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Site Investigation Work Plan Nella Oil Station#21, 40052 Highway 41, Oakhurst, California groundwater collected from MW-4 at 26,300 ug/I. Dissolved Total Petroleum Hydrocarbons as Diesel (TPH-D) and Total Petroleum Hydrocarbons as Motor Oil (TPH-MO) were detected in groundwater collected from MW-2 at 10,900 ug/I and 12,600 ug/l, respectively. Fuel oxygenates were detected in groundwater collected from MW-2 and MW-4, however, only MW-4 had all four BTEX constituents and at significantly elevated values compared to the other wells; benzene was detected at 735 ug/l, toluene was detected at 700 ug/I, ethylbenzene was detected at 900 ug/l, and total xylenes were detected at 3,375 ug/l. The fuel oxygenate MtBE was detected at 1.02 ug/I in MW-2 and DIPE was detected at 21.1 ug/I in MW-2 and 330 ug/I in MW-4. The residual TPH-D and TPH-MO portion of the contaminant plume is therefore constrained to the area directly surrounding the former motor oil USTs and contamination extends downgradient("s-outheast). The residual TPH-G portion of the contaminant plume is constrained 4 —to the area extending downgradient (southeast) of the former motor oil LISTS towards MW-4 under South El Dorado Street. Reviewing the analytical results of the CPT groundwater samples collected October 2007 and the Rockworks© modeled Iithology of the site, the depth of the TPH-G plume likely ranges anywhere from approximately 20-35 feet bgs, but could possibly be present until confirmatory non-detection at 57 feet bgs. 2 The petroleum hydrocarbon constituents appear to be weathered, with benzene detected in only CPT groundwater sample from CPT boring P-2 at 35 feet bgs and DIPE only detected in CPT boring P-2 at all four depths. Benzene and DIPE were only detected at elevated concentrations during the second quarter monitoring event in monitoring well MW-4. 2.3 Potential Pathway Analysis 2.3.1 Utility Corridors Although there are some utility corridors located near the site along South El Dorado Street, the depth to groundwater (typically 25 to 35 feet below grade) precludes utility trenches as a preferential pathway of any significance for migration of petroleum hydrocarbons. 2.3.2 Geologic Pathways The silty and sandy soils and limited clay identified from CPT boring activities at the site do not provide a significant barrier to petroleum hydrocarbons constituent migration, although some natural attenuation will occur within the unsaturated zone as the petroleum hydrocarbons disperse in the soil. Rockworks©was used to interpret preferential lithologic pathways for contaminant migration between the California Tank Lines and Pacific Pride site (3147 South El Dorado Street), and a Iithology profile is presented as Figure 7. At the interval of contamination on the California Tank Lines site, likely approximately 20-35 feet bgs, there exists a relatively continuous zone of clayey silt with sand interbeds between the most contaminated regions from both sites (MW-4 and P-2 from CTL and MW77 near borings P-7, P 8 and P-9 from Pacific Pride). The a litholo types can facilitate i ration of TPH-G ' nd enzene t ough the 9Y Y P 9 ' groundwa r zone at these depths. From 3 -40 feet bgs, there s Iso likely a c ntinuous clay bed, whin does provide a significant barr r for petroleum hydr c rbon vertical igration. or Sari 1 n6f$ I� i S 4 -�I {