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I samples designated "MW-7" (duplicate samples from MW-2) and MW-5 samples taken from the apparent downgradient direction of the former tank excavation 1,2 DCA has not been detected in wells MW-1, -3, -4 or MW-6 PCE has been found in wells MW-4, -5, and -6 Since MW-6 is in the apparent upgradient direction of this site, it appears that the source of the PCE is offsite This theory is strengthened by our understanding that this phenomenon may be an area-wide groundwater PCE problem (RWQCB, 1992) Laboratory personnel at California Water Service Co also indicated that PCE has also been ' detected occasionally in Wells 77 and 71 SECOR contacted the City of Stockton Public Works Department for information on the sanitary sewer distribution Based on a review of flow line ' elevations, the sanitary sewer wastewater flows south along West Lane and westward along Alpine Avenue(Figure 7) It appears that the source of detectable PCE in wells MW-4, -5 and -6 may be due to leakage from the local sanitary sewer system, which carries wastewater from numerous upstream industrial facilities, several of which are potential users of PCE as discussed below 4.3.2 Review of Potential Source Areas A radius search of adjacent properties and properties within one mile of this site was performed by Environmental Data Research (EDR) Within the one mile radius, several potentially suspect facilities have been identified These facilities have been designated "potentially suspect" because of processes inherent to their operations which may have utilized PCE or related halogenated solvents These facilities include a dry cleaner, a transmission shop, a PG&E Service Center, a US postal service vehicle maintenance facility, a millwork facility, a cabinet shop, and a chemical ' company Portions of the EDR Report are presented in Appendix G In summary, it appears that the presence of PCE found in Site groundwater samples is the result of offsite sources Given the immediate proximity of site monitoring wells to the sanitary sewers, this phenomenon, as discussed in RWQCB documents (RWQCB, 1992), is known to have contributed to an area-wide problem regarding PCE It also appears likely that the 1,2 DCA source may have ' been onsite, occurring as a possible gasoline additive in the product which may have been stored in the former underground tanks 4.4 ESTIMATES OF AFFECTED GROUNDWATER VOLUMES ' Laboratory results from the Final Quarterly Monitoring Event (October 1994) indicated no- detectable BTEX compounds in any groundwater samples collected from the Site 1,2 DCA was ' detected in the samples collected from MW-2 at 1 6 ug/l and MW-5 at 1 0 ug/l A conservative estimate of the total residual mass of BTEX and 1,2 DCA compounds in Site groundwater was calculated as 0 0010 gallons as presented in Table 7 This estimate includes 0 0002 gallons of benzene, 0 0002 gallons of toluene, 0 0002 gallons of xylenes, 0 0002 gallons of ethylbenzene, and ' SECOR Intemational Inc (SECOR®) CRC Environmental Risk Management,Inc N0026 001 01 add 15 Navember 30 1994