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V CJ` Alternative 4: N Pro'ect Under this alternative, groundwater levels would be expected to remain well below the existing ground surface and would not cause inundation of ground-surface or impacts to F crop productivity. Mitigation-Measure 4..2=6a=-Altematives-1I2 and 3) The maximum depth of mining shall be a minimum of six feet above the seasonal high water level. As a basis for initial and ongoing mining depths, the proponent shall submit annually the results of quarterly monitoring reports to the Department Services, Division which document the groundwater elevations for a min►murrit_o.fen was supplylwells-around-the-general Perimeferof-t pem�itted"mining area. In the event _. - - that the proponent is unable to obtain or retain measurements from at least ten existing water wells, the proponent shall procure new wells for monitoring purposes at locations acceptable to the Development Services Division. The proponent shall provide a verifiable bench mark at each well site that can be referenced for compliance monitoring. Based on the water depth monitoring reports, Development Services Division may recalculate the expected high groundwater elevation, thereby adjusting the maximum depth of mining. Level of Significance After Mitigation: 9 9 Following implementation of Mitigation Measure 4.2-6a, the impact of fluctuating groundwater levels would be reduced to a less-than-significant level. i All Quar1y_Excavation Alternatives (1, 2. and 3l Impact 4.2-7 sDeyeilopment ofj.PerchedGroundwater Alternatives 1, 2, and 3 propose the construction and operation of processing wastewater basins. The initial basin would be constructed within the Phase IV mining area.. The depth of the basin is not presented in the reclamation plan. The basin would be unlined allowing percolation of the processing water into the underlying sand and gravel deposits. Complex stratigraphy in the area of the initial basin indicate that fine-grained layers of sediment could impede downward migration of the percolating water and possible development of perched groundwater. The perched groundwater could potentially be discharged along the excavation slopes, possibly resulting in erosion or failure of the slopes. In addition, perched groundwater could flow under the Delta Mendota Canal, located 800 feet east of the proposed basin under Alternatives 1 and 2 or the California Aqueduct (under Alternative 3 only). If significant pore water pressure develops, the structural integrity of the canals could be compromised. This is a significant impact. Draft Environmental Impact Report 4-22 ER-96-3