ARCHIVED REPORTS -GROUNDWATER MONITORING REPORTS

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I Subsurface Investigation Report ' Installation of Wells MWS,MW6, and MW7 640 N El Dorado Street, Stockton, CA ' Groundwater samples collected from newly installed wells MWS, MW6, and MW7 contained several VOCs, including PCE In three sampling events conducted since the wells were installed, PCE was ' detected at levels ranging from 7 9 to 5 0 ppb in well MW5, from 60 to 34 ppb in well MW6, and from 120 to 77 ppb in well MW7 (Table 7) ' 5.0 CONCLUSIONS Petroleum hydrocarbon contamination, in the form of free product (oil) in well MW3, is apparent in groundwater within close proximity to the former UST, although the thickness of free product has been greatly reduce through the deployment of a passive oil-wick skimmer The contaminant plumeassociated with the former UST appears to be stable as evidenced by the lack of free product and/or dissolved ' petroleum hydrocarbon constituents in any perimeter wells PCE is present at levels exceeding its MCL of 5 ppb in each site well except well MW3, which was ' completed at the location of the former waste oil UST, and well MW5, which is upgradient of the former UST Groundwater samples collected from newly installed wells indicate that PCE is present upgradient (well MW5), downgradient(well MW6), and cross-gradient(well MW7) from the former waste oil UST I However, PCE concentrations in all site wells have steadily declined since groundwater monitoring began The distribution of PCE in groundwater beneath the site, with the highest concentration detected in the cross-gradient well nearest the intersection of Park and El Dorado Streets, suggests that there is an off- site source of PCE contamination in groundwater City sewer lines in the vicinity of the site flow from I east to west beneath Park Street, which would contain discharges from 715 Hunter Street (has housed several repair facilities) and from 248 E Park Street (Canepa's Carwash) City sewer lines also flow from north to south beneath El Dorado, which would contain discharges from PIP Printing (907 N EI I Dorado) and Unifirst Corporation (804-820 N EI Dorado/125 E Flora), currently a uniform rental/cleaning facility The Unifirst site was previously operated by Parisian Laundry from approximately 1953 until it was acquired by Unifirst in the late 1990s I The apparent presence of PCE in the a discre groundwater sample collected from a deeper sand zone in boring SB10/MW6 suggests that there has/may have been some downward vertical migration of PCE, although it is unclear if the deeper sand units encountered in boring SB10/MW6 (48-51 feet bsg) and I boring SB9/MW5 (40-43 feet bsg) are connected with the upper water-bearing units encountered in each site well -7-