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�4 City of Stockton A68104 02A June 6, 1996 page 2 shallow subsurface geology in the site vicinity consists of a heterogeneous mix ofg ravel, sand, silt, and clay (Hackel, 1966) 2.2.2 H_, dY rology Sediments containing fresh groundwater are largely unconsolidated silts g Y and sands which were derived from river channel, flood plain, and alluvial fan deposits of ' Pliocene to Recent age (Davis and others, 1959) Local groundwater flow may be influenced by pumping of water wells and recharge from various sources The effect of these influences on groundwater beneath the site is beyond the scope of this investigation According to the 1986 San Joaquin County Flood Control and Water Conservation District map, seasonally high groundwater is approximately 15 feet below site grade (BSG), and the regional flow direction in the Stockton area is toward the east Groundwater was encountered beneath the site at approximately 25 feet BSG during the ' drilling of the soil boring for monitoring well MW-1 on June 22, 1995 On June 29, 30, and September 26, 1995 during the development and sampling events, groundwater was measured at approximately 22 feet BSG ' 2.3 Pro.ect History Twining g un derstands that a 350-gallon gasoline underground storage tank (UST) and fuel dispenser was removed from the site on December 3, 1992 The analytical results of for a soil sample collected at that time from approximately 10 feet BSG beneath the UST indicated toluene at a concentration of 220 micrograms per kilogram (,ug/kg), Xylenes at a concentration of 420 ug/kg and total petroleum hydrocarbons - gasoline range (TPH-G) at a concentration of 13 milligrams per kilogram (mg/kg) (GeoAnalytical Laboratories, Inc , 1992) Excavation work was conducted on July 7, 1994 in an attempt to remove the soil impacted with petroleum product constituents (PPCs) The results of boundary soil samples collected from the excavation are summarized in Table 1 t