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The water level hydrograph presented in Figure 5 indicates groundwater elevations generally follow the same seasonal fluctuation trend in all monitoring wells. Groundwater elevations have shown a decreasing trend since 2006,presumably due to drought conditions and possibly from water table drawdown from agricultural water use on the adjacent parcels. L4.5 Extent of Petroleum Hydrocarbon Impacted Soil The results for soil samples analyzed during historic site investigations are discussed in this section. L Figure 6 presents a site plan with historic soil analytical results. Appendix D,Figures D-2 and D-3 show soil concentrations plotted on geologic cross sections. Historic soil analytical data is included in Appendix B,Table B-3. In general,petroleum hydrocarbons detected in soil samples from the site consist of longer chain petroleum hydrocarbons (TPHK,TPHD,and/or TPHMO). Shorter chain petroleum hydrocarbons (TPHG) were detected in soil samples collected from several borings;however,these samples had L much higher concentrations of TPHK and/or TPHD in the soil samples. The TPHG results are likely from the heavier end of the gasoline range,which overlaps with the lighter end of the TPHK or TPHD hydrocarbon range. Additionally,the relatively few and infrequent detection of BTEX components indicate that the release most likely originated from the diesel and liquid asphalt cutback storage tanks. The longer chain petroleum hydrocarbons detected at this site are relatively L immobile and of lower toxicity than the shorter chain petroleum hydrocarbons typically found at gasoline UST sites. The lateral extents of petroleum hydrocarbon impacts to soil have been defined by data from borings B-5, B-3,C-3,MW-3,MW-6,and MW-5. The vertical extent of petroleum hydrocarbon impacted soil was defined in all soil borings. Soil samples designated E-#were collected from the exploratory trenches installed in January 2004,and were extended to a maximum of 14 feet BGS; however several soil borings in the vicinity of these samples have deeper soil samples collected. The approximate extents of impacts to soil are shown in Appendix D,Figures D-2 and D-3. The majority of the identified petroleum hydrocarbon mass to date appears to be limited to soils within the upper 35 feet BGS in the vicinity of the former USTs and consists primarily of TPHD range petroleum hydrocarbons. TPHD was detected in the soil sample collected from 40 feet BGS in boring C-2,however was non-detect for TPHD at 45 feet BGS. Petroleum hydrocarbons in soil have been identified in the subsurface vicinity of the former USTs and ASTs;however, there is approximately 8 to 20-feet of vertical separation between identified soil impacts and the highest groundwater elevations recorded in site wells during historic monitoring events. Furthermore, these soils are isolated from surface influxes as the facility is generally capped with asphalt or chip seal. The cap and the physical nature of the material insure that the risk to human health and the environment from leaving this material in place is acceptable. 4.6 Extent of Petroleum Hydrocarbons in Groundwater L The results of historic well point and monitoring well sample analyses are summarized in this section and indicate that groundwater beneath the site contains low levels of material that is detected in the TPHD range. Figure 7 shows the historic concentrations of TPHD in groundwater at \\Eureka\projects\2007\007152-WOS\PUBS\rpt\20100324-1stHa1f2010GWMR&RFC.doc c'_�I.A_/ 8