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the Storm Water Master Plan Update,the drainage plan prepared for the College Park site(MacKay& Somps 2004),describes the proposed drainage facilities that would be constructed to safely control and convey stormwater runoff from the project site.The plan reflects the changes in land uses that would occur under the proposed buildout of College Park compared to the land uses that were addressed in the Mountain House Storm Water Master Plan Update(MHCSD 2003).The principal change between the proposed and original land use _ plans with respect to stormwater drainage is the inclusion of Delta College in an area previously proposed for residential use; it is currently designated as a commercial/industrial development. As described in Chapter 4.7,"Drainage,"the stormwater drainage system for the College Park area would consist of a gravity conveyance system that directs drainage north to other facilities already under construction for the entire Mountain House community. In accordance with the Mountain House Storm Water Master Plan Update (MHCSD 2003)and Mountain House Storm Water Master Plan Update, Addendum (May 2004),most of the — drainage from the eastern portion of Neighborhoods B and D would flow to Water Quality Basin No. 1 (WQBI), which is located immediately northwest of the intersection of Mountain House Parkway and Byron Road. Drainage from the western portion of Neighborhoods A and B would flow to a new water quality basin that would be located in the southwest quadrant at the intersection of Central Parkway and Grant Line Road. A new water quality basin,located in the southwest quadrant at the intersection of Central Parkway and Grant Line Road,shall conform with the first flash treatment required for commercial and industrial drainage from the western portion of Basins A and B as designated in the College Park at Mountain House Specific Plan Ill,Figure 15-1 (MacKay& Somps 2004).Treated runoff shall then merge with the`clean"storm drain system within Central Parkway. As also described in Chapter 4.7,the modeled stormwater runoff volumes for the proposed project are not appreciably higher than those estimated for development under the MHMP.WQB1 may need to be slightly enlarged to accommodate the additional flow from the College Park site to achieve acceptable levels of water quality treatment. Table 4.8-2 shows the estimated stormwater runoff volumes for the 24-hour, 100-year storm event from the project site based on model results presented in the College Park drainage plan and the Mountain House Storm Water Master Plan Update(MHCSD 2003). Table 4.8-2 Estimated 24-hour, 100-Year Stormwater Runoff Volume on the College Park Site and Contributing Watershed under Existing and Developed Conditions Existing• Included in MHMPb Included in Proposed Projectc Acreage modeled 1,368 1,194 1,368 Runoff volume(acre-feet) 250 220 261 Notes: Existing runoff volume calculated based on ratio of total existing runoff in Mountain House watershed to runoff with Mountain House development as identified in Mountain House Storm Water Master Plan Update(MHCSD 2003). " Modeled land uses in the Mountain House Storm Water Master Plan Update(MHCSD 2003);total land area included in the model for the College Park area and in the watershed located upstream of College Park was slightly less than modeled for the College Park area drainage plan(MacKay&Somps 2004). ` Modeled land uses in the College Park drainage plan(MacKay&Somps 2004 . The constituent mass loading analysis was based on the"Simple Method"developed by the EPA and currently being used in the PLOAD constituent-loading module of the EPA BASINS modeling program for watershed runoff evaluation(EPA 2001).The EPA method provides a mass-loading estimate based on runoff volumes and research-based estimates of event mean concentrations of constituents in stormwater runoff.The constituent mass loading analysis was conducted using the following equation: L=QxCx0.226 L=constituent load(pounds) Q=runoff volume(acre-feet) — C=event mean constituent concentration(mg/L) 0.226=units conversion factor EDAW College Park at Mountain House Specific Plan III Draft EIR Water Duality 4.8-10 San Joaquin County