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7A - ATTACHMENT NO.3 Saltworks Proposal- Water Group Summary Report (22 January 2010) Page 28 3.5 Groundwater Supply The DMB Supply Report states that most of the project's potable water demands will be satisfied with up to 700 AFY of groundwater from an onsite production well. DMB subsequently proposed a surface water transfer as the source of potable water supply, thereby reducing or eliminating the proposed reliance on groundwater. But since the Saltworks project may need to rely on a range of water supply sources, this analysis provides a preliminary assessment of the feasibility of a groundwater supply and identification of unresolved issues surrounding the sustainability of such a supply. These issues will need to be investigated further if reliance on groundwater supply is reinstated at some point in the future. 3.5.1 Description of Groundwater Occurrence and Use in the Area The Saltworks site is located near the confluence of Redwood Creek and Westpoint slough near the Port of Redwood City (see Figure 5). The Saltworks site covers 1,436 acres or 2.24 square miles (mi2). A portion of the site is a former salt production facility that has been in operation since 1901. Current access to the site is via Seaport Boulevard at the interchange of U.S. 101 and Woodside Road (SR 84). The site is bordered on the west by Redwood Creek, on the north by Westpoint and First sloughs, and on the east by Flood slough; these water courses discharge into the adjacent San Francisco Bay. Networks of levees surround and divide the site into salt evaporator compartments. The levees were used to control and flood the salt evaporators with Bay water for salt production. 3.5.1.1 Hydrogeologic Setting The site is located in an eastward facing intertidal and wetland area of San Francisco Bay. The average annual rainfall at the site ranges from 14 to 16 inches (Rantz, 1969). Current surface elevations on the site range from sea level to about 10 feet above mean sea level (msl). The mean range of tidal fluctuations is approximately 6 feet (USGS Redwood Point Quadrangle map). The shipping channel of the adjacent Port of Redwood City is about -18 feet msl while Westpoint slough is about -6 feet msl. Greco and Bair Islands are adjacent to the site on the northeast and northwest, respectively. The Hetch Hetchy Bay Division Pipelines, owned and operated by the SFPUC, are located between 3,000 and 6,000 feet due south of the Saltworks site. The Saltworks site is in the San Mateo Subbasin, which is part of the larger Santa Clara Valley Groundwater Basin (DWR, 2004). This subbasin resides in a long structural trough bounded by the Santa Cruz Mountains on the southwest and San Francisco Bay on the northeast. The basin has been infilled with eastward-dipping, alluvial sediments of Plio-Pleistocene and Quaternary age. The subbasin is bounded by the Westside Subbasin on the north and San Francisquito Creek on the south. Depth to bedrock (Franciscan Complex) at the project site is about 400 feet below the land surface (Fio and Leighton, 1995). Surface geology at the site consists of bay mud and artificial fill (Brabb and Pampeyan, 1983). Bay mud contains unconsolidated silty clay and clay with abundant organic matter and interspersed lenses of sand, peat, gravel, and shell fragments. The thickness of the mud varies from zero at landward edge to as much as 130 feet. Artificial fill contains loose to consolidated gravel sand, silt, clay, rock fragments, organic matter, and manmade debris in various combinations. Fill thickness is variable and may exceed 21