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4. ENVIRONMENTAL ANALYSIS '^ 4.8 HYDROLOGY AND WATER QUALrrY • 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 significant impact without mitigation prescribed as part of this EIR. Background Agricultural and domestic groundwater users extract groundwater from the upper unconfined aquifer for their primary source of water or to supplement surface water supplies. Although it .. has lower water quality,the upper,unconfined zone is a beneficial use aquifer and is an important water resource. The water level of the upper unconfined zone fluctuates seasonally or over the long term due to regional groundwater conditions. During the winter months, groundwater levels will increase in an unconfined aquifer and gradually decrease with the onset of summer. Temporary drought conditions can directly affect the unconfined water table due to a lack of precipitation and recharge or due to over-pumping to supplement reduction in surface ` water supplies. Over the long-term,excessive groundwater pumping for irrigation, such as that observed during the early to mid 1900s,caused groundwater levels to decline and then slowly recover with availability and use of surface water sources later in the decade. With the combined use of surface water sources,conservation,and reduced irrigation groundwater pumping,groundwater levels in both unconfined and confined aquifers are gradually recovering throughout the San Joaquin Valley. Large mining operations that intersect or extend below groundwater levels can cause groundwater to seep into the active training pits and rise to near the approximate level of the RMC Pacific Vernalis Quany Mining and Reclamation Project 4.8-19 ESA/203015 Admwstmtive Draft EIR Much 2004