ARCHIVED REPORTS_SECOND SEMIANNUAL 2002 GROUNDWATER MONITORING

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' /� GEOMATRIX groundwater samples is included in Table 1. Analytical results from the November 2002 sampling event are summarized on Figure 2. Laboratory data sheets and chain-of-custody records are included in Appendix A. BTEX,TPHg, TPHd(without and with silica-gel preparation), and PAHs were not detected in ' the sample collected from MW-5. The concentrations of BTEX, TPHg, TPHd (without silica- gel preparation), and PAHs detected in the groundwater samples collected from wells MW-6, MW-7,MW-8, and MW-12 during this monitoring event are less than or similar to the previous ' monitoring data (Table 1). The groundwater monitoring data collected to date indicate that the trends of petroleum hydrocarbon concentrations are stable and/or declining,indicating a stable or shrinking plume (Figure 5). ' TPHd concentrations of the sample extracts from wells MW-6, MW-7, MW-8, and MW-12 treated using a silica-gel preparation procedure were less than those analyzed without the silica- gel preparation (Table 1). These data indicate the presence of positive interferences in the samples, likely due to the presence of soluble byproducts associated with the intrinsic bioremediation of residual petroleum hydrocarbons at the site. 3.0 INDICATOR PARAMETERS FOR INTRINSIC BIOREMEDIATION Indicator parameters in groundwater were measured to evaluate ongoing intrinsic bioremediation in the subsurface at the site. These parameters included field measurements (pH, dissolved oxygen, and oxidation-reduction potential [ORP]) and laboratory analyses (alkalinity, dissolved iron, nitrate, and sulfate). A discussion of these parameters is presented in Appendix B. Laboratory analyses for inorganic constituents were conducted by Agriculture &Priority Pollutants Laboratories, Inc., of Fresno, California. The measurements of intrinsic bioremediation parameters are summarized in Table 2 and on Figure 3; laboratory data sheets ' and chain-of-custody records are included in Appendix A. The spatial relationships of these parameters with BTEX concentrations in groundwater were evaluated for evidence of biological activity (Figure 4). ' The results of these measurements indicate the depletion of electron acceptors (dissolved oxygen, nitrate, and sulfate), the accumulation of metabolic byproducts or intermediates (dissolved iron and alkalinity), and reducing conditions (negative ORP)in the wells with higher • BTEX detections (i.e., MW-6 and MW-7). These results and the declining BTEX concentrations in the groundwater samples (Figure 5) are evidence of ongoing intrinsic 1:NROJECn2000s\2463\DOCUMENR2463 12-02 rpt.d- 4