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' Fourth Quarter/Annual <br /> 2008 Groundwater <br /> ' ARCADIS Monitoring Report <br /> Chevron Bulk Fuel Terminal <br /> 100-1621 <br /> ' 22888 Kasson Road <br /> Banta, California <br /> northwest and northeast of the site. However, based on quarterly groundwater <br /> ' monitoring data, it appears that petroleum hydrocarbon impacts have not significantly <br /> migrated in the B zone. MTBE was not detected in groundwater samples collected <br /> from the B zone during the Fourth Quarter 2008 Monitoring and Sampling Event <br /> ' conducted by G-R or the grab groundwater samples collected from the newly installed <br /> wells. <br /> ' The newly installed monitoring wells have not been surveyed;therefore,were not used <br /> for preparing the isoconcentration maps.A site plan illustrating the approximate <br /> locations of the newly installed wells is provided in Figure 11. <br /> Biotransformation of MTBE <br /> ' Of the four monitoring well locations used to indicate the existence of a <br /> biotransformation occurring from MTBE to TBA(MW-1 UA, MW-3UA, MW-50A, <br /> MW-68A),only monitoring well MW-50A was sampled during the fourth quarter of <br /> ' 2008.The TBA/MTBE ratio in monitoring well MW-50A was 340. Studies have <br /> indicated that TBA/MTBE ratios greater than 10 suggest that biotransformation of <br /> MTBE to TBA is occurring(ETIC,2005. Development of TBA Plume Conceptual <br /> ' Models).The TBA/MTBE ratio observed in the groundwater sample taken from <br /> monitoring well MW-50A is an indication that biotransformation of MTBE to TBA is <br /> occurring in the A zone(Appendix G, Figures 1 through 4). <br /> ' Monitoring Wells MW-34UA and MW-35UA Trends <br /> In response to a RWQCB request dated July 2, 2004, the following text discusses the <br /> ' effect of the excavation of contaminated soil on the nearby shallow groundwater <br /> monitoring well concentrations. Between May 3 and 7, 2004,approximately 775 cubic <br /> yards of impacted soil were excavated from the area around monitoring wells <br /> ' MW-34UA and MW-35UA. <br /> Following the excavation,concentrations of TPH-DRO(the only petroleum <br /> hydrocarbon constituent detected since 1999)have remained stable in groundwater <br /> samples collected from MW-34UA and MW-35UA(Appendix F, Figures 18 and 19). <br /> Groundwater data will continue to be collected in accordance with the MRP and <br /> ' reviewed quarterly to assess any effects of groundwater concentrations due to the <br /> excavation of affected soil. <br /> 10 of 17 <br />