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i I 28 July 1995 AGE-NC-95-0134 Page 2 of 6 Icontaminant plume for remediation A vapor extraction system (VES) was installed in August 1991 and was operated under pernvt requirements of the San Joaquin Aar Pollution Control Distnct order ' number 91-61 Between August I991 and July 1993, the VES was in operation for approximately 13,500 hours Progress has been summarized in reports submitted between March 1992 and February 1993 by GeoAudit rHydrocarbon 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 I samples were collected on 22 February and 28 April 1993 for laboratory analysis The sample results were then correlated with the field readings This information was used to determine the volume and efficiency of hydrocarbon removal Soil samples from borings CB-1 and CB-2, installed in June 1994, I indicated the removal of 94% of petroleum hydrocarbons from the impacted soil The results of the samples were described by GeoAudit in a Quarterly Report dated 17 July 1993 2 4 REGULATORY MEETING As a result of a 20 August 1993 meeting with 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 operated since I The concentration of vapors removed from the soil at the site during the re-start period from p g p { I September to November 1993) was not significant It appeared that the continued operation of the vapor extraction unit would not significantly enhance soil mitigation at the site GeoAudit recommended that the unit be permanently disconnected and vapor extraction at the site terminated I 2 5 ADDITIONAL SOIL AND GROUND WATER INVESTIGATION Seven groundwater 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, groundwater depth was approximately 42 feet Subsequently, groundwater depth fell below 55 feet and all wells were dry On 13 and 14 June 1994, four soil borings were drilled at the site Borings MW-8 and MW-9 were I advanced to depths of 75 feet and completed as groundwater monitoring wells Borings CB-I and CB-2 were advanced to a depth of 65 feet and backfilled with Portland cement Petroleum hydrocarbons were detected in boring CB-1 at depths of 20 feet and 35 feet and in boring CB-2 at depths from 30 feet to 40 feet No petroleum hydrocarbons were detected in the samples analyzed from MW-8 or MW-9 No benzene was detected in any of the samples analyzed Two soil