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REQUEST FOR SITE CLOSURE REPORT Former Beacon Station #610 3300 Waterloo Road, Stockton, Califorma AMV Project No 19042 04 3.2 Current Distribution of Petroleum Constituents in Ground Water -5- Ground water monitoring wells MW -5, MW -6, and MW -7 were sampled on October 13, 1994 Because the depth to ground water currently exceeds 60 feet below grade, ground water samples were not collected for analysis from monitoring wells MW -1 through MW -4 during this sampling event. Ground water samples collected on October 13, 1994 (indicative of recent ground water conditions), were submitted for laboratory analysis of BTEX, TPHg, and TPHd The sampling results are compiled in Table 4 Concentrations of dissolved benzene were not detected in ground water samples collected from monitoring well MW -5, MW -6, or MW -7. The current inferred distribution of dissolved benzene in ground water is illustrated on Figure 7. 3.3 Mass Balance of Petroleum Constituents in the Soil and Ground Water Active remediation of the took place as part of the UST removal process Approximately 900 cubic yards of soil were excavated and removed from the site during UST removal in October 1993. An initial 150 cubic yards of soil, consisting of overburden material removed from the is excavation prior to the UST removal, were transported to a local landfill Four soil samples for every 50 cubic yards of soil were collected from the stockpiled soil (approximately 750 cubic yards) and composited in the laboratory for analysis of BTEX, TPHg, TPHd, EDB, total lead, and organic lead The 750 cubic yards of soil extracted during the overexcavation process reportedly contained an average TPHg concentration of 136 mg/kg (approximately equivalent to parts per million (ppm)) and an average TPHd concentration of 2,200 mg/kg (Table 6) Assuming a soil density of 2,800 pounds per cubic yard, approximately 285 6 pounds of TPHg and 2,464 pounds of TPHd were removed from the subsurface of the site as of November 1993 These values are equivalent to approximately 46 1 gallons of TPHg and 347 gallons of TPHd Values were approximated using the following equation (n parts of TPHg or TPHd/1,000,000 parts of soil) x (2,800 lbs of soil/yd' of excavated soil) x (v yd3 of excavated soil) = lbs of TPHg or TPHd where, n = average concentration of TPHg or TPHd in soil (from Table 6) v = volume of stockpiled soil used to determine the average concentration of TPHg or TPHd in soil Based on the results of soil sampling conducted during October 1993 and September 1994, some soil impacted with petroleum constituents remains along the southwest extent of the UST excavation and at approximately 35 to 40 feet below grade in the vicinity of vapor extraction wells VE -1 and VE -2 As illustrated on Figure 8, two geologic cross-sections illustrating the rptl M mj BQ011525