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k August 6, 1993 Page 4 PACIFIC analyzed five sets of groundwater analytical data between 1989 and 1993 Isoconcentration contours at 1,000 ppb TPH-g and 10 ppb benzerne were constructed These data are presented on Figure 4 and as Attachment B The contours indicate that the configuration of the hydrocarbon plume has remained relatively stable over a 4-year period PACIFIC attributes this finding to the combination of the fine-grained sediments underlying the site, the relatively shallow gradient, and the effects of natural attenuation and degradation processes PROPOSED SCOPE OF WORK The site is underlain by fine-grained material and the groundwater gradient has consistently been relatively shallow Because of these conditions, the hydrocarbon plume has remained almost unchanged since 1989 All underground tanks and lines have been removed from the site, along with affected soil in the vicinity of the tanks, thereby removing much of the previous source of hydrocarbons to the groundwater Based on the most recent meeting with the PHS/EHD and Unocal, two additional monitoring wells are being proposed at the site to complete the groundwater assessment Field and laboratory procedures for these well installations are presented as Attachment C The wells are proposed at the following locations (Figure 4) o One well south of the site (along Pacific Avenue) to complete lateral defrrutron o One well southwest of the site, upgradrent, since no upgradrent groundwater quality data currently exists Following installation of these wells, they will be developed and added to the existing quarterly groundwater program After completion of the assessment, a remedial action plan (RAP) will be prepared for the site The RAP will consider source removal options for the prod- uct island areas (which includes the area of Well MW-2) and the area surrounding Well MW-6 The RAP will also consider the potential risk to receptors in the area • 3104001\WORKPLN