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The model presented is interpretational and will be modified as more data is collected-The four vertical zones identified are as follows (depths are approximate and can vary by several feet across the site) I Vadose Zone-Extending from surface grade to 54 feet bsg,the depth to the water table The vadose zone is predominantly fine grained material from surface gra-le to 54 feet bsg, with a coarse gram sand unit occurring between 20 feet and 25 feet bsg across a portion of the site The zone is characterized by low pore pressure Hydrological Unit 1 (HU1), at the top of the current saturated zone, extends from 54 feet to 80 feet ' bsg and is comprised of silty sand to sandy silt and characterized by predominantly high pore pressure Hydrological Unit 2 (HU2)extends from 80 to 96 feet bsg, it is composed of an upper 10-foot layer of fine-grained soil underlain by an approximately 5-foot sand and gravelly sand interval HU2 is characterized by low pore pressure ' ' Hydrological Unit 3 (HU3) extends from 96 feet bsg to and possibly beyond 134 feet bsg Silt and clay are the dominant soil type in the interval The interval is characterized by pore pressure ' variations from moderate to high pressures IUNDERGROUND]ND STORAGE TANK REMOVAL Two 550-gallon USTs were removed from the site on 03 October 1986 by Stockton Service Station Equipment Company According to correspondence from the PHS-EHD,both USTs were reportedly used to store gasoline Upon removal of the USTs, one soil sample was collected from the floor of the excavation The sample was analyzed by California Water Labs,Inc ,a state-certified analytical ' laboratory, for total petroleum hydrocarbons quantified as diesel (TPH-d), benzene, toluene and xylene TPH-d and xylene were detected in the soil sample at concentrations of 3,000 parts per million (ppm) and 0 188 ppm, respectively Benzene and toluene were not detected above the analytical method reporting limit of 0 010 ppm Based upon results of the soil sample analysis,the PHS-EHD required assessment of the lateral and vertical extent of hydrocarbon-impacted soil and/or ground water in the area of the former UST M1 PREVIOUS SUBSURFACE INVESTIGATION On 18 September 1997,five soil probe borings(B-1 through B-5)were advanced at the site to assess the lateral and vertical extent of petroleum-impacted soil TPH-g was detected in samples from B 1, ' B3 and B5 at concentrations as high as 14,000 ppm(reported as milligrams per kilogram) Benzene was detected in only one sample(B 1-40), which contained a concentration of 0 042 ppm Toluene, ethylbenzene and xylenes were detected in samples from B1, B3 and B5 at concentrations as high as 1,600 ppm (xylenes in B5-25) TPH-d was detected in samples from B-1, B-3 and B-5 at Advanced GeoEnvironmental,Inc I