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• remediation activities A decision was made to re-start the vapor extraction motor about September 20, 1993 and monitor vapor levels closely for several days to determine whether additional benefit could be obtained by continuing to operate the system 3.0 RESULTS The combustion unit was re-started on September 20, and stabilized by September 24. A PID reading of 191 ppm was collected on September 24 (13,624 hours) The unit was then shut down for repairs to the carburetor, propane tank and exhaust heat shield on September 26 The unit was again re-started on November 8 (13,706 hours) PID readings of 145 ppm, 166 ppm, 242 ppm and 187 ppm were collected on November 10, 11, 29 and December 1, respectively The unit was shut-off on December 7, 1993 and is currently not running Using data obtained from the air samples collected during the first half of 1993, actual concentrations of petroleum hydrocarbons were calculated to be 43% of the PID readings The last five PID readings ranged from 145 ppm to 242 ppm, this theoretically represents actual petroleum hydrocarbon concentrations of 62 ppm to 104 ppm 4.0 CONCLUSIONS 4.1 Vapor Extraction Unit The concentration of vapors removed from the soil at the site during the re-start period (from September to November) was not significant It appears that the continued operation of the vapor extraction unit would not significantly enhance soil mitigation at the site We recommend that the unit be permanently disconnected and vapor extraction at the site terminated Furthermore, we recommend the drilling of one or two soil borings within 10 to 15 feet of the extraction well to confirm the successful removal of contaminants from the soil These borings should be done in conjunction with a groundwater investigation (see below) 4.2 Groundwater Investigation The PHS/EHD is requiring a groundwater investigation to determine if contamination is still present Groundwater depth at the site is currently 65 feet Since all existing monitoring wells at the site are dry, we recommend the installation of three groundwater monitoring wells to determine groundwater quality, gradient and flow direction At least one proposed monitoring well should be drilled in the area of the former tank pit (near VE-1) and can be sampled to confirm the soil clean-up level achieved by the vapor extraction process 2