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site, is not typically treated or passed through BMPs to remove contaminants.Consequently,the conversion of agricultural land within the College Park site to urban land uses would be expected to generally reduce the quantity of contaminant discharges in stormwater runoff relative to existing conditions for a majority of the constituents of concern. Stormwater runoff from commercial and industrial land uses proposed for the project site, if not treated with BMPs, could have similar or slightly greater concentrations or loading rates of some constituents(e.g., biochemical oxygen demand, oil and grease, copper,lead,mercury, selenium, zinc,and chlorpyrifos)in comparison with existing conditions(refer to Tables 4.8-3 and 4.8-5).However, discharges of these constituents would not be expected to cause a substantial adverse water quality effect for several reasons.Primarily, implementation of nonstructural and structural BMPs pursuant to the proposed project would generally result in less constituent loading than under existing conditions as shown in Table 4.8-5. In addition,based on analysis of _ the mixed land uses for the proposed project,the loading rates for several constituents would be less under the proposed project than under existing conditions even without implementation of BMPs.Despite the overall increased volume of stormwater discharges after project development as a result of the increase in impervious — surfaces,the lower concentrations in runoff for these constituents would result in less loading. Under existing conditions,some constituents typically present in runoff from irrigated or other agricultural operations(e.g., dairies)can be discharged throughout the year and also during the warmer spring and summer months when irrigation runoff is generated.Discharges of nutrients during summer, for example, can stimulate nuisance algae growth in response to higher concentrations and warmer temperatures and sunlight conditions. Discharges of sediment,inorganic salts,and bacteria during the summer can be detrimental to aquatic life or the quality of source water used for municipal supplies.However,under the postproject condition,these discharges would generally decrease during summer and only occur during the winter stormwater runoff season when conditions are less suitable for the potential adverse ecological effects and background streamflow rates are higher. Higher background streamflow generally provides greater dilution of waste discharges. The available data indicate that the concentration of chlorpyrifos associated with urban runoff could be higher than the concentration in background nmoff from the existing land uses.The EPA is currently phasing out diazinon for over-the-counter sale,and residential outdoor use is expected to decrease by about 75%according to the EPA. A TMDL program for diazinon has not been completed by the Central Valley RWQCB for the Delta.Upon passage of — stormwater runoff through the BMPs,and in combination with dilution provided in the receiving water,the projected concentrations resulting from developed land uses under the proposed project should not cause adverse water quality effects or result in a measurable increase in the receiving waters.Consequently,the proposed project is _ not anticipated to create or contribute substantially to adverse pesticide runoff. As demonstrated in Table 4.8-5,the proposed changes in land uses for College Park compared to the land uses previously evaluated in the Mountain House Storm Water Master Plan Update,including the substitution of the development of the community college for previously identified residential uses,are not expected to appreciably change estimated constituent loading rates in stormwater runoff. Consequently,within the margin of error of the constituent loading analysis,there is very little difference in stormwater quality under the proposed land use plan compared to the assumptions for the MHMP. hi addition,the MHMP and Mountain House Storm Water Master Plan Update(MHCSD 2003)did not include the water quality treatment basin currently being proposed for Neighborhood A at Central Parkway and Grant Line Road.Therefore,this water quality treatment basin would — likely result in reductions of constituent loading in stormwater runoff in this subbasin,compared to the loading that would have occurred under the Mountain House Storm Water Master Plan Update(MHCSD 2003). Because the constituent loading analysis shows that the implementation of nonstructural and structural BMPs would substantially reduce the potential adverse effects associated with discharges of contaminants into stormwater,the potential long-term water quality impact associated with the proposed project is considered less _ than significant. EDAW College Park at Mountain House Specific Plan 111 Draft EIR Water Quality 4.6-16 San Joaquin County