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Groundwater. It is not known to what degree the study area functions as a natural groundwater recharge area. The increase of impervious surfaces would reduce the size of the groundwater recharge area. Conversely, inclusion of retention basins to accommodate storm water runoff would improve the current groundwater recharge situation by aiding percolation of rainwater. If new development is placed on private wells, additional drafting of groundwater could accelerate the rate at which the water table is dropping in the area. Groundwater Quality. The implementation of private wells in the study area may have an adverse impact on groundwater quality. Additional drafting of groundwater in the vicinity may exacerbate the existing salt water intrusion problem and increase the concentration of pollutants already present in the groundwater. In addition, withdrawal of groundwater beneath the study area may cause a migration of TCE contaminated groundwater from the Tracy Defense Depot area to the project area. This could cause a well water contamination problem in the project area. However, if wells are of sufficient depth and properly cased, the potential for TCE contamination in well water could be minimized. Refer to Section 3.4., Public Services, Water Supply, for regulations governing water well installation and operation. The implementation of septic facilities in the study area could lead to contamination of shallow groundwater supplies. The potential for shallow groundwater contamination due to septic usage increases with housing density. A potential exists for contamination of well water due to septic seepage. However, if wells are properly cased, the potential for well water contamination is slight. All water well construction shall conform to San Joaquin Local Health District standards and guidelines. Refer to Section 3.4, Public Services, for regulations governing septic tank installation and operation. MITIGATION MEASURES. The following subdivision -stage measures are recommended to minimize potential hydrologic impacts of the proposed project and should be the responsibility of the developer. - Roadway designs should consider and not impede existing surface runoff drainage patterns. Retention basins should be implemented at terminal drainage sites within the study area and designed to ensure adequate capacity to accommodate additional runoff generated by future area buildout. Soil percolation tests should be conducted on every lot within the project area to ensure adequate septic suitability. - Detailed groundwater testing should be conducted on the project site prior to water well implementation with special concern for TCE and nitrate contamination. C/ All wells should be properly cased at appropriate depths to ensure protection from contaminated groundwater. VAlternative means of obtaining water (other than private wells) should be considered to minimize adverse impacts on the currently depleting groundwater supply. 3.5-3