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MOUNTAIN HOUSE PROPOSED MACHADO TENTATIVE MAP 4. ENVIRONMENTAL CHECKLIST 4.8 HYDROLOGY AND WATER QUALITY 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 _ 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. I 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. A potentially significant impact would occur. MITIGATION MEASURES IDENTIFIED IN 2005 SPIHEIR Mitigation Measure 4.7-2 requires developers of Specific Plan III to expand WQB No 1 as required to accommodate runoff from their proposed projects or provide on-site retention storage in lieu of the WQB No. 1 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 — flooding and temporary construction-related water quality impacts to a less-than-significaut levels. DISCUSSION REGARDING PROPOSED PROJECT a) Violate any water quality standards or waste discharge requirements? During construction of the proposed project, grading operations would result in the removal _ of vegetation and topsoil and create the potential for erosion of on-site soils and off-site siltation. The project applicant would have to comply with the Phase I National Pollutant Discharge Elimination System (NPDES) permit program. Among other things, the Phase I NPDES program regulates storm water discharges from large and medium-sized municipal separate storm sewer systems (those serving more than 100,000 persons) and construction sites that disturb five or more acres of land. Under the program, developers of projects that 4-44