SITE INFORMATION AND CORRESPONDENCE

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DDJC-Tracy UST NFA Memo• -5 - • 24 June 2004 hydrocarbons. TPHD was detected at a concentration of 15,000 mg/Kg during the 1995 investigation. BTEX was not detected in any soil samples. The soil hotspot was resampled in 1998 and a TPHD concentration of 1,000 mg/Kg was detected. This sample was also analyzed for leachable TPHD (DI WET extraction) and a concentration of 50,000 µg/L was detected. A groundwater sample collected from the 1995 soil hotspot area contained 14,000 µg/l,TPHD. The same area was resampled in 1998 and TPHD was not detected. Assuming no natural attenuation has occurred since 1998,the residual TPHD mass is estimated to be 23 kilograms and the affected volume of soil is estimated to be 89 cubic yards. The extent of the diesel plume is considered negligible. ** Site 24 UST Site 24 is located adjacent to Building 255 and in the northern portion of the Depot. A 2,000-gallon steel UST was installed prior to 1956 and was used to store No. 2 fuel oil for a steam boiler in Building 255. The UST was closed in-place in 1957 and removed in 1988. Subsurface investigations were conducted in 1993, 1998, and 2003. The maximum TPHD concentration of 1,400 mg/Kg was detected in 1998. Concentrations of TPHD in other soil samples collected in 1993 and 1998 ranged from below detection to 872 mg/Kg. Concentrations of 173,000 and 52,000 gg/L TPHD in groundwater were detected in 1993 and 1998 near the former UST. These two hotspots and three down gradient locations were sampled in June 2003. Concentrations in June 2003 ranged from below detection to 490 µg/L. The maximum historical concentration decreased from 173,000 to 490 µg/L. The remaining mass of TPHD in soil is estimated to be 82 kilograms. The volume of impacted soil is estimated to be 661 cubic yards. This volume estimate is considered very conservative as it assumes no degradation of the TPHD has occurred since 1998. Based on June 2003 data,the extent of the TPHD plume is estimated to be 6,200 square feet. ** Site 27 UST Site 27 is located adjacent to Building 250 and in the northern portion of the Depot. Two steel USTs were installed in 1974 and were removed in 1988. One tank had a capacity of 250 gallons and was . used to store lube oil. The second tank had a capacity of 4,000 gallons and was used to store diesel fuel for a backup generator in Building 250. Subsurface investigations were conducted in 1993, 1997, 1998, and 2003. The 1997 investigation included removal of approximately 170 cubic yards of contaminated soil. Concentrations of 2,500 mg/Kg TPHD and 1,200 mg/Kg were detected in soil samples collected from the base of the excavation. Additional excavation was not performed due to concern over undermining the foundation of Building 250. In 1998, soil beneath the base the 1997 soil excavation was sampled and a maximum TPHD concentration of 5,800 mg/Kg was detected. The soil contamination detected beneath the 1997 soil excavation may be part of a groundwater smear zone. **Natural attenuation studies conducted at DDJC-Tracy and the documented rapid decline in petroleum hydrocarbons in both soil and groundwater at DDJC-Tracy UST sites indicate residual petroleum hydrocarbon contamination should attenuate naturally.