SITE INFORMATION AND CORRESPONDENCE

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DDJC-Tracy UST NFA Memo• -3 - 24 June 2004 Site 10 UST Site 10 is located beneath the Building 201 awning in the northern portion of the Depot. A 500- gallon steel UST was installed at this site in 1956 and was used to store No. 2 fuel oil. The tank was removed in 1989. Subsurface investigations were conducted in 1991, 1992, 1993, 1995, 1998, and 2003. The original tank excavation was enlarged in 1991 and some of high concentration TPHD mass was probably removed at this time. Accurate records of this soil removal action were not kept. Numerous borings have been advanced since 1991 and the highest residual concentration of TPHD in soil is 3,750 mg/Kg. However, this sample is believed to be from the groundwater smear zone. Data collected from the soil borings suggest there is was a significant smear zone near the location of the former UST,but actual vadose zone soil contamination is limited. Concentrations of TPHD in groundwater grab samples collected from the area of the former tank have declined from 710,000 µg/Lin 1996 to 2,400 pg/L in June 2003. Concentrations of TPHD in down gradient grab samples collected in June 2003 and monitoring well LMl 11A range from ND to 50 µg/L. BTEX contamination was detected in one groundwater grab sample in 1995,but has not been detected in subsequent investigations. The estimated residual TPHD mass is 50 kilograms and the estimated volume of impacted soil is 148 cubic yards. Based on June 2003 data, the extent of the groundwater plume is estimated to be 1,200 square feet. ** Site 14 UST Site 14 is located along 5s'Street and near former Building 25 on the Depot. The site consisted of a 1,000 gallon steel tank that was used to store gasoline. The tank was installed in 1956, abandoned in- place in 1961, and removed in 1988. Subsurface investigations were conducted in 1993 and 1998. Maximum concentrations of 2,320 mg/Kg TPHG, 8.68 mg/Kg benzene, .12.2 mg/Kg toluene, 11.9 mg/Kg ethylbenzene, and 30.5 mg/Kg total xylenes were detected during the 1993 investigation. BTEX was not detected during the 1998 investigation. In 1998, a TPHG concentration of 2,200 gg/L was detected in a DI WET sample collected from the 1993 hotspot. Three groundwater grab samples were collected immediately down gradient from the historical TPHG hotspot,but TPHG and BTEX were not detected. These results suggest the 1998 DI WET results may have been spurious. Assuming little or no biodegradation has occurred since 1998,the residual petroleum hydrocarbon mass is estimated to be 75 kilograms and the volume of affected soil is approximately 205 cubic yards. No groundwater contamination has been detected at this site. ** Site 18 This UST site is located near Building 231 in the northern portion of the Depot. A 500-gallon tank was installed prior to 1956 and was used to store No. 2 fuel oil for a water boiler in Building 231. The tank was abandoned in place in 1957 and removed in 1988. **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.