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LW bioremediation is probably the predominant bioremediation occurring in the contaminant plume. Thus, it appears that nitrate and sulfate are the primary electron acceptors in the `' groundwater beneath the site. Some studies say that benzene is not degraded when nitrate and sulfate are the electron acceptors, and other studies say that benzene can be degraded with �. these electron acceptors (Borden et al, 1995). However, nitrate and sulfate are known to aid in biodegradation of toluene, ethylbenzene and xylenes. z. Based upon geotechnical and geochemical data collected from groundwater and/or soil underlying the project site, this study concludes intrinsic bioremediation is taking place in the LM contaminantlume beneath the site. Biodegradation P g appears to be continuing in an aerobic and anaerobic environment. A review of groundwater chemical data suggests the contaminant &- plume is stable since it appears that petroleum hydrocarbon concentrations are decreasing in the wells. This can be verified with further groundwater monitoring at the site. LN Additional evidence of bioremediation occurring in groundwater under the site is found in the soil vapor analytical data. Carbon dioxide concentrations are higher in vapor samples from borings inside the plume than those outside the plume, suggesting bioremediation of the i petroleum hydrocarbons in the capillary fringe. Also, methane was detected in the soil vapor L.'__ sample from boring SB2, which is inside the plume, suggesting this is a byproduct of ' bioremediation of the petroleum hydrocarbons and floating product in the capillary fringe. 7.0 POTENTIAL RECEPTORS AND PATHWAYS 7.1 Potential Receptors At the request of the PHS/EHD, Smith Technology conducted a search of the Department of Water Resources (DWR) groundwater well construction logs to identify any groundwater wells located within one half mile of the site. The search revealed thirteen wells located in the vicinity of the site. Well logs for the on-site water supply well were not found in the DWR `' records. The well locations are shown on Figure 8. The wells were located on the map according to "well location" description information provided on the well construction logs. If +- the description was not helpful, the township, range, and section were used to locate the wells on the map. The well construction logs are contained in Appendix VI. The on-site.well is the closest to the plume (within approximately 100 feet of the plume). At the writing of this report, no data was able to be obtained from the property owner, tenant or { PHSIEHD concerning the well depth, screen interval or depth of seal. The petroleum affected w:1951221Plumerpt 15 LW • SMTH