SITE INFORMATION AND CORRESPONDENCE

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DDJC-Tracy UST NFA Memo• -7 - 24 June 2004 maximum concentration of 630 µg/L in down gradient grab samples. PCE was detected in one of the down gradient grab samples at a concentration of 2.1 pg/L;however,there is an existing pump &treat system located down gradient that is expected to capture/treat PCE that may originate from the Building 201 sump. The sump and down gradient hotspots were resampled in June 2003 and TPHD was not detected. The estimated volume of soil impacted by the sump is 9 cubic yards and the remaining TPHD mass in soil is approximately 12 kilograms. Based on 1998 data,the diesel plume is estimated to be 1,650 square feet;however,the June 2003 data suggest the plume extent is significantly less. ** Oxygenates, 1,2- Dichloroethane, and Ethylene Dibromide At our request,DDJC—Tracy sampled all existing UST sites for oxygenates (including MTBE), 1,2- dichloroethane (1,2-DCA), and ethylene dibromide in November 1999. Groundwater grab samples were collected from the hotpsot at each site. If a hotpot was not identified by previous site investigations,the sample was collected in the vicinity of the former UST. Oxygenates, 1,2-DCA, and ethylene dibromide were not detected at the 12 sites. Threat to Receptors There are several private drinking water wells in the vicinity of DDJC-Tracy. The nearest supply wells are located more than 3,500 feet down gradient. The potential for TPH migrating from the 12 sites to these wells is considered exceedingly low for the following reasons: • The residual contamination is primarily weathered diesel. Gasoline and BTEX are not present in groundwater beneath most of the sites. • The petroleum hydrocarbon plumes are limited in extent and are shrinking. • The more mobile oxygenates have not been detected. • Natural attenuation studies at DDJC-Tracy have demonstrated it is a viable remedy. • There is an existing extraction well network equipped with a GAC treatment system(for treatment of pesticides and chlorinated solvents)between the 12 UST sites and potential down gradient receptors. This system is located on DDJC-Tracy and is expected to operate for at least 10 more years. • Local groundwater flow velocities outside the influence of the extraction wells are very low. Conclusions and Recommendations Soil and groundwater contamination at the 12 sites is considered adequately characterized. All sites have shown a decline in petroleum hydrocarbon concentrations in both soil and groundwater since the initial investigations were conducted in the early 1990s. Residual contamination appears to be attenuating rapidly and is not expected to reach any receptors or migrate off the facility. No further action is recommended for the 12 sites. **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.