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• Based upon field and analytical d - - Po y data obtained from B 4, B-5 and B 6, these borings do indeed represent a zero line for the soil contaminant plume The locations of bonngs B-4, B-5, and B-6 are shown on Plate 2 Soil boring logs for B-4, B-5, and B-6 are included in Attachment 1. The former tank pit which contained T1/T2/T3 falls within the area bounded by borings B-4, B-5, and B-6, and therefore falls within the zero line of contamination established by those borings. Based upon the data collected by and provided to SMTH, it appears that the estimated radii of influence of existing vapor extraction wells encompasses the area of known and suspected hydrocarbon impact. Once the vapor extraction system is installed and operating, actual operating parameters can be evaluated and the need for additional vapor extraction wells can be explored Groundwater Characterization As stated in your correspondence, the outer lying wells at the site (MW-2, MW-3, MW-4, MW-5) have shown intermittent groundwater contamination at very low levels However, the detected contaminant in each case, with the exception of apparently anomalous data from MW2 in September 1994, has been total petroleum hydrocarbons as diesel (TPHd), which is not amenable to vapor extraction Further, although vapor extraction tends to have the effect of somewhat reducing dissolved contaminants in the uppermost saturated zone, this effect would be minimized or rendered negligible in wells with very low dissolved contaminant levels such as the outer lying wells at the site Soil Vapor Extraction Based upon the data collected by and provided to SMTH, it appears that the estimated radii of influence of existing vapor extraction wells encompasses the area of known and suspected hydrocarbon impact It should be noted, however, that the estimated radii of influence of the existing vapor extraction wells are estimated based upon data obtained during the vapor extraction test A vapor extraction test is a field test used to obtain data upon which to base a system design. The design of a system should be sufficient so as to accommodate moderate expansion in the number of wells Once the vapor extraction system is installed and operating, actual operating parameters can be monitored and adjusted accordingly for maximum system efficiency The need for additional vapor extraction wells should be explored and addressed on an ongoing basis The inclusion of passive air inlet wells should be considered on a site specific basis Passive air inlet wells promote preferential pathways of vapor movement, or "channeling", in the vadose zone and therefore serve a specific purpose Passive air inlet wells are typically used • 38009-10/FRP-ADD dm I