ARCHIVED REPORTS XR0001493 - Laserfiche WebLink

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