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' Site Conceptual Model Frontier Transportation June 30, 2003 Page 7 Historically concentrations of dissolved BTEX above maximum contaminant levels (MCLs) set by the California Department of Health Services were identified in monitoring wells MW-1, MW-2, MW-3, MW-3R, MW-4, and MW-13 Maximum dissolved BTEX concentrations were 302 6 µg/L, 145 1 µg/L, 8,374 µg/L, 846 7 µg/L, 6 µg/L, and 62 µg/L, respectively,'Dissolved BTEX concentr tions in monitorin wells MW-1, MW-2, and MW-4 have remained non detect since at least'ay,6?Id rOnIy monitoring well MW-3, which was replaced by MW-3R, contained dissolved BTEX from every groundwater sampling episode Dissolved BTEX over time was graphed for those wells (MW-1, MW-2, MW-3, MW-3R, ' and MW-13) that demonstrate significant trends in contaminant concentrations Graphs of dissolved BTEX concentrations over time are in Appendix E Based on the regression line, all five monitoring wells identified an overall attenuation of dissolved BTEX concentrations also ' All other monitoring wells identified nondetectable concentrations of dissolved BTEX historically Laboratory analysis indicates that historically the dissolved BTEX plume has been for the most part contained within the site boundaries Please see Appendix E for estimated plume extent using the maximum dissolved BTEX concentrations from all sampling episodes Historically concentrations of dissolved MTBE above its MCL of 13 µg/L set by the • California Depart of Health Services were identified to monitoring wells MW-1, MW-3, MW-3R, MW- d`MW-13 Maxmu m dissolved MTBE concentrations were 5,800 µg/L,29,400pg/L, 5,500 µg/L, 42 µg/L,fa46 µg/L, respectively A maximum concentration of ' dissolved MTBE in MW-3 was detected using EPA method 8020, which is not a quantitative LI WO method for detecting MTBE This may not be representative of true subsurface concentrate_ ons M Vj Dissolved MTBE co n ra ions in monitoring wells MW- a�W-13 have remained non detect since at least.of 2002 Ol'y Monitoring well MW-3,,wliich was replaced by MW-3R, , All contained dissolved MTBE from every groundwater sampling episode A Dissolved MTBE over time was graphed for those wells (MW-1, MW-3, and MW-3R) that demonstrate significant trends in contaminant concentrations Graphs of dissolved MTBE concentrations over time are in Appendix E Based on the regression line, only monitoring well MW-3R identified an overall increasing trend in dissolved MTBE concentrations Monitoring well MW-3R was installed within the excavation backfill material, which was coarser than the native soil The permeability difference between the backfill material and native soil can cause groundwater to accumulate more than normal within the backfill material and can cause fluctuations in MTBE All other monitoring wells identified nondetectable concentrations of I dissolved MTBE historically Laboratory analysis indicates that historically the dissolved MTBE plume has been for the most part contained within the site boundaries Please see Appendix E for estimated plume extent using the maximum dissolved MTBE concentrations from all / sampling episodes