SU0004094 - Laserfiche WebLink

Home Browse Search

4. ENVIRONMENTAL ANALYSIS 4.8 HYDROLOGY AND WATER QUALrrY 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 surfaces areas and hot temperatures can accelerate evaporation rate and increase evaporative losses. If groundwater seeps into an excavation and is then evaporated,it can constitute a loss to the groundwater supply. These 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 can 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 which alters natural groundwater flow. This effect may be difficult to detect or monitor over the short term but over the long term, alteration in groundwater flow direction and gradients can, in some cases,result in a noticeable effect to local recharge and the natural radial flow to wells within the same aquifer. Potential Project-Related Effects _ The proposed project would excavate material to depths just above the water table and the excavation plan does not include situations in which the excavation would extend into or below the groundwater. Considering this plan, it is unlikely that groundwater would be exposed in the active mining areas during normal mining operations and under existing groundwater conditions. However,if the regional groundwater levels gradually rise due to a seasonal or long-term, regional increase in groundwater levels,the groundwater could eventually inundate the bottom of the active mining phase and expose groundwater to evaporative losses. Groundwater level increases above the lowest aggregate extraction depths within the active mining phase could hinder mining operations,contribute to slope instability, and limit access due to the presence of surface water. Long term increases of groundwater levels could also result in the inundation of the bottom of the completed,reclaimed mining phases depending on the depth of cover placed over the mined area during reclamation. If this were to occur, the surface soils in the reclaimed phases would become saturated and depending on the reuse(i.e.,agriculture or grazing),could result in unfavorable conditions. Although this would not necessarily constitute an impact under CEQA (see Significance Criteria,above),it could be considered long term flooding and adversely affect the proposed mining and reuse plans. Groundwater seepage and inundation within the active or reclaimed mining phase could cause a — groundwater quality issue as surface water and pollutants from the surface commingle with the incoming groundwater. Because of the size of the excavation and volumes of incoming groundwater,the degradation would be localized in areas where surface pollutants are present and are allowed to enter the water. Furthermore,the amount of unpolluted surface and groundwater would likely be sufficient to dilute pollutant loads to below regulatory action levels. However, if a large concentrated spill of a potential pollutant(i.e. chemicals related to mining operations including fuel,oil,grease, and sediment)occurs and is not contained, the resultant plume could degrade the surface water and migrate into the groundwater. Although RMC Pack Vemalis Quarty Mining and Reclamation Project 4.8-20 ESA/203015 Administrative Draft Eat March 2004