RES 06035 - Laserfiche WebLink

Chapter 4 Environmental Setting, Impacts, and Mitigation Measures H. Hydrology, Drainage, and Water Quality In addition, it is possible that there are old water supply wells associated with historic farming at the Plan area. If the sanitary seals associated with the wells have been damaged since installation, or were to be damaged during grading and construction, surface water (potentially containing pollutants) could seep into the wells and the underlying aquifer, causing water quality degradation. In addition, it is possible that the water supply wells were not fitted with a proper seal when they were installed. It is possible that, if the detention basins were to expose groundwater at the surface (i.e., if the basins were deepened under the proposed project), evaporation from them could reduce available groundwater resources. Based on available data, the depth to groundwater in the vicinity of the detention basins is approximately 20 feet or more below the ground surface. The basins are approximately ten feet deep. Therefore, approximately ten feet of unsaturated sediments underlie the basins. As water infiltrates through these sediments, pollutants are adsorbed to soil particles and largely broken down by biochemical processes. Ten feet of separation between the bottom of an infiltration feature and the groundwater table is generally considered adequate dis- tance to be protective of groundwater quality. As stated by the RWQCB, "A water table dis- tance separation of ten feet depth in California presumptively poses negligible risk for storm water not associated with industrial activity or high vehicular traffic."JO As noted above, the potential agriculture club would be designed to channel animal waste to the City's sanitary sewer, or to otherwise manage that waste to avoid water pollution. The City of Pleasanton recognizes impacts related to groundwater resources could occur as a result of the type of development proposed. Therefore, the Specific Plan states that the design of any modification to the basins would maintain this ten foot separation, unless alternative measures approved by the RWQCB are identified that are equally protective of groundwater quality. In addition, the Specific Plan requires that any runoff generated at upstream locations that might contain pollutants (e.g. runoff from roadways or parking lots) must be pretreated prior to being discharged to the main detention basins. Pretreatment features may include shallow settling basins, bioswales, vortex-type separators, or equivalently effective BM Ps. The purpose of these pretreatment BMPs is to minimize the pollutant loading of the detention basins and to collect pollutants in areas where accumulations can be more easily removed. The Specific Plan includes provisions that require any and all wells identified prior to or during development are properly managed (either protected and maintained or destroyed in accordance with state regulations). 30 California Regional Water Quality Control Board, Waste Discharge Requirements (WDRs) for Storm Water Discharges from Small Municipal Separate Storm Sewer Systems (General Permit). Order No. 2003.0005- DWQ,2003. 91