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Atl:umc kids icid coutpam 5 Fomier ARCO 4493,205 \ Conter Street,Stodton.CA 5otemhor 5.2000 • well MW-6 at concentrations of 1.87 ppb, 171 ppb, 17.6 ppb, and 33.28 pph, respectively Well MW-7 was non detect for all constituents analyzed. • Fuel oxygenates (TAME, DIPS, EtBE, TBA, EDB and 1,2-DCA) were reported as nondetect in all soil and groundwater samples analyzed. • Groundwater flow was to the east at a gradient of 0,006 ft/ft. • Approximately 2 cubic yards of stockpiled soil generated during drilling activities was disposed of at Vasco Road Landfill in Livermore, California. CONCLUSIONS AND RECOMMENDATIONS Based on the data collected during this assessment, petroleum hydrocarbons have impacted soil to approximately 25 feet bgs and possibly groundwater to 60 feet bgs in the area of boring B-1 . Additionally, MtBE and VOCs (cis-1, 2-dichloroethene and TCE) in shallow groundwater are not defined to the southeast as indicated by groundwater analytical results from wells MW-5 and MW-6. Based on the above data, SECOR recommends that a multi-completion well be installed onsite in the vicinity of boring B-I to confirm the vertical distribution of hydrocarbons in groundwater onsite. Since the soils encountered in boring B-i were saturated from approximately 16 to 56 feet bgs, groundwater "grab" samples collected from boring B-I during drilling may not be representative of actual groundwater conditions at the various sample depths due to the potential for cross-contamination to occur during drilling operations. Once the vertical distribution of the hydrocarbon plume is confirmed, the need for additional multi-completion wells on- and offsite should be evaluated. To confirm the lateral extent of hydrocarbons in groundwater downgradient of the site, SECOR recommends that the newly installed monitoring wells be added to the quarterly groundwater monitoring and sampling program. Based on the results of this confirmation sampling, the need for additional wells downgradient of wells MW-5 through MW-7 will be evaluated. As previously reported, VOCs in groundwater in the area of the site are likely from an offsite source. Given the extensive nature of this VOC (including cis-1, 2-dichloroethene and TCE) plume up- and downgradient of the site, SECOR suggests that a meeting between ARCO, the San Joaquin County Environmental Health Department, the Regional Water Quality Control Board, and the Department of Toxic Substances Control be held to determine ultimate site usage and regional groundwater cleanup goals. If you have any questions or comments regarding this report, please feel free to call us at (916) 861-0400. 077.41883-001well install2.doc