ARCHIVED REPORTS_XR0007498 CASE 1

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Mr Wade Broughton General Mills, Inc, January 26, 1995 Page 3 the maximum allowable TPH as diesel No 6 concentrations at 15 teet bsg (tank basin floor) was 1,000 ppm On April 15, 1993, representatives trom the County, Delta, and General Mills discussed appropriate actions for the site It was concluded that the variation between the County's and Delta's Leaching Potential Analyses was based on the depth to ground water The County utilized a minimum depth to ground water trom the soil samples containing TPH as diesel No 6 of less than 25 teet resulting in a maximum in place hydrocarbon concentration of 100 ppm Delta utilized a depth to ground water in excess of 25 feet, resulting in a maximum allowable in place hydrocarbon concentration of 1,000 ppm Due to the discrepancy, the County suggested that Delta present its case for leaving soil in place with concentrations of TPH as diesel No 6 over 100 ppm to the Technical Advisory Committee Delta pertormed a General Risk Appraisal, which is considered by the LUFT manual guidelines to be "more sophisticated than the leaching potential analyses" for evaluating the risk of ground water pollution at an underground fuel tank site for BTEX concentrations in soil The Risk Appraisal Analysis conducted for toluene, xylenes, and ethylbenzene indicated that the acceptable concentrations for these constituents at a depth of 15 feet below grade were 8, 30, and 40 ppm, respectively Delta also pertormed a "worst case scenario" Risk Appraisal Analysis which assumed that toluene, xylenes, and ethylbenzene had migrated vertically at the same concentrations detected at 15 feet, these findings indicated that the concentrations detected at 15 feet would still be acceptable at 20, 25, and 30 teet bsg A Risk Appraisal Analysis for benzene was not performed since benzene had not been detected in the soil samples collected at the site Delta submitted a Request for Site Closure Report, dated September 3, 1993, to the County which summarized Delta's findings, including Leaching Potential Analysis and Risk Appraisal Assessments However, the County did not approve the request for closure and required that additional soil samples be collected to turther evaluate the vertical extent of petroleum hydrocarbons in soil immediately beneath the tormer tank basin In response to this request, Delta submitted a Work Plan for Additional Drilling, dated April 12, 1994, which proposed the installation of three additional soil borings in the vicinity of the former tank basin The County approved the work plan in a letter dated April 28, 1994, in which they stated that "based on site specific conditions, soil type, location and seasonal depth to ground water, up to 100 ppm TPH-diesel could be left in place at 25 feet below grade without posing a threat to groundwater " Additional Soil Boring Results On July 26, 1994, three soil borings were advanced to 40 feet bsg using a truck-mounted, hollow-stem auger drilling rig As proposed in Delta's work plan, soil samples were collected at 5-foot vertical intervals, in addition, soil samples were also collected from a depth of 17 5 feet bsg (approximately 2 5 feet below the bottom of the tormer tank basin) to assess the extent of petroleum constituents in soil immediately below the tormer tank basin Delta also collected a grab sample of the drill cuttings generated trom soil boring SB-2, between the depths of 2 5 to 4 teet bsg (sand backfill material), because petroleum hydrocarbon odors were detected in soil borings SBA and SB-2 at that depth during drilling The locations of the three additional soil borings are shown on 16 Figure 4 The methods used to drill and sample the soil borings are described in Enclosure A Soil boring logs are included in Enclosure B