Guadalupe Dam Mercury Investigation Project

Guadalupe Dam Mercury Investigation Project

The Santa Clara Valley Water District (Valley Water), as the lead agency, certified the Dam Maintenance Program (DMP) Program Environmental Impact Report (PEIR) in January 2012, for the routine maintenance and repair of fourteen regulated water retention facilities that are operated under licenses granted by the State of California Department of Water Resources, Division of Safety of Dams (DSOD). Guadalupe Reservoir is one of the fourteen facilities included in this PEIR. The DMP PEIR covers up to 67 different maintenance activities, subdivided into fourteen categories. Among those categories is inspections, monitoring, and exploratory work. Since adoption of DMP PEIR, Valley Water has been developing design alternatives for the Guadalupe Dam Seismic Retrofit (GDSRP) to meet DSOD and Valley Water requirements. The Addendum to the DMP PEIR proposes to conduct additional investigations and environmental sampling as part of a Mercury Assessment Work Plan for the GDSRP, with the purpose of determining subsurface conditions and the concentration of chemical constituents (primarily mercury and other metals) within surficial and subsurface soils. These activities are required to inform the final design for the GDSRP and will also be used to determine, prioritize, and monitor the ongoing maintenance needs for Guadalupe Dam and the appropriate methodologies to implement those maintenance needs over time. The supplemental investigation may include environmental sampling in up to 71 locations in and around the Guadalupe Dam and Reservoir and may be undertaken over multiple phases. Of the 7 locations, 11 are within the Area of Routine Maintenance Effects (ARME) already evaluated by the DMP PEIR. The proposed exploratory sampling will include three general approaches: • Surficial soil grab samples from 0 to 1 feet below ground surface (bgs), using hand equipment such as a trowel or small shovel. Samples collected using this method would occur in areas downstream of the dam or in dry areas of the reservoir, below the elevation of the spillway and many of which would be below the DSOD-restricted level of El. 601 NAVD 88. Sampling in areas below the DSOD-restricted level would occur when reservoir levels are low, to allow access, such that all sampling activity would occur in dry areas of the reservoir and no in-water work would be required. • Hand augers or subsurface borings from 1 to 5 feet bgs, using either hand equipment (e.g., hand augers) or mechanical equipment (e.g., drill rig). Sampling locations utilizing these methods would occur in areas at or downstream of the dam or upstream of the dam above the elevation of the spillway (El. 619.3 feet NAVD 88). • Subsurface borings or test pits from 4 to 50 feet bgs, advanced using mechanical equipment (e.g., drill rig, excavator, or backhoe). All of the sampling locations utilizing these methods would occur on the dam embankment or in areas upstream of the dam above the elevation of the spillway (El. 619.3 feet NAVD 88). Any surficial ground disturbance resulting from vehicular access in the hillside and shoreline areas will be in accordance with the applicable BMPs and requirements provided Santa Clara Valley Habitat Plan (VHP) (Santa Clara Valley et al. 2012), Valley Water’s DMP PEIR (SCVWD 2012), and BMP Handbook (SCVWD 2014).

Santa Clara

Santa Clara