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i Attachment l Depth to water information is summarized in Table l Development and purge water generated during this event were stored onsite pending receipt of analytical results Groundwater ' samples collected from monitoring wells MW-4 through MW-12 were submitted under chatn-of-custody to Argon Labs of Ceres, California (DOHS #1873) and analyzed for TPHg and BTEX using EPA Methods 8015 and 8020, respectively Table 2 summarizes the results of laboratory analyses for groundwater samples Laboratory reports aid chain-of-custody documentation for these samples are ' included in Attachment 2 i ' On May 16, 1995 Ground Zero surveyed the top-of-casing elevations of newly installed monitoring wells MW-9 through MW-12, tying them to the existing network The survey data was combined with the depth to groundwater measured on May 15, 1995 to determine groundwater elevation On May 15, 1995, groundwater flow at the site, as measured in (lie wells, was generally to the northeast at an ' average gradient of approximately 0 003 ft/ft 'Tile groundwater flow direction and gradient appear to change significantly offsite to the east and north (Figure 3) Groundwater Analytical Results Elevated levels of TPHg and BTEX were detected in groundwater samples collected from wells located northeast of the former USTs The highest concentrations of TPHg and BTEX were detected in samples collected from wells MW-5 and MW-8, located in the northeast corner of the site and immediately offsite to the northeast, respectively No hydrocarbon analytes were detected in samples from wells MW7, MW l l and MW 12 i Conclusions. Groundwater has been impacted by gasoline hydrocarbons and the plume may have migrated off site. Review of the PHS/EHD file for the Top Filling Station, located across Wilson Way and upgradient of the Rock property, revealed the presence of substantial concentrations of gasoline hydrocarbons in groundwater The Top Filling Station site may be a contributing source to the plume Recommendations Deepen temporarily plugged borings SB-25 and SB-26 to depths of approximately 55 feet bgs, construct vapor extraction test wells in borings 513-24, SB-25, SB-26, and S13-29, and perform a I day duration ' vapor extraction test at the site Deepen boring SB-37 to a depth of approximately 75 feet and construct an ambient at r/grou nd water monitoring well in the boring r Resurvey the well head elevation of MW8 and tie all wellhead elevations to those at the Top Filling Station across Wilson Way Perform at least four quarterly monitoring and sampling events at the site in order to verify concentrations of gasoline hydrocarbons and groundwater elevations observed during this investigation 2