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MOUNTAIN HOUSE PROPOSED MACHADO TENTATIVE MAP 4. ENVIRONMENTAL CHECKLIST 4.8 HYDROLOGY AND WATER QUALITY The MEIR indicates that,with construction of the storm drain system planned in the Mountain House Master Drainage Plan and implementation of mitigation measures identified in the MHMP, development of the Mountain House Community(including the proposed Lproject) would result in less-than-significant flooding impacts. Mitigation Measures The proposed project would increase the rate and amount of runoff, but the increase would not result in flooding on or off the project site. No mitigation measures are required. L e) Create or contribute runoff water which would exceed the capacity of existing or planned stormwater drainage systems or provide substantial additional sources of polluted runoff? Runoff quantity and the adequacy of the hydraulic capacity of the storm drain system are addressed in Item d)above. Since the project site was formerly used for a dairy and irrigated pasture land, it is likely that the non-point source (NPS)pollutants formerly found in runoff from the site were sediment,nutrients, agricultural chemicals, and oxygen-demanding constituents. When completed,the proposed land uses will be residential, commercial, office,and limited industrial. The conversion of the project site would likely result in a decrease in sediment, nutrients, agricultural chemicals, and oxygen-demanding constituents in the site runoff while increasing levels of oils, grease, metals,and petroleum hydrocarbons. All "urban" runoff originating from the project site would be treated in WQB No.I prior to discharge to Mountain House Creek and Old River. This treatment is particularly important for the first flush that occurs at the beginning of a strong storm when pollutants that are on the streets are washed into the storm drains. Mitigation Measures ` The Mountain House Community storm drainage system includes facilities to adequately treat the storm runoff to reduce pollutants to a less than significant level. No additional mitigation measures are required. L fJ Otherwise substantially degrade water quality? L Refer to discussions for Items a),c), d) and e)above. Mitigation Measures No additional mitigation measures are required. g) Place housing within a 100 year flood hazard area as mapped on a federal Flood Hazard Boundary or Flood Insurance Rate Map or other flood hazard delineation map? I The project site is not within a 100-year flood hazard area as mapped in the MHMP(Figure 15.7) or on a FEMA Flood Insurance Rate Map. No impact would occur(EDAW,2005b). 4-47