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EXECUTIVE SUMMARY The protect objectives were to further define the impact to groundwater below the water table, and to establish a groundwater monitoring capability downgradient of the plume of impacted groundwater To achieve these objectives, cone penetration test (CPT) borings were advanced ' for discrete groundwater sampling at six on-site locations, a two-well monitoring well cluster (MW-8A and MW-813) was installed downgradient of the existing monitoring well network, and the groundwater monitoring well network was resurveyed, monitored and sampled ' Soils observed to the total drilling depth of approximately 160 feet, consisted of massive intervals of clay and silt mixtures, interrupted by saturated sandy intervals mostly present between 70 and 80 feet, 110 and 125 feet, and 140 and 160 feet bgs The sands between 110 and 125 feet were the thickest, most continuous and present over the greatest aerial extent The depth-to-groundwater beneath the site ranged from 39 20 feet bgs in MW-4 to 42 88 feet bgs in MW-7 Separate groundwater surfaces calculated using the shallow-constructed and deep-constructed wells were relatively similar, and confirmed previous north and northeast groundwater flow directions and shallow gradients No significant vertical groundwater gradient was observed MTBE and 1,2-DCA were the only gasoline compounds detected in the groundwater samples ' collected from the monitoring wells The groundwater samples collected from MW-8A and MW- 8B and from Irrigation Well#2 did not contain detectable concentrations of gasoline compounds In the deeper groundwater samples collected during the CPT investigation, only 1,2-DCA was detected, except for a questionable detection of TPH-as-gasoline MTBE in groundwater appears to be limited to the groundwater table to the northeast (downgradient) of the former UST The plume of groundwater impacted by MTBE is defined downgradient of the former UST by MW-4, MW-8A and MW-813 1,2-DCA at the groundwater table also appears to be defined downgradient of the former UST ' The 1,2-DCA below the groundwater table appears to be concentrated in the 110 to 125 foot sand interval south and west of the former UST The concentrations of 1,2-DCA in groundwater beneath the water table are relatively low (0 5 to 1 2 µg/I), but slightly above the Maximum Contaminant Level (MCL) for 1,2-DCA of 0 5 µg/l Although 1,2-DCA in groundwater beneath the water table is not completed defined to below the MCL, it does appear to be delineated vertically and laterally ' Based on these conclusions, Ramage Environmental recommends that the monitoring well network continue to be monitored and sampled on a quarterly basis to further monitor plume concentration and stability If concentrations of gasoline compounds remain stable over the next three quarters, the case should be considered for regulatory closure • 4 r