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Al i..o INTRODUCTION 'The diesel locomotive maintenance and fueling facility for the Stockton Terminal& Eastern Railroad (ST&E)-is'located at 1282 Shaw Road in Stockton,Califorriia (Figure 1): An underground storage tank (UST) for diesel fuel was formerly located on the property, and diesel-contaminated soil was detected beneath the UST during its removal in: 1998. A subsurface drilling investigation was subsequently undertaken.in 1999 to determine the extent of contamination. Upgradient Environmental Consultants concluded in its Site Investigation crud Closure Report that contamination was very limited both laterally and vertically and that groundwater was.not impacted. Upgradient recommended that the case be closed, 'since it_met the conditions fnr site closure under Appendix B of. the Tri-Regional Board Recommendations-for Preliminary Investigation and Evaluation of Underground Tanks Sites. San Joaquin County'Public Health Services, Envh`onmental Health Division(PHS/EHD) is the lead agency responsible for overseeing the investigation of the site. After reviewing the , Upgradient report, PHS/EHD issued a request for additional information supporting the recommendation for site closure. Specifically, PHS/EHD requested ST&E to calculate the mass of hydrocarbons remaining in place and model the fate and transport of this contamination in the subsurface environment. SME subsequently retained U0gradient Environmental to perform these activities. C/ 2.0 BACKGROUND - S The SME diesel facility is located in an industrial area in east Stockton, on a parcel that is several acres in-size aid is adjacent to the SME freight switching yard (Figure 2). ,The former UST-was located alongside the first engine-house track, and the locomotives were fueled from a dispenser that was located above the UST: The area was surfaced with crushed rock(railroad b{allast), and the UST was buried at a depth of approximately'l5 feet. After the UST was',exposed and removed with a backhoe,the excavated soil was returned to the cavity. This soil was then covered with a sheet of.plastic, and additional soil was imported to bring the excavation back up to surface grade. Four borings were drilled in and around the UST excavation.and soil samples were . collected at 5-foot intervals to identify the lithology and determine the extent of diesel contamination.: Diesel was detected only in samples from the boring that was drilled through-the excavation;,'hydrocarbons were present at 15 and 20 feet below grade. Total' Petroleum Hydrocarbons in the diesel range were detected at a concentration of 8,500 parts per million (ppm) at 15 feet and 9,100 ppm at 20 feet_ Trace amounts of two aromatic compounds (ethylbeitzene and xylene)were reported at 0.89 and 1.98 ppm' respectively. The hydrocarbons were contained within relatively fine-grained (silty) soil, . which was present in alll borings in the depth range between about 10 and 30 feet. No hydrocarbons were detected in sandy soil below this depth,,suggesting that the low- I' permeability silt layer inhibited downward infiltration of the hydrocarbons.- The depth of r