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' Ms Diane Hinson September 14, 1994 Page 5 has been consistent since 1991, and currently it does not appear that dissolved hydrocarbons have impacted the downgradient and offsite monitoring well (MW-6) The groundwater gradient at both sites are relatively shallow having limited potential for rapid plume migration Subsurface materials encountered beneath the ARCO Lodi site contain more sand than material beneath the ARCO Stockton site The increased sand content equates to higher permeabilities and higher leaching potential than material beneath the ARCO Stockton site Based on the information gathered from the well survey and current site condition s, there does not appear to be any identifiable water supply wells within the immediate area of the ARCO Stockton site which ' would be adversely impacted by hydrocarbons in the near future A well survey will be conducted for the ARCO Lodi site in the next phase of work ' CONCLUSIONS It is EMCON's opinion that the ARCO Lodi site warrants the next available vapor extraction ' treatment system and should initiate soil remediation immediately following system installation This decision is based of the following o the greater mass of residual hydrocarbons beneath the Lodi site o the inconsistent groundwater flow direction at the Lodi site o the higher permeability values associated with the sand units present at the Lodi site ' o the possibility of further impacting groundwater at the Lodi site which currently contains only low concentrations of dissolved hydrocarbons (benzene below MCLS) o the absence of known water supply wells in the immediate vicinity of the Stockton site ARCO intends to address soil and groundwater remediation at the ARCO Stockton site as soon ' as a vapor extraction treatment system becomes available in San Joaquin County Quarterly groundwater monitoring will continue and ARCO will closely monitor hydrocarbon concentrations in the groundwater and the groundwater flow direction 1