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Former Beacon Station No 12474 June 7, 2005 Stockton California Results of AddittonaE Groundwater Assessment at 92 and 124 feet bsg from the monitoring well MW-15 boring suggests that the source of DCA is the impacted groundwater at that location and level Quarterly groundwater monitoring through first quarter 2005 indicates a trend for dissolved hydrocarbons at the groundwater table to generally migrate towards the southeast corner of the property (Figure 7) Dissolved TPHg and DCA in groundwater also appear to have migrated from the source area south and southeast to levels below the groundwater table Dissolved TPHg and DCA in groundwater detected in hydropunch samples collected at 90 and 108 feet bsg from boring B-9 and at 95 feet bsg from boring B-10 indicate TPHg and DCA are not laterally delineated at these intermediate depth intervals in the southern portion of the site However, groundwater analytical results from monitoring wells MW-15 and MW-16 indicate vertical delineation of hydrocarbons in groundwater in the southern portion of the site To monitor or potentially delineate the lateral extent of TPHg and DCA at these intermediate idepth intervals in the southern portion of the site, Horizon recommends installation of groundwater monitoring wells approximately 100 feet southeast of monitoring well MW-15 in the Broadway right-of--way and in the southwest corner of the site near monitoring well MW-16 It is recommended that the proposed well southeast of monitoring well MW-15 be screened over the approximate interval 95 to 100 feet bsg and the well near monitoring well MW-16 be screened over the approximate interval 105 to 110 feet bsg Field lithological data from soil samples recovered from the monitoring well boring would be considered in the selection of the screen interval The updated SCM identifies volatilization and enclosed space accumulation of volatile vapors from subsurface soil and groundwater through paved surfaces, leaching and groundwater migration as transport mechanisms that have the potential to complete exposure pathways to sensitive receptors at and in the area of the site However, site data and the sensitive receptor survey indicate that it is unlikely that the exposure pathways to potential • sensitive receptors are complete at this time AddtnlGWassnini Project No 147415 13 HORIZON ENVIRONMENTAL INC