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i PROBLEM ASSESSMENT REPORT 14800 West Frontage Road Manteca California I4 3 Groundwater Conditions Groundwater elevations have risen, since Aegis began recording measurements (January 1991), a maximum average of approximately 3 21 feet and fallen a maximum average of approximately 0 71-feet (Table 5) The rise of groundwater occurred between the months of February and September 1991 (Figure 6) The direction of groundwater flow for the period of January through August 1991 was north-northeast Beginning in October 1991 through January 1992, groundwater flow had no apparent direction Beginning with the February 1992 measurements and continuing to the present (April 20, 1992), groundwater flow is to the southwest (Figure 7) 4 4 Distribution of Petroleum Hydrocarbons in Groundwater Figure 8 indicates the distribution of concentrations of TPH, as gasoline, and benzene in groundwater As shown on the figure, the highest concentrations of petroleum hydrocarbons were reported in the water sample collected from MW-4 on January 31, 1992 Concentrations of TPH, as gasoline, and benzene shown on Figure 8 are also reported in Table 3 Floating liquid hydrocarbons have not been observed in any of the nine wells at the project site Figure 8 indicates an inferred "zero line" for the extent of dissolved hydrocarbons in groundwater 5.0 REMEDIAL ACTION ALTERNATIVES The following sections discuss appropriate sod and groundwater remedial alternatives iEach section provides a general description and technical critique of the respective remediation alternative 5.1 Soil In general, sal remediation alternatives are dictated by site size, subsurface conditions, contaminant types, contaminant concentrations, and the volume of contaminated soil Sal remediation alternatives can be broken into two broad categories, ex-situ and in-situ Ex-situ sal remediation involves excavation of the soil and the selection of an appropriate I treatment and/or disposal option In-situ remediation options generally include the prevention of the migration of contaminants or destruction/removal of the contaminants ' while still in the subsurface Typically, sal treatment options are effective for a limited number of organic compounds The presence of certain organic compounds (polychlorinated byphenyls, certain pesticides, etc ) and certain inorganic compounds I (metals, acids, etc ) above regulated concentrations may limit treatment and disposal options In some cases where multiple contaminants are involved, treatment is utilized to reduce contaminant concentrations of a select number of compounds in order to increase disposal options 90-097A PAR 8