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<br />7A - ATTACHMENT NO.3 <br />Saltworks Proposal- Water Group Summary Report (22 January 2010) Page 38 <br /> <br />would be needed to meet project goal of 700 AFY. The number of onsite production <br />wells would likely range from two to five wells, depending on actual site-specific <br />conditions. <br /> <br />The amount of pumping that could be conducted on a sustainable basis from a number <br />of onsite production wells is unknown. A preliminary subbasin water balance indicated <br />that the groundwater resource could potentially be more developed given the amount of <br />outflow estimated for the freshwater aquifer systems. Although the potential exists for <br />capturing some amount of outflow from the groundwater basin, the actual amount <br />depends on pumping location and ability of the wells to capture the water. Wells at this <br />site would not likely be capable of capturing a significant portion of the perennial (safe) <br />yield of the groundwater basin. Over-pumping to capture a significant quantity of outflow <br />increases the risk of inducing poor-quality water into the aquifer along a variety of <br />potential migration pathways. This issue is discussed in more detail below. <br /> <br />3.5.2.2 Groundwater Quality <br /> <br />Unresolved water quality issues focus on the potential for groundwater pumping to <br />induce inflow into these freshwater aquifers of poor quality water. Three potential <br />mechanisms exist that could result in water quality deterioration: regional lateral <br />saltwater intrusion, vertical "upconing" of poor quality water from depth, and vertical <br />downward migration of poor quality shallow groundwater. In addition, several sites with <br />potentially contaminating land uses are nearby, and documented use and release of <br />chemicals represent a potential for vertical and horizontal migration of contaminated <br />groundwater into the site. <br /> <br />Saltwater Intrusion <br />Saltwater intrusion is the movement of saline water into freshwater aquifers. Intrusion is <br />most often caused by groundwater pumping from wells but can result from decreases of <br />rainfall recharge that produce a groundwater imbalance. Saltwater intrusion reduces <br />fresh groundwater storage and can lead to the abandonment of supply wells. Even if <br />saltwater only represents about two percent of the groundwater, a supply well can be <br />considered contaminated (Todd, 1980). Although groundwater treatment was <br />envisioned at the Saltworks site, increasing salinity from seawater intrusion would likely <br />require significant changes to the treatment system. <br /> <br />Three migration pathways for saltwater intrusion are possible at the Saltworks site <br />including lateral, vertical downward, and vertical upward movement of saltwater. <br />Historical seawater intrusion has been documented in many areas of the South Bay and <br />probably has occurred in the vicinity of the proposed project under historical pumping <br />conditions (Metzger and Fio, 1997). <br /> <br />Lateral Migration. The position of the saltwater wedge is a dynamic balance between <br />the saltwater and the freshwater interface, which is controlled by the outflow of <br />freshwater at the interface and the density contrast between saltwater and freshwater. <br />Less outflow (i.e., more pumpage or less groundwater recharge) causes further inland <br />movement of the saline water, while less pumpage and more groundwater recharge <br />repels the position of the saltwater wedge to near the shoreline. Saltwater and <br />freshwater have a density of about 1.025 gm/cm3 and 1.000 gmllcm3, respectively; <br />saltwater is heavier then freshwater. A hydrostatic balance must exist between saltwater <br />and freshwater. In general, onshore water level elevations that remain above sea level <br /> <br />31 <br />