ARCHIVED REPORTS XR0000152 - Laserfiche WebLink

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detectable volatile petroleum hydrocarbon concentrations and a significant decrease in residual diesel fuel concentrations (7,100 ppm diesel in 1991, 2,900 ppm diesel in 1993) In November 1994, the former tank location was overexcavated in an attempt to remove residual diesel fuel concentrations remaining beneath the original tank excavation backfill at approximate depths of 10 to 20 feet below grade Approximately I8 yd' or 25 tons of excavation fill material was segregated from the approximately 33 yd' or 50 tons of native soil generated during excavation procedures Both the fill material and native soil were found to be impacted by petroleum hydrocarbons and were subsequently disposed offsite Following the removal of the approximately 51 yd' or 75 tons of soil from the former tank location, residual diesel concentrations of 740 ppm (excavation floor) and 900 ppm (southwest side wall) were encountered in verification soil samples These residual concentrations of diesel fuel are significantly less than the 2,900 ppm observed in 1993 9.2 Leaching Potential Analysis Table 2-1 of the State of California's Leaking Underground Fuel Tank Task Forces "Leaking Underground Fuel Tank (LUFT) Field Manual", February 1989, was consulted in order to estimate the concentrations of residual diesel fuel that can be left in place without threatening groundwater The depth to groundwater employed in calculation was based on the water level in the onsite well (135 ft) The average annual precipitation data was derived from a thirty year annual average of 14 04 inches Iisted in the National Oceanic and Atmospheric Administrations "Local Climatological Data - Annual Summary with Comparative Data - Stockton, California" Unique site feature's, which lend to contaminant migration, were considered to be coarse-grained site soils, near by agricultural/industrial wells, the Calaveras River and the Mokelumne Aqueduct The score tabulated utilizing the referenced table was 44 points Based on this score the maximum allowable diesel concentration is 1,000 ppm A copy of the Table 2-1 worksheet is included in Appendix D for reference Table 2-3 of the LUFT field manual for calculation of acceptable cumulative soil contamination levels was reviewed, however, due to the lack of detectable volatile hydrocarbon concentrations (the constituents with the greatest leaching potential) The worksheet was not applicable No health standards are currently established for diesel fuel concentrations, only for the volatile components of fuel M7J01-27-95/UST/95-0011 nbr 12