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November 12, 1992 Working To Restore Nature ARCO Station No. 6100, Tracy, California To evaluate the extent of gasoline hydrocarbons in soil, ARCO requested that RESNA perform a subsurface investigation In October and November 1991, RESNA observed i drilling of 14 soil borings (B-6 through B-19) and construction of rune vapor-extractions wells The boring and vapor-extraction well locations are shown on Plate 2. Residual gasoline hydrocarbons were detected in soil beneath the dispenser islands, between the underground gasoline storage tanks and the dispenser islands, and along the north and west side of the former gasoline tank cavity (RESNA, January 31, 1992, Report 30041-2) The lateral extent of residual gasoline hydrocarbons in soil was delineated at the site except for the area near the northeast corner of the station building and south and southeast of the dispenser islands Although trace to low concentrations of TPHg and BTEX were detected below 70 feet bgs, the vertical extent of residual gasoline hydrocarbons in soil appeared to be limited to a depth between 50 and 70 feet bgs, dependent upon location. RESNA performed a vapor-extraction test at the site on October 14, 1991, using vapor-extraction wells VW-1, VW-2, VW-3, and VW-4 (RESNA, January 31, 1992, Report 30041-2) The results of the test indicated that vapor extraction is a feasible means to remove residual gasoline hydrocarbons from the soil CURRENT WORK Soil Boren s A geologist from RESNA was at the site June 1 through 4, 1992, to observe Woodward Drilling Company(Woodward) of Rio Vista, California,drill six 8-inch-diameter soil borings (B-20 through B-25) The boring locations are shown on Plate 2 The purpose of drilling borings B-21, B-22 and B-24 was to install additional vapor-extraction wells (VW-11, VW-12 and VW-14, respectively) at selected locations within the interior of the plume The purpose of drilling borings B-23 and B-25 was to perform lateral delineation at locations beyond the southeast property line Soil samples were collected during drilling at approximately 5-foot intervals and were identified using visual and manual methods and classified according to the Unified Soil Classification System (Plate 3) Descriptions of earth materials encountered are presented in the Logs of Borings (Plates 4 through 18) The materials consisted predominately of silt and silty clay The borings were terminated at depths ranging from 26-1/2 to 90 feet bgs Ground water was not encountered in any of the borings Woodward constructed vapor-extraction wells VW-10 through VW-15 in borings B-20 through B-25, respectively Details of vapor- extraction well construction are shown on the Logs of Borings and are discussed in more detail in an attachment to this report 1� 0623cear 30041-3 2