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I . 12 December 1995 AGE-NC Project No 95-0134 Page 2 of 6 contaminant plume for remediation A vapor extraction system (VES) was installed in August 1991 and was operated under permit requirements of the San Joaquin Air Pollution Control District order number 91-61 Between August 1991 and July 1993, the VES was in operation for approximately 13,500 hours Progress of remediation activities was summarized in reports subnutted between March 1992 and February 1993 by GeoAudrt Hydrocarbon vapor concentrations were measured at the inlet to the VES, using a photo-ionization detector (PID), on a weekly basis to monitor the removal of hydrocarbons from the soil Vapor samples were collected on 22 February and 28 April 1993 for laboratory analysis The sample results were then correlated vAth the field readings This information was used to determine the volume and efficiency of hydrocarbon removal Sod samples from borings CB-1 and CB-2, installed in June 1994, indicated the removal of 94% of petroleum hydrocarbons from the impacted soil The results of the samples were described by GeoAudrt in a Quarterly Report dated 17 July 1993 2 4 REGULATORY MEETING As a result of a 20 August 1993 meeting tenth the PHS/EHD, the vapor extraction unit was re-started and operated from 20 September to 26 September and again from 8 November to 7 December 1993 The unit was shut-off on 7 December 1993 and has not been put in operation since then The concentration of vapors removed from the soil at the site duan the re-start period (from p g September to November 1993) was not significant It appeared that the continued operation of the vapor extraction unit would not result in additional removal of considerable volume of hydrocarbons GeoAudrt recommended that the unit be permanently disconnected and vapor extraction at the site terminated 2 5 ADDITIONAL SOIL AND GROUND WATER INVESTIGATION Seven ground water monitoring wells had previously been installed on or near the site The wells were installed to depths of 55 feet At the time of installation, ground water depth was approximately 42 feet Subsequently, ground water depth fell below 55 feet, which caused the wells to become dry On 13 and 14 June 1994, four soil bonngs were drilled at the site Borings MW-8 and MW-9 were advanced to depths of 75 feet and completed as ground water monitoring wells Borings CB-1 and CB-2 were advanced to a depth of 65 feet and backfilled with Portland cement IPetroleum hydrocarbons were detected in boring CB-1 at depths of 20 and 3 5 feet and in boring CB- 2 at depths from 3 0 to 40 feet Samples CB 1-15 and CB 1-20 were found to contain total lead at Iconcentrations of 12 ppm and 9 ppm, respectively Soil samples collected from MW-8 and MW-9 did