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These wells are constructed so that representative water samples can be collected directly at the groundwater surface, where a surficial 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 September 1999, the reported ' concentrations ranged from 380 ugll to 780 /jg/l in samples collected at DMW-6, and 82 ugll to 660 yg/I 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, based on the chromatograms attached in Appendix B 4 0 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 39 feet bgs Continued monitoring of TPH-D concentrations is recommended in order to assess potential seasonal or long-teem trends in concentrations ' Dames & Moore recommends that the water samples be analyzed for TPH-D using method 8015M with the Northwest Cleanup The Northwest Cleanup is recommended in order to determine of 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 ' DAMES&MOORE 7 J 1Af P\REPORT1991 1 01 doc