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`S Department of Health Services, a state agency, required that the contaminated soil be <br /> demonstrated as being non-hazardous pursuant to the State of California, California <br /> Administrative Code, Title 22, Division 4 To accomplish this, a representative sample of <br /> the diesel-saturated soil was collected, sent to a State-certified laboratory, and subjected <br /> to a toxicity analysis A static acute hazardous waste bioassay analysis was performed on <br /> the sample which determined that the LC-50 value of the contaminant (a value which, <br /> basically, defines the lethal concentration of a substance) was such that the contaminant <br /> was defined as being non-hazardous. The LC-50 calculation method used was non-linear <br /> interpolation The Bay Area Regional Water Quality Control Board, a state agency, had <br /> a memorandum of understanding with the Alameda County Water District (ACWD), a <br /> county agency, therefore, ACWD was the responsible and lead agency with respect to <br /> water quahty at the subject site The Alameda County Water District required that the <br /> site be characterized so as to determine the type, extent, and concentration of <br /> contamination. <br /> Defining the Problem and Di in It Out <br /> The extent of contamination was determined using a series of four north-south trending <br /> trenches dug from the southern boundary of the footprint in Octooer of 1989 (Figure 1) <br /> Soil samples were taken at 15 foot intervals in each of the trenches from a layer of <br /> contaminant-saturated soil, and the samples were analyzed for diesel Chemical analysis <br /> of the soil samples and visual inspection of the four, 4-5 feet deep trenches showed that <br /> an approximately 1 foot thick lens of diesel-saturated soil was present beneath the <br /> footprint of the former building. The lens was sandwiched between an upper layer of soil <br /> not-saturated-with-diesel,-and a Iower clay unit (see Figure 2) <br /> Akparently, during the initial excavation work done in preparation for the building of the <br /> the plant, approximately a two-foot thickness of the native soil, a dark bay mud, was <br /> scraped off and removed. A coarse aggregate base rock, a silty gravel, was laid down in r r <br /> place of the clay as a more suitable bed for the building. The spilled diesel seeped into <br /> the ground through the relatively permeable gravel base rock, but its downward migration <br /> was-checked by the relatively impermeable native clay, hence, lateral migration beneath <br /> the building took place. The coarse gravel which served as the base rock for the building <br /> acted as an ideal fluid reservoir. <br /> The resultant diesel levels determined by the chemical analysis of the soil samples were <br /> plotted on a map of the four trenches, and isoconcentration contours v,ere drawn showing <br /> lines of equal contamination (Figure 3) From this data an estimate of the extent of <br /> contamination beneath the footprint and the approximate volume of diesel-saturated soil <br /> was obtained The isoconcentration map indicated that the contamination probably <br /> extended to the east of the building footprint <br /> Excavation of the diesel lens began in Januar, 1990 Further trenching around the <br /> excavation to further define .he extent of contaminarion Vias carried out in late January <br />