SFPUC 2001 Alameda Watershed Management Plan

III. ENVIRONMENTAL SETTING AND IMPACTSD. HYDROLOGY AND WATER QUALITYthe other geologic units in the area. The alluvium is located in the Sunol Valley and alongstream channels to depths of about 60 feet and is composed of coarse sand and gravel depositswith high permeability. Typically located below the alluvium, the Livermore Gravels are foundin the Livermore Valley and Sunol Valley to depths of at least 500 feet; this geologic unit hasbeen shown to have low water transmissivity and to contain limited groundwater. Bedrock islocated at depths greater than 500 feet below the ground surface. Groundwater development inthe Sunol Valley has been limited to historical operation of the Sunol Infiltration Galleries and tothe shallow dewatering wells associated with mining operations. 1 Both activities are locatedalong Alameda Creek in the secondary Watershed.The Sunol Infiltration Galleries (sometimes referred to as the Sunol Filter Galleries, and shownin Figure III.D-1) and the Sunol Water Temple were constructed by the Spring Valley WaterCompany in 1901. The system was originally designed to provide passive use of shallowgroundwater by capturing downstream flows in Alameda Creek from groundwater that wasbacked up behind the Sunol Dam. The Sunol Dam was built in 1889 and is located downstreamof the Infiltration Galleries and the Sunol Water Temple. The Infiltration Galleries areessentially subsurface concrete tunnels with holes in the side walls, constructed at depthsbetween 10 and 20 feet below the ground surface. They are approximately 9,000 feet long andrun parallel to Alameda Creek, beginning at the Sunol Dam and a point just west of I-680, andmeet at the Sunol Water Temple. Groundwater seeps into the Infiltration Galleries, wherecollected groundwater flows downstream into the Sunol Water Temple and eventually to theSunol Aqueduct and Niles Reservoir.Seepage into the Infiltration Galleries was increased through the installation of perforated pipesdirectly beneath the Alameda Creek channel and through the construction of gravel dams in thecreek channel. In the 1960s, the Sunol Pump Station was constructed to pump flows from theInfiltration Galleries into San Antonio Reservoir and to the Sunol Valley WTP. Since theconstruction of San Antonio Reservoir in 1965, flows to the Infiltration Galleries have beenreduced, and the SFPUC has ceased construction of gravel dams in Alameda Creek. Water in theshallow alluvium that percolates into the Infiltration Galleries is pumped to San AntonioReservoir. The Sunol Aqueduct has been decommissioned.In addition to the Infiltration Galleries, the majority of historical groundwater use in the SunolValley has been associated with gravel mining. Mining operations that affect the groundwatersystem include pumping groundwater for use in processing, diverting groundwater to emptymining pits or Alameda Creek in order to dewater the pits, and constructing slurry cutoff walls tolimit groundwater flow to the mining pits. The dewatering wells are located almost exclusivelywithin the upper layer of alluvium. Slurry cutoff walls made of bentonite (a clay material) havebeen installed within the alluvium to depths of 50 feet around the perimeter of most of themining excavations to seal the upper alluvium and to prevent the flow of groundwater to the pits.While the mining pits extend to depths of 100 to 140 feet, the need for groundwater control is1 It should be noted that although it is referred to as shallow groundwater in this section, the groundwater interceptedby the Infiltration Galleries is considered by the California Department of Health Services to be surface water dueto the connection these waters have with the shallow alluvium in Alameda Creek.NOP 96.223E: Alameda Watershed Management Plan III.D-6 ESA / 930385January 2001