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s <br /> E CLEARWATER <br /> G R O U I', I N C. - <br /> Point 4. USTCF. -It is unfortunate that the PHS/EHD has acted to suspend <br /> reimbursement at the. subject site. The issues to be resolved are.,fairly clear,and itis " <br /> hoped ,that the information provided in this letter.will facilitate thei resolution: :. <br /> Point 5. -'The system is not in need of any redesign, as wells VW-1, 2, and 3 have <br /> been,shown to be unnecessary (VW-3 in it's current configuration; 'refer .to <br /> recommendations section of this letter for.additional clarification). Piping to these " <br /> wells should be valved shut i <br /> Point 6. Clearwater. will apply thick plastic to the currently .uncovered formef -. <br /> excavation backfill surface.::;, <br /> Point 7. .1t is 'apparent that the PHS/EHD will not consider the measurement of <br /> ., biogenic gasses as an indicator of remediation effectiveness (in addition to extracted <br /> vapor analysis). Clearwater will: not initiate any such measurements; though.: <br />? continues to recommend same in an effort -to avoid premature confirmation sample <br />�. collection. <br /> Recommendations <br /> 1) With .the possible exception of the shallow contamination in the northeast � <br /> quadrant of. the former excavation, wells VW-1, 2, and 3 are not productively close <br /> to areas of known subsurface contamination and serve no useful purpose regarding <br /> the extraction of hydrocarbon vapor from the subsurface. While it has been <br /> demonstratedthat VW-2 produces vapors under vacuum conditions, it likely does <br /> so at the expense of those capable of being produced by VW-4, which is centrally <br />+ located .with 'respect to contaminants beneath the former dispenser island. <br /> Clearwater therefore recommends that an evaluation of the effectiveness of the <br /> combined operation of VW-2, 4 and 5 be completed in order to determine whether <br /> the continued operation of VW-2 is appropriate. This evaluation would be simple, <br /> and would need only compare the mass of contaminants removed by the combined <br /> operation of VW-2, 4 and S ,for a fixed period of time to the mass generated by the <br /> operation of VW-4 and 5 alone fora similar length of time. If it is shown that the _ <br /> operation of VW-2 enhances the rate of cleanup, it will remain on-line. If it shown <br /> to be detrimental to the rate of cleanup, it will be taken off-line. Clearwater further <br /> recommends that VW-1 and VW-3 be taken off-line as soon as possible, as data ! <br /> clearly show that these wells do not serve any useful purpose,_ and indeed dilute the <br /> effectiveness of the existing system. <br /> 2) Some concern regarding the hydrocarbons detected in soil samples collected in the . <br /> northeastern quadrant of- the .former excavation following UST removal may be <br /> warranted. The contaminants detected "in the northeastern quadrant 'of the <br /> excavation may still be present in the, subsurface. Due to the screened interval of <br /> VW-3, they will likely not be removed by the vapor extraction system in it's current <br /> i <br /> I <br /> k <br />