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Feasibility Study/Corrective Action Plan 4 The brake Property 26485 Thornton Road Thornton California November 26 1996 Analytical results of groundwater samples obtained from soil borings B-1 through B-10 by Groundwater Technology to February 1995 are summarized in Table 2 All groundwater samples collected were below MDLs for BTEX and TPH-G with the exception of the sample from B-3 which contained 0 51 pg/L ethylbenzene and 0 57 pg/L total xylenes The detection limits were raised during the analysis of four (B-1, B-2, B-3, B-9) of the 10 groundwater samples due to the presence of non-target analyte interferences (Groundwater Technology, 1995) Historical groundwater monitoring and sampling data are presented in Table 3 Analytical results of groundwater samples collected from the monitoring wells indicate the presence of dissolved hydrocarbons in MW-2 and MW-3 With the exception of the August 22, 1994 sample from MW-2 (0 5 ug/L benzene), groundwater samples from ail wells have historically not contained detectable benzene concentrations Non-detectable benzene concentrations in this well for each of the three sampling events preceding August 22, 1994, and all subsequent quarterly sampling events, as well as the trend of decreasing TPH-G concentrations from May to November 1994, suggest that the near MDL concentration of benzene detected in MW-2 may be due to sampling or laboratory error BTEX and TPH-G have historically been below MDLs in MW-1 Historical TPH-G concentrations in MW-2 range from 130 ug/L to 300 ug/L, and in MW-3 from below the MDL to 620 ug/L Impacted groundwater in MW-3 does not appear to be originating from the UST-pit area despite its downgradient location The abnormal chemistry noted in some samples appears to be widespread and recurring Chromatograms not typical of gasoline have been noted in previous analyses of groundwater samples collected during quarterly groundwater sampling events (Table 2) The previous site owner has confirmed that gasoline was the only fuel stored in USTs at the site, therefore, the non-gasoline compounds may be naturally occurring, or originating from a secondary on-or off-site source 30 SITE LITHOLOGY AND HYDROGEOLOGY As noted in the tank pit during site reconnaissance and in subsequent soil borings, soils to a depth of 20 feet BGS consist of silty clays and clayey silts with occasional thin, fine sand layers (Wallace-Kuhl, 1994) Groundwater was encountered at depths ranging from 6-15 feet BGS in borings B-1 through B-10, and ranges from 6-11 feet BGS in wells MW-1 through MW-3 Groundwater flow direction is to the north- northwest at a hydraulic gradient ranging from 0 001 to 0 01 feet per feet (ft/ft) (Wallace-Kuhl, 1996) o191FEAR PTiMisc 151 FLUOR DANIEL GTI �_5