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4.8 Hydrology and Water Duality --J considered significant if it would result in any of the following criteria, which are adapted from the CEQA Guidelines. • Violate any water quality standards or waste discharge requirements or otherwise — substantially degrade surface and groundwater water quality; • Substantially deplete groundwater supplies or interfere substantially with — groundwater recharge such that there would be a net deficit in aquifer volume or a lowering of the local groundwater table level(i.e.,the production rate of pre-existing nearby wells would decline to a level which would not support existing land uses or planned uses for which permits have been granted); — • Substantially alter the existing drainage pattern of the site or area, including through the alteration of the course of a stream or river, in a manner which would deplete — flows to downstream receiving water bodies,reduce surface water available for localized infiltration, or eliminate adequate water flow for aquatic and riparian habitats; • Create or contribute runoff water which would exceed the capacity of existing or planned stormwater drainage systems,cause flooding on and offsite, or provide substantial additional sources of polluted runoff; — The following impacts and mitigations are presented in the general order of the significance criteria listed above. Impacts and Mitigation Impact 4.8.1: Groundwater seepage that enters the mine excavation and inundates the active and reclaimed mining phases would cause groundwater losses due to evaporation, threaten groundwater quality,and disrupt mining and future reuse operations. This would be a potentially significant impact. Background — Large mining operations that intersect or extend below groundwater levels can cause groundwater to seep into the active mining pits and rise to near the approximate level of the surrounding — groundwater.The influx of groundwater inundates the bottom of the mining excavation. Once the groundwater becomes surface water within the excavation, it is available for evaporation. Large surface areas and hot temperatures can accelerate the evaporation rate and increase evaporative — losses, which can constitute a loss to the groundwater supply. Evaporation rates are relatively small compared to groundwater pumping but, in some cases,can represent a measurable groundwater loss. Long-term evaporative losses to a groundwater aquifer could contribute to — long-term, local groundwater level declines that could eventually impact local groundwater wells that extract from the same aquifer. Groundwater seepage into a large excavation can also influence groundwater flow direction and flow gradients within an aquifer that could thereby alter natural groundwater flow. This effect may be difficult to detect or monitor over the short term, but over the long term, alterations in RMC Pacific Vernalis Quarry Mining and Reclamation Project 4.8-21 ESA 1203015 Draft Environmental Impact Report May 2006