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the groundwater surface, where a surticial TPH-film would be expected,, if present DMW-6 and DMW-7 were installed near the centers of the two zones of residual TPH soil impacts associated with the former USTs Therefore, the concentrations detected at these two wells should be representative of the worst-case scenario for TPH-D concentrations in groundwater at the Site From December 1998 to February 2000, the reported concentrations ranged from 110 ug/L to 780 pg/L in samples collected at DMW-6, and 82 Ng/L to 660 ug/L in samples collected at DMW-7 The weathered diesel observed at DMW- 6 and DMW-7 does not appear to be present in samples collected from wells DMW-1 through DMW-5, with the exception of the sample collected during February 2000 at DMW-3 (53 Ng/L) ' FUTURE ACTIVITIES Continued quarterly groundwater monitoring at wells DMW-1 through DMW-7 is recommended in order to monitor trends in water level elevations and the reported unknown hydrocarbon concentrations Continued water level measurements are recommended in order to monitor for a potential further increase in water levels, which could bring groundwater closer to the residual impacted soil present at depths of approximately 20 to 25 feet bgs Groundwater is currently at a depth of approximately 37 feet bgs Continued monitoring of TPH-D concentrations is recommended in order to assess potential seasonal or long-term trends in concentrations Dames & Moore recommends that the water samples continue to be analyzed for TPH-D R Y using method 8015M with the Northwest Cleanup The Northwest Cleanup is recommended in order to determine if the reported detections are resulting from non- petroleum hydrocarbons derived from organic material The removal of any non-petroleum hydrocarbons would allow for a more accurate assessment of petroleum hydrocarbon concentrations 1 ' Dames & Moore 6 t wFP\REPORT1g1-2000 DOC