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Table 4.8-4 Percentage Cover of Proposed Land Uses on the College Park Site Existinge Included in MHMP^ Included in Proposed Project Subbasin Area %by Land Area %by Land Area %by Land (acres) Use (acres) Use (acres) Use Undeveloped 693 85.0 34.7 4.3 91.8 11.3 Residential 122 15.0 505.55 62.0 448.0 55.0 Commercial 0 0 185.05 22.7 118 14.5 Industrial 0 0 86.5 10.6 139 17.0 E>asting land uses identified in MacKay&Somps(2004) ° Modeled land uses in MHCSD(2003) Modeled land uses in MacKay&Somps(2004).The Delta College area eras assumed to reflect approximately 50%commercial land uses and 50%undeveloped landscaped areas. Sources:MHCSD(2003)and MacKay&Somps(2004) The constituent loading analysis was also conducted to assess the reduction in loading that would follow implementation of urban runoff BMPs.Development implemented in the College Park area would be required to implement permanent nonstructural and structural postconstruction BMPs,pursuant to the Mountain House SWMP and as identified in both the Mountain House Storm Water Master Plan Update(MHCSD 2003)and the MacKay& Somps(2004)drainage plan for buildout of the proposed College Park development.The most effective SWMPs combine both preventive(nonstructural)and treatment(structural)BMPs.Key drainage features for water quality — protection include routing all flow from developed surfaces to a structural BMP.In addition,all"fust-flush" drainage up to the volume occurring under a 10-year event would be routed to the water quality basins.The first- flush flow is the initial runoff during each storm event that tends to contribute the majority of constituent loading. Once flows exceed the rate corresponding to a statistical 10-year event rate,the majority of constituents would have already been transported to the structural BMPs,and the storm drainage piping system is designed to route any additional increased flows directly to the storm drain outfalls located in the Mountain House Creek channel. For the constituent loading analysis,typical BMP effectiveness coefficients for contaminant reduction following implementation were based on the method used for similar development proposed for the nearby Mossdale Landing project in Lathrop(RBF Consulting 2002).Contaminant removal efficiency values reflect sequential application of BMPs: first,nonstructural practices, such as education and pollution prevention,then smaller structural practices such as on-lot treatment,and finally, larger scale structural BMPs, such as extended detention basins. A range of effectiveness is typical when BMPs are implemented because the effectiveness of individual _ measures depends on a variety of site-specific environmental and management factors.For the purposes of this analysis,the coefficients reflecting the conservative lower range of BMP effectiveness were used. The constituent loading analysis described above was used to evaluate runoff conditions associated with the following four conditions: Existing condition(i.e., existing land use at the project site), Developed conditions at the College Park site without BMPs under the MHMP and Mountain House Storm Water Master Plan Update(MHCSD 2003)(i.e.,previous land use proposal), — Developed conditions without BMPs under the proposed College Park project and revised drainage plan for the College Park site(MacKay&Somps 2004),and _ Developed conditions with BMPs under the proposed College Park project and revised drainage plan for the College Park site. EDAW College Park at Mountain House Specific Plan III Draft EIR Water Quality 4.8-12 San Joaquin County