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4.8 Hydrology and Water Quality housekeeping or standard equipment maintenance practices. The majority of inadvertent, deliberate spills to the surface would be absorbed by the surface soils, would degrade over time, and not represent a threat to the environment or trigger regulatory action. However, larger releases (i.e., quantities greater than 25 gallons) such as ruptured fuel tanks or aboveground storage tanks would constitute a significant release requiring immediate attention and prompting regulatory action. Mitigation Measures Measure 4.8.2: Implement Mitigation Measure 4.11.2. Impact Significance After Mitigation: Measures to mitigate this impact involve source control and leak containment to reduce the potential that a petroleum or fuel spill would — reach surface or groundwater. In addition, media that is impacted by petroleum fuel is to be removed from the site. The prescribed mitigation would limit the opportunity for hazardous materials to contact surface water and groundwater and would ensure that water — quality impacts associated with petroleum fuel releases would be less than significant. Impact 4.8.3: Excavation that disturbs and/or truncates the existing groundwater well shafts on the project site could increase the potential for contaminants to enter and pollute the groundwater through the wells.This would be a potentially significant impact. The proposed project converts agricultural lands to an open quarry to depths of approximately 100 feet. Since agricultural operations have occupied this area for many years,there is a possibility that unused, unidentified,or abandoned groundwater wells remain within the boundaries of the property. Unidentified useable or abandoned groundwater wells remaining on — the proposed project site after project implementation could be damaged or truncated by excavation equipment. A truncated or improperly abandoned groundwater well would act as a preferential pathway to the underlying aquifer allowing surface water, sediment, and surface- _ bome contaminants to degrade water quality. According to the significance criteria, a significant impact would occur if the project caused a condition that would lead to groundwater quality degradation. There are five known wells located within the project boundaries, three agricultural wells and two domestic wells. The proposed project would continue to use the two of the agricultural wells and one of the domestic wells. The two unused wells,as well as others that may not yet be identified, would not be operated as part of the project and therefore,could be either truncated or destroyed during quarry operations. As part of the project,the project applicant would decommission — unused wells. Mitigation Measures Mitigation Measure 4.8.3: Prior to the commencement of excavations, the project applicant shall conduct a comprehensive well sYrvey to locate, identify, and confirm all existing groundwater wells on the project site:Information for well locations shall be RMC Pacific vernalis Quarry Mining and Reclamation Project 4.8-25 ESA 1203015 Draft Environmental Impact Report May 2006