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18 November 1996 AGE-NC-Project No 95-0134 ' Page 4 of 8 Iextraction activities at the site be terminated 2 5 ADDITIONAL SITE INVESTIGATION 2 5 1 SOIL INVESTIGATION Seven ground water monitoring wells had previously been installed on or near the site The wells were installed to 55 feet bsg At the time of installation, ground water depth was approximately 42 feet Subsequently, the water table fell below 55 feet, causing the wells to become dry On 13 and 14 June 1994, four soil borings were drilled at the site by GeoAudit Two borings, MW-8 and MW-9, were advanced adjacent to MW-1 and MW-3, respectively (Figure 2) These borings were drilled to 75 feet bsg and converted into monitoring wells to replace the respective dry wells Borings CB-1 and CB-2 were advanced to a depth of 65 feet on the side walk near the former tank location (Figure 3) to evaluate the success of the vapor extraction system Upon completion of 5oxl sampling, these borings were backfilled with Portland cement Petroleum hydrocarbons were detected n1 boring CB-1 at depths of 20 and 35 feet and in boring CB- 2 at depths from 30 to 40 feet Samples CB i-15 and CB 1-20 were found to contain total lead at concentrations of 12 and 9 ppm, respectively Soil samples collected from MW-8 and MW-9 did not indicate the detection of petroleum hydrocarbons Analytical results are summarized on Table 4 Details of the soil boring and monitoring well installations were summa>>zed in a quarterly report prepared by GeoAudit, dated 03 August 1994 During the installation of EW-1, no samples above 55 feet bsg were analyzed However, PID readings were collected during drilling PID readings ranged from 1,719 ppmv to 2,679 ppmv in the interval from 15 feet to 45 feet in EW-1, indicating the presence of significant concentrations of petroleum hydrocarbons PID readings from CB-1 ranged from 0 ppm to 451 ppm in the same depth range Organic vapor was detected only from 35 feet to 45 feet bsg in CB-2 This represents a considerable reduction in organic vapor concentrations within the hydrocarbon plume The original estimation of the volume of gasoline in the soil was based on the assumption of a cylindrical plume with a diameter of 25 feet, the thickness of the impacted soil being 40 feet and the concentration of hydrocarbon from the laboratory analyses The volume of gasoline was estimated to be 3,000 gallons The current dimension of the impacted soil is approximately 10 feet thick, extending from 30 to 40 feet A pocket of impacted soil is also present near CB-1 at a depth of 20 feet This indicates that the current plume is approximately one quarter the volume of the original plume Concentrations of remaining petroleum hydrocarbons are also approximately one quarter that of the original concentrations Figures 5 and 6 are cross sections illustrating the estimated extent of