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... <br /> C" VVATC <br /> M <br /> A S S O C I A T E S I N C <br /> stabilize before roundwater sam les were collected. Samples were then collected from each well ! <br /> g p <br /> using disposable bailers. Purge water was contained in 55-gallon DOT approved drums. Purge <br /> water from monitoring well MW-1 from the previous sampling event did not contain detectable <br /> concentrations of petroleum hydrocarbons and was discharged to the ground surface. <br /> The groundwater samples collected from each well were submitted with Chain of Custody Record <br /> to State-certified American Environmental Network (AEN) laboratory in Pleasant Hill, California <br /> :1 for analysis of total petroleum hydrocarbons as gasoline (TPHg), benzene, toluene, ethylbenzene, <br /> xylenes (BTEC and methyl tertiary butyl ether (MTBE) utilizing EPA methods 8015/8020. <br /> Groundwater well purge and sample logs are provided in Appendix B. <br /> AnalyticaI results indicated that petroleum hydrocarbon concentrations have increased in wells <br /> MW2 and MW3 since the last monitoring event. Petroleum hydrocarbons concentrations in well <br /> % i MW1 remained nondetectable, <br /> 2.2 Natural Attenuation Monitorin <br /> Between September 1996 and June 1997, groundwater samples collected from each of the site's <br /> I <br /> monitoring wells were analyzed for specific geochemical parameters to determine if intrinsic <br /> bioremediation is occurring at the site. In addition, grab groundwater samples were collected <br /> from soil boring s duringthis time period and were also analyzed for specific geochemical <br /> r---. parameters. <br /> To determine if intrinsic bioremediation is occurring, the groundwater wells and soil borings must <br /> be categorized as inside the contamination plume or outside the contamination plume. The wells <br /> and borings located inside the plume of petroleum hydrocarbon affected groundwater are: wells <br /> __' MW2 and MW3, borings S132, S134A, and SB6. Wells and borings located outside the plume are: <br /> - -t well MWl, and borings SB 1, S133, SBS, and SB7. The low concentrations of toluene and MTBE <br /> `._ in borings SBI and SB3 mark the extent of the petroleum hydrocarbon plume, and thus, these <br /> borings are considered outside the plume. <br /> Eh readings, nitrate, sulfate and total alkalinity levels do not indicate a clear pattern consistent <br /> with the distribution of petroleum hydrocarbons present in the wells. However, the data show <br /> �J; that over time dissolved oxygen levels have decreased, and carbon dioxide and ferrous iron levels <br /> Fir� have increased indicating that bioremediation may be occurring. Generally, throughout the site, <br /> dissolved oxygen concentrations were below detection limits (<I.0 ppm) in the groundwater <br /> G==a samples collected from the borings. <br /> Based upon geotechnical and geochemical data collected from groundwater and/or soil underlying <br /> the project site, this study concludes intrinsic bioremediation may be taking place in the <br /> contaminant plume beneath the site. Biodegradation appears to be continuing in an aerobic and <br /> =' anaerobic environment. A review of groundwater chemical data suggests the contaminant plume <br /> is stable since it appears that petroleum hydrocarbon concentrations are not significantly <br /> Cw:1951221reportslparcap.doc 3 <br /> C`� <br />