 0i flooding and temporary construction-related water quality impacts to a less-than-significant levels. Fi DISCUSSION REGARDING PROPOSED PROJECT I a) Violate any water quality standards or waste discharge requirements? F! 4-42