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9.0 SUMMARY 1 The following pharagraphs summarize the results of the subsurface investigation performed at the project site ' 1 A review of cumulative laboratory analytical data collected from the project site to date indicates the February 1995 over-excavation successfully removed the majority of soil contamination in the vicinity of the former tank pit This is evidenced by confirmation soil results and generally decreasing concentrations of dissolved petroleum hydrocarbon constituents in groundwater samples collected from wells MW2 and MW3 located hydraulically down gradient of the former UST pit (Tables 1 and 2) Due to physical constraints, including underground utilities and the adjacent municipal well compound and post office, the over excavation was impeded in the northern direction Laboratory data and noted field observations indicate petroleum affected soil remains at relatively low concentrations in an area less than approximately 115 feet by 70 feet to a maximum depth of approximately 26 feet bgs and is centralized in the vicinity of well MW4 (Figures 4 and 5) 2 Groundwater data indicate the dissolved petroleum hydrocarbon plume is laterally distributed with the highest concentrations centered in the vicinity of well MW4 Groundwater samples collected from wells MW3 and MW4 continue to exhibit benzene concentrations which exceed California's Maximum Contaminant Level (MCL) of I part per billion (ppb) Petroleum hydrocarbon concentrations in wells MW3 and MW4 have generally decreased over time, however this does not appear to be the case for benzene in well MW4 Benzene concentrations of water samples collected from MW4 have fluctuated from a high of 110 ppb (October 199 1) to below laboratory method detection level of 0 5 ppb (July 1993 to December 1994) During the most recent event MW4 contained benzene at 34 ppb These concentrations are most likely attributed to ' fluctuating water levels in conjunction with hydrocarbons remaining in the vadose zone 3 March 25, 1997 fluid level measurements indicated a westerly groundwater flow direction beneath the site with an average gradient of approximately 0 0009 ft/ft, or approximately 4 7 feet/mile which is generally consistent with gradients observed in the past The rise in ' groundwater detected in March 1997 is most likely the result of a seasonal trend as a rise in the water table was also observed during the Spring of 1995 and 1996 4 Site boring logs and sediments encountered while drilling indicate that the shallow soil beneath the site is consistent with soil encountered in previous borings on-site The upper 5 to 10 feet of the soil column generally consists of silt Below 10 feet, fine to medium ' grained sand with interbedded silt layers were encountered to a depth of 45 5 feet, the maximum depth explored Groundwater was encountered in the borings at approximately 10 to 15 feet bgs during drilling activities . w 194326Vcports\sumrptdoc 6 • 1 Sh*H