SU0004094 - Laserfiche WebLink

Home Browse Search

5. ALTERNATIVES r Large mining operations that intersect or extend below groundwater levels cause groundwater to seep into the excavation and will rise to 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 evaporation rate and increase evaporation.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 alter groundwater flow direction and flow gradients within the aquifer which thereby 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 adversely impact recharge and the natural radial flow to wells. Groundwater seepage and inundation within the active mining phase could cause a groundwater quality issue as surface water and pollutants from the surface commingle with the incoming groundwater. The degradation would be localized in areas where surface pollutants are present and are allowed to enter the water. 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 migrate into the groundwater. Although possible under certain conditions,reintroduction of pollutants to the groundwater from the open excavation of a mine is less likely because the seepage pressures and flow gradient that cause groundwater to enter the mine excavation would prevent infiltration. However, if those seepage pressures were overcome and the flow gradient reversed by,for example,excessive groundwater pumping adjacent to the mine excavation, there is a potential that surface borne contaminants could enter the groundwater aquifer and adversely impact groundwater quality. With a deeper excavation below the groundwater level, the temporary and permanent slopes would be longer,thereby increasing the potential for slope instability. A portion of the slopes ~ would be saturated,potentially increasing safety issues. The geologic material,its structure and attitude,and amount of groundwater can affect the potential for slope failure. Slope stability and ro the potential for slope failure would be increased with this alternative. With a deeper excavation below the groundwater level,the mining pits would he filled with water and the ability to reclaim the mined areas as agricultural land would be eliminated. This would result in a greater loss of productive agricultural land than the Proposed Project. Due to the potentially significant environmental impacts to groundwater hydrology that exceed those associated with the proposed project, as well as the inability to reclaim the mined areas as agricultural land,this alternative was not evaluated as a viable alternative. RMC Pacific Vernalis Quarry Mining and Reclamation Project 5-5 FSA/ZD3015 AdmiNslradve Draft QR Mamh 2004