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City of Stockton A68104 02B <br /> August 28, 1996 Page 2 <br />' shallow subsurface geology in the site vicinity consists of a heterogeneous mix of gravel, <br /> sand, silt, and clay (Haskel, 1966). <br /> 2.2.2 Hydrology Sediments containing fresh groundwater are largely unconsolidated silts <br /> and sands which were derived from river channel, flood plain, and alluvial fan deposits of <br /> Pliocene to Recent age (Davis and others, 1959) Local groundwater flow may be <br />' influenced by pumping of water wells and recharge from various sources The effect of <br /> these influences on groundwater beneath the site is beyond the scope of this investigation <br />' According to the 1986 San Joaquin County Flood Control and Water Conservation District <br /> map, seasonally high groundwater is approximately 15 feet below site grade (BSG), and the <br /> regional flow direction in the Stockton area is toward the east <br /> Groundwater was encountered beneath the site at approximately 25 feet BSG during the <br /> drilling of the soil boring for monitoring well MW-1 on June 22, 1995. On June 29, 30, and <br /> September 26, 1995 during the development and sampling events, groundwater was <br /> measured at approximately 22 feet BSG On August 19, 1996 prior to the destruction, <br />' groundwater wasmeasured at 19 2 feet BSG <br /> 2.3 Project History Twining understands that a 350-gallon gasoline underground <br /> storage tank (UST) and fuel dispenser was removed from the site on December 3, 1992 <br /> The analytical results of for a soil sample collected at that time from approximately 10 feet <br /> BSG beneath the UST indicated toluene at a concentration of 220 micrograms per kilogram <br /> (pg/kg), xylenes at a concentration of 420 ,ug/kg and total petroleum hydrocarbons - <br /> gasollne range (TPH-G) at a concentration of 13 milligrams per kilogram (mg/kg) <br /> (GeoAnalytical Laboratories, Inc., 1992) Excavation work was conducted on July 7, 1994 <br />' in an attempt to remove the soil impacted with petroleum product constituents (PPCs) The <br /> results of boundary soil samples collected from the excavation are summarized in Table 1. <br /> 1 <br />