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December 22, 1998 41017 002 Harding Lawson Associates Mr Michael Kith ' PHS/EHD Page 9 ' excavation, continued carefully from 1 0 to 15 feet vertically and 2 to 3 feet laterally Overexcavation on the northeast side of the excavation, continued from 1 5 to 2 0 feet vertically and 2 to 3 feet laterally Groundwater was not encountered ' On July 21, 1998, Mr Knoll of the Site Mitigation Group arrived for the final round of confirmation sampling Samples T-17 through T-20 were collected along the ditch in the northwest side of the excavation and samples ' T-21 through T-23 were collected along the ditch in the northeast side of the excavation On the northwest, samples T-17, T-18, and T-19 were collected to constrain the lateral extent and sample T-20 was collected to constrain the vertical extent of impacted soil On the northeast, samples T-21 and T-22 were collected to ' constrain the Iateral extent and sample T-23 was collected to constrain the vertical extent of impacted soil Plate 4 shows TPHd and TPHmo concentrations of soils remaining in place following the final phase of ' excavation None of the samples shown in Plate 4 contained detectable concentrations of BTEX above normal method reporting limits Impacted soils remain in two locations The analytical data on confirmation samples from these two areas are adequate to determine both the vertical and lateral extent of impacted soils To ' assess/determine whether the remaining TPHd-and TPHmo-impacted soils pose a potential threat to groundwater quality, selected samples (T-18, T19, and T-20) were analyzed for the soluble fractions of TPHd and TPHmo Soil types of these and other samples collected were similar consisting of borderline silty clay/clayey silt The results show that none of these samples contained detectable soluble TPHd or TPHmo and therefore do not pose a potential threat to groundwater quality Additional soluble data are provided by soluble analyses of samples T-8, T-9, and T-12 The results of these analyses also indicated that soluble TPHd and TPHmo were not detectable in the leachate from these samples The soluble data from these six samples (T-8, T-9, T-12, T-18, T19, and T-20) provide an empirical data set of total concentrations versus soluble concentrations for the site This data set indicates that TPHd ' concentrations ranging from 230 to 8,100 mg/kg did not contain detectable concentrations of soluble TPHd, and TPHmo concentrations ranging from 540 to 9,600 mg/kg did not contain detectable concentrations of soluble TPHmo ' Based on these data, approximately 25 cubic yards of the stockpile around samples location SP-4, and 25 cubic yards around location SP-I were utilized as backfill material Approximately 25 cubic yards of soil around sample location SP-2 were disposed of offsite This was a conservative approach used because of the higher concentrations detected in SP-2 (540 mg/kg TPHd and 2,000 mg/kg TPHmo) and apparently higher sand contents in the soils near SP-4 In addition, approximately 25 cubic yards of soil around sample location SP-3 (nondetect for TPHd and TPHmo) were utilized as backfill material BACKFILL, COMPACTION, AND DISPOSAL ACTIVITIES After receipt of the analytical results, backfilling the excavation with clean imported fill (and the 75 cubic yards of the stockpiled soils which were not disposed of offsite) proceeded in lifts approximately one foot thick Lifts ' were compacted mainly with the track-mounted excavator The backfill process continued in lifts until the excavation was filled to approximately 0 25 to 0 5 feet above grade This was done to avoid potential ponding of rainwater over the area of the former excavation ' r,