AgdaPkt 2010-02-01 clsd and regular

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7A - ATTACHMENT NO.3 Saltworks Proposal- Water Group Summary Report (22 January 2010) Page 39 will reduce the chances of saltwater intrusion, while water levels below sea level will increase the chances of saltwater intrusion. Downward Migration of Poor Quality Groundwater. The potential also exists for vertical downward migration of poor quality from shallow to deeper groundwater. As noted (EKI, 2006), shallow groundwater is very saline, up to three to four times saltier and denser than seawater. This higher salinity results from infiltration of brine from the overlying salt evaporation ponds and salt management on the site as well as lateral migration of saltwater from the watercourses of the estuary. While surficial bay muds and shallow clay aquitards have been documented in Redwood City along the Bay (Todd, 2003) and are inter-bedded with freshwater aquifers, the lateral continuity of these low-permeability zones in preventing downward saline water migration has not been established at the project site. Because of the density contrast between saltwater and freshwater, saltwater will tend to sink while freshwater will tend to float. Production well pumping beneath the site will increase the vertical hydraulic gradient and will promote the overlying denser brines to migrate downward. Several downward migration paths could exist at the site. The disposition of several abandoned wells in the vicinity of Saltworks is unknown and may provide direct leakage of shallow brackish water to the underlying system; these are referred to as conduit wells. Poland and Garrett (1943) documented several of these wells with structural failures (see Table 4). Installation of deep pilings to mitigate liquefaction and stabilize building foundations also could compromise the effectiveness of clay layers as vertical barriers. In addition, downward vertical migration of shallow brine or other shallow poor- quality groundwater may result from desiccation of underlying clays and widening of micro-fractures that may become potential pathways. Much of the site is composed of artificial fill which has a propensity for piping and associated conduits. Finally, any clay layers that currently provide separation of shallow low-quality groundwater and deeper freshwater aquifers may be compromised once pumping is increased beneath the site. Upward Migration of Poor Quality Groundwater. Water quality deterioration also could occur as a result of intensive pumping that induces upward migration or "upconing" of poor quality groundwater from deep zones. The underlying saltwater rises beneath pumping wells to compensate for the density differences between saltwater and the overlying freshwater aquifers, which must be in hydrostatic equilibrium. While water quality data from deep zones at the site are not available, a recent study (Newhouse, et aI., 2004) included sampling and analysis of the Eleanor Park multiple-well monitoring site in Palo Alto (about three miles southeast of Saltworks). The Eleanor Park site includes two deep (greater than 720 feet deep) monitoring wells; the concentrations of TDS in water from the deepest and second-deepest wells were 1,500 and 2,700 mg/L, respectively. As noted (EKI, 2009), occurrence of such poor quality water at the project site would represent a potential for upward migration. Other Potentiallv Contaminatina Activities EKI (2009) provides a brief review of adjacent chemical uses and release sites, indicating the existence of about 50 sites within one-quarter mile of the Saltworks site. Specific sites noted as a primary concern are the former Menlo Park Sanitary Landfill, Pilot Petroleum plume, and a chlorinated vac plume. Figure 5 shows the location of two sewage disposal areas, one landfill, and industrial activities at the Port of Redwood City. EKI (2009) notes that the release sites are not a major problem to a potential 32