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Ms. Margaret Lagorio, SJCEHD January 5,2005 <br /> Work Plan for Additional Site Assessment and Project No. 2033-15600-01 <br /> Evaluation of Remedial Alternatives <br /> Joe's Truck Plaza,Lathrop,California <br /> Page 3 <br /> SUBSURFACE CONDITIONS <br /> i <br /> Geology <br /> Soils beneath the study area consist primarily of fine-grained material (predominantly <br /> silty clay, clayey silt, and clay, with minor amounts of silt, and sandy silt). Three <br /> laterally extensive sand strata (identified as Sand A, Sand B, and Sand C) were <br /> encountered within this fine-grained unit, along with several thin, laterally restricted sand <br /> bodies. Sands A, B, and C do not appear to be stratigraphically connected. <br /> i <br /> 1 Sand A is encountered approximately 5 to 8 feet bgs, and is approximately 3 to 10 feet <br /> thick. Sand B is encountered at depths between 25 and 35 feet bgs, and is approximately <br /> 7 to 18 feet thick. Sand B appears to extend north to south across the entire study area, <br /> but may be limited laterally to the southwest. Sand C is encountered at depths between <br /> 50 and 65 feet bgs, and is approximately 15 to greater than 25 feet thick <br /> Hydrogeology <br /> All existing groundwater monitoring wells except, MWO-1X, appear to be screened in <br /> Sand A. Groundwater flow has generally been towards the northeast and east, although <br /> north and northwest flow has sporadically been observed. Groundwater gradients have <br /> been relatively flat, between 0.00083 and 0.0015 ft/ft. Groundwater beneath the site <br /> fluctuates from approximately 6 to 10 feet bgs, with quarter-to-quarter variations of 1 to 2 <br /> feet. Fluctuations in groundwater elevation do not appear seasonal. Groundwater in <br /> Sands A, B, and C appear to be under similar hydraulic head. <br /> Extent of Petroleum Hydrocarbon Impact in Soil <br /> The vertical extent of MTBE impact in soil has been adequately characterized, and <br /> extends to a maximum depth of approximately 25 feet bgs in the immediate vicinity of, <br /> and downgradient from, the former northern UST's. Laterally, impacted soil appears <br /> restricted to the immediate vicinity of, and in the capillary fringe downgradient from, the <br /> former northern UST's. Impact beneath the former southern UST appears to extend no <br /> deeper than 20 feet bgs, and laterally appears restricted to the immediate vicinity of the <br /> former UST. <br /> Extent of Petroleum Hydrocarbon Impact in Groundwater <br /> Historical monitoring data indicate the chemicals of concern dissolved in groundwater <br /> include total petroleum hydrocarbons as gasoline (TPHG) and MTBE. Low <br /> concentrations of tert butyl alcohol (TBA) and tert amyl methyl ether (TAME) have also <br /> been reported on a sporadic basis. <br /> i <br /> K:Uoe's Truck Plaza\Work PlansUoes workplan 010505.doc <br />