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Section 5 RESULTS SUBSURFACE CONDITIONS The soils underlying the Thornton Gas Dehydrator Station include interbedded layers of silt, sand, and clay (see boring logs in Appendix A and the geologic cross sections in Figure 4). Silt and sand are present in the upper 5 to 7 feet and underlain by medium brownish-gray clay to a depth of at least 10 feet. The interbedded nature of these soils is typical of fluvial depositional systems. Groundwater was not encountered in the boreholes drilled at the site. CHEMICAL ANALYTICAL RESULTS Table 1 summarizes the results of the laboratory analyses performed on the soil samples collected from the boreholes. The formal laboratory reports and chain-of-custody records are presented in Appendix B. Petroleum hydrocarbons were reported in soil samples collected from 13 of the boreholes at the site (B-1 through B-10 and B-12 through B-14). The highest levels of petroleum hydrocarbons were reported from the borings performed in the drainage ditch located immediately west of the dehydrator station. Boring B-10 contained the highest levels of petroleum hydrocarbons,with total oil and grease up to 4,400 mg/kg and TPH-D up to 9,300 mg/kg. BTEX compounds represent a very minor portion of the petroleum hydrocarbons identified at the site, and when detected were generally reported in the parts-per-billion range. The petroleum hydrocarbons identified in these samples consist primarily of diesel-range hydrocarbons. Discussions with the analytical laboratory revealed that the oil and grease results probably represent the heavier hydrocarbon compounds reported as TPH-D because the oil and grease analysis is sensitive to C 15+hydrocarbons and the TPH-D analysis is sensitive to C 12 to C 19 hydrocarbon compounds. cca09/17/93(9784a.doc)cp45) 5-1