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0 0 <br /> 2.4 Regional Groundwater Quality <br /> According to the Northeast San Joaquin County Groundwater Banking Authority <br /> (NSJCGBA, 2002), an estimated 60% of the water needs of the Stockton area were met <br /> by pumping from groundwater wells. Increasing groundwater pumping in the region, due <br /> to increased water demand caused by a combination of increased population in the <br /> Central Valley and periodic droughts, resulted in over-pumping of the regional aquifer, <br /> with an all-time low in regional groundwater levels in 1993. As a result of this over- <br /> pumping, saltwater from the San Francisco Bay in the western area of the Delta has <br /> intruded into the regional aquifer from the west. <br /> The saline front' extends under the city of Stockton (including the Site) and is advancing <br /> eastward at an estimated rate of 150 - 250 feet per year (NSJCGBA, 2004). Due to the <br /> seawater intrusion from the west, high TDS concentrations render groundwater beneath <br /> the Site not suitable for use as drinking water, or for use in commercial crop production. <br /> 2.5 Constituents of Concern at the Site <br /> The MGP-related residues at the Site were lampblack, tar, and petroleum fuel. The <br /> compounds that have been detected in groundwater samples collected from the Site <br /> include polycyclic aromatic hydrocarbons (PAHs), diesel- and gasoline-range petroleum <br /> hydrocarbons, and benzene, toluene, ethylbenzene, and xylenes (BTEX). The main <br /> source areas included the locations of two former gas holders, the former lampblack pit, <br /> and the former tar pit and tar sump within Area I. The primary constituents of concern <br /> (COCs) in groundwater at the Site are benzene and PAHs. <br /> 2.6 Completed Remediation Activities <br /> Various phases of soil and groundwater investigations and remedial activities have <br /> been conducted at the Site since 1987. Approximately 20,600 tons of affected soil was <br /> excavated from Area I for offsite disposal between 1999 and 2000. Following excavation <br /> activities, Oxygen Release Compound (ORC) was injected through the bottom of the <br /> excavation of the gas holder in Area I, and an engineered cap was placed over the <br /> backfilled excavation area. <br /> The DTSC-approved remedial action for the Site includes maintenance of the cap and <br /> regular groundwater monitoring. The current monitoring network at the Site includes a <br /> The saline front is defined as 300 milligrams per liter(mg/L) of total dissolved solids (TDS) in <br /> groundwater. <br /> 5 ENVY <br /> A M E A 1 C A <br /> PAPGM PGE-0S-0t Stockton MW26R Investigation\MW26 Work PIan1PGE Stockton MW26R WP.doc <br />