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4 0 CONCLUSIONS AND RECOMMENDATIONS <br /> i <br /> ' Sandy sod intervals were identified below the groundwater table, with the thickest and most <br /> extensive between 110 and 125 feet bgs The relatively discontinuous sandy soil intervals and <br /> ' general soil stratigraphy are characteristic of fluvial deltaic deposition <br /> Groundwater exists beneath the site underwater table conditions The separate groundwater <br /> surfaces calculated using the shallow-constructed and deep-constructed wells were relatively <br /> similar, and confirmed previous north and northeast groundwater flow directions and shallow <br /> gradients No significant vertical groundwater gradient was observed <br /> MTBE and 1,2-DCA were the only gasoline compounds detected in the groundwater samples <br /> collected from the monitoring wells In the deeper groundwater samples collected during the CPT <br /> investigation, only 1,2-DCA was detected, except for a questionable detection of TPH-as- <br /> gasoline <br /> ' MTBE in groundwater appears to be limited to the groundwater table to the northeast <br /> (downgradrent) of the former UST As shown on an isoconcentration map (Figure 6), the plume of <br /> groundwater impacted by MTBE is defined to the east at MW-4 and to the northeast at MW-8A <br /> and MW-8B <br /> 1,2-DCA at the groundwater table also appears to be defined downgradrent of the former UST by <br /> MW-4, MW-8A and MW-813 In the deeper groundwater samples 1,2-DCA was detected in only 3 <br /> of the 19 samples analyzed, at concentrations ranging from 0 5 to 1 2 µg/I The 1,2-DCA below <br /> ' the groundwater table appears to be concentrated in the 110 to 125 foot sand interval south and <br /> west of the former UST Although the concentrations of 1,2-DCA are slightly above the Maximum <br /> Contaminant Level (MCL) for 1,2-DCA of 0 5 µg/l, they are relatively low and appear to be <br /> delineated vertically and laterally <br /> ' Based on these conclusions, Ramage Environmental recommends that the monitoring well <br /> network continue to be monitored and sampled on a quarterly basis to further monitor plume <br /> concentration and stability At the site, the sources of impacted groundwater have been removed <br /> and/or remediated, the plume of impacted groundwater has been delineated and appears to be <br /> stable, and nearby sensitive receptors do not appear to be threatened If concentrations of <br /> gasoline compounds remain stable over the next three quarters, the case should be considered <br /> for regulatory closure <br /> r <br /> . 14 <br />