ARCHIVED REPORTS_XR0007498 CASE 1

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Mr Wade Broughton General Mills, Inc January 26, 1995 ORPage 2 The three 12,000-gallon USTs were used to store No 6 heating oil The tanks were removed between October 11, and 17, 1991 Alter removing the tanks, soil was excavated to a depth of approximately 15 teet below surtace grade (bsg} Following excavation, Water Works of Escalon, California, collected nine soil samples from the base of the tank basin excavation and submitted them to a certified laboratory for analyses of benzene, toluene, ethylbenzene, xylenes (BTEX), and high boiling point hydrocarbons (TPH as diesel No 6) Benzene was not detected in any of the soil samples, however, concentrations of TPH as diesel No 6 ranged from below the laboratory's reporting limits to 820 parts per million (ppm) The approximate soil sample locations are presented in Figure 3 and the laboratory analyses results are summarized in Table 1 Following the excavation of soil, the tank excavation was backfilled to approximately 5 feet bsg with well-graded gravel, and the remaining 5 feet was backfilled with silty sand On May 12, 1992, Delta oversaw the advancing of three soil borings to 50 teet bsg These borings were completed as ground water monitoring wells MW-1, MW-2, and MW-3 The locations of the ground water monitoring wells are shown in Figure 4 Soil samples were collected at 5-toot intervals from borings MW-1, MW-2, and MW-3 and were classified according to the Unified Soil Classification System Soil types encountered beneath the site consisted predominantly of poorly graded sand with gravel, clay, silt, well graded sand with silt, and sandy lean clay Field screening of the soil samples included both visual inspections and analyses for the presence of organic vapors with an hNu photoionization detector (PID) Soil staining indicative of fuel contaminated soils was not observed in the soil samples inspected Organic vapors in the soil samples were not detected with the PID The soil samples collected from 35 teet beneath the surtace in borings MW-1, MW-2, and MW-3 were submitted to a certified laboratory for analyses of BTEX and TPH as diesel No 6 No analytes were detected at or above the laboratory's reporting limits for the Soil samples collected at 35 feet Analytical results are presented in Table 2 The results of Delta's subsurface investigation are presented in Delta's report dated June 9, 1992 Ground water monitoring wells MW-1, MW-2, and MW-3 have been monitored since their installation in May 1992 The depth to ground water has ranged at the site between approximately 39 6 and 42 teet bsg during this time Cumulative ground water table measurements are compiled in Table 3 The direction of ground water flow has consistently been toward the southwest at an approximate hydraulic gradient of 0 003 to 0 005 Quarterly ground water sampling was conducted at the site for four consecutive quarters (May 1992 through March 1993), at which time the County, in their letter dated July 27, 1993, agreed to a request to reduce the monitoring wells sampling frequency to semi-annual, provided that no contaminants were detected Ground water samples have been submitted to a state-certified laboratory for analysis of BTEX and TPH as diesel No 6 To date, no analytes have been reported at or above the laboratory's reporting limits Cumulative ground water sample results are presented in Table 4 In a letter dated February 17, 1993, the County requested that additional site assessment be performed at the site In this letter, the County stated that after evaluating soil with hydrocarbon concentrations remaining in place using the Leaching Potential Analysis, that additional investigation was required between 15 teet bsg and the top of ground water Delta responded to the County's request by presenting a Leaching Potential Analysis, in a letter dated March 18, 1993, utilizing intormation collected during the hydrogeologic investigation conducted at the site Delta's analysis indicated that