SFPUC 2001 Alameda Watershed Management Plan

III. ENVIRONMENTAL SETTING AND IMPACTSD. HYDROLOGY AND WATER QUALITYlimited to the upper 50 feet. Below 50 feet, there is limited groundwater inflow to the pits. Thedewatering wells used in the gravel mining operations have essentially diverted shallowgroundwater in the alluvium to empty quarries, settling ponds, or Alameda Creek so thateventually it is returned to the groundwater system. In general, gravel mining has not affectedthe pattern of groundwater flow beneath the valley. However, not all mining pits in the SunolValley have slurry cutoff walls to prevent migration of shallow groundwater into the pits.Accumulated groundwater in the pits is sometimes used as process water for the miningoperations, but such use could result in the need to discharge turbid water into Alameda Creekduring the rainy season. Direct discharges to Alameda Creek are subject to requirements of theRegional Water Quality Control Board in order to protect water quality in the creek.A review of data from approximately 18 existing monitoring wells in the Sunol Valley indicatesthat shallow groundwater levels in the alluvium typically occurs 20 to 30 feet below the groundsurface, and groundwater flow is parallel to Alameda Creek. Two production wells in the valleyextend into the Livermore Gravels. One well is used for small-capacity nursery irrigation, andthe other well was formerly used for plant process water by one of the mining operators. Fieldtesting of one of the production wells was conducted for the Alameda Creek Water ResourcesPlanning Study and confirmed that there is limited groundwater availability or storage capacityin the Livermore Gravels (deeper than 50 feet). It was also determined that a network of 20 to60 wells (drilled to a depth of 50 feet) would be needed to effectively pump water from thealluvium. Therefore, the study concluded that there is a low potential for groundwaterdevelopment in the Sunol Valley for water supply. Groundwater quality testing for the studyindicated that the groundwater generally meets primary drinking water standards. There is noevidence that gravel mining has affected groundwater quality, but there are locally elevatednitrate concentrations in groundwater in the vicinity of historical farming in the area.1.6 GRAVEL MINING•Historical and current gravel mining operations in the Sunol Valley have removed a largequantity of the valley’s alluvium, which has altered surface and ground water flow as well asgroundwater storage. Mining operations involve major earthmoving and excavation activities,and historical mining has resulted in several excavations along Alameda Creek between theSan Antonio Pump Station and I-680 (see Figure III.B-2). Before mining operations began in the1960s, Alameda Creek apparently flowed naturally through an area now occupied by one of thelargest excavations. The current creek alignment has been relocated along the western edge ofone of the excavations, and mining has extended to depths of 100 to 140 feet. As part of SMP-24mining operations, slurry cutoff walls made of bentonite have been constructed in the upper50 feet or so around the perimeter of the excavations on three sides of SMP-24 to limit the inflowof shallow groundwater to the pits. The fault trace along Calaveras Road acts as an impermeablebarrier to groundwater and serves as a fourth wall. SMP-24 requires minimal dewatering toreach the total pit depths (Luhdorff and Scalmanini Consulting Engineers, 1993). Other miningoperations in the Sunol Valley area remove groundwater inflow out of the pits through pumping.NOP 96.223E: Alameda Watershed Management Plan III.D-7 ESA / 930385January 2001