SITE INFORMATION AND CORRESPONDENCE

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DDJC-Tracy UST NFA Memo• -2 - • 24 June 2004 declined significantly in the four monitoring wells,but it is sporadically detected in Well LM171AU at concentrations ranging from 0.5 to 2.1 µg/L. The last sampling event was conducted in May 2003 and benzene was detected at a concentration of 0.5 gg/L. The residual soil contamination and residual TPH mass at UST Site 6 is estimated to be 1,733 cubic yards and 228 kilograms. However,the estimate of residual soil contamination is based on 1998 soil data and is considered very conservative. This volume includes all soil exceeding TPHG or TPHD detection limits. Natural attenuation studies at this site suggest the contamination would degrade quickly, so the present volume of contaminated soil is expected to be much lower than 1,733 cubic yards. The groundwater plume has almost attenuated completely since 1998, which indicates the residual soil contamination is not continuing to impact groundwater. Site 7 UST Site 7 is located near former Building 206 on the northwest portion of the Depot. Two 12,000- gallon steel USTs were installed in 1942 and were used to store diesel fuel until 1977. The tanks were abandoned in-place in 1977 and were later removed in 1988. The subsurface beneath Site 7 was investigated in 1993, 1995, 1998, and 2003. The maximum diesel concentration in soil (1,900 mg/Kg)was detected during the 1988 tank removal activities. Subsequent soil investigations did not encounter residual TPHD contamination. TPHD was detected at a maximum concentration o f 60,000 µg/l,in a 1998 groundwater grab sample. BTEX was detected at trace concentrations in 1993,but was not detected in samples collected in 1995 or 1998. The hotspot was resampled in June 2003 and TPHD was not detected. The highest concentration of TPHD detected in grab samples and monitoring wells immediately down gradient from the historical hotspot was 54 µg/l,in 2003. The estimated volume of impacted soil (defined as soil exceeding detection limits) is considered negligible. Based on the groundwater sampling conducted in 2003, the extent of the plume is estimated to be 630 square feet. ** Site 9 UST Site 9 is located near Building 201 on the north side of the Depot. The site was a 1,000-gallon steel fuel tank used to store No. 2 fuel oil. The tank was reportedly installed in 1956 and was abandoned in 1977. The tank was removed in 1988. UST Site 9 was investigated in 1993, 1995, and 1998. The maximum concentration of TPHD in soil was 1.1 mg/Kg. BTEX was not detected in any soil samples. TPHD was detected at a maximum concentration o f 730 µg/L in a 1995 groundwater grab sample. Resampling of the hotspot area indicates TPHD declined to 180 pg/L by 1998. BTEX has not been detected in groundwater samples collected from this site. The residual soil contamination at Site 9 is negligible. The extent of the groundwater plume is estimated to be 390 square feet, assuming little or no attenuation has occurred since 1998. ** **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.