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amec�9 <br /> Nitrate and boron concentrations were highest in the water table (B-zone) groundwater <br /> samples (up to 52 and 0.59 mg/L, respectively) and concentrations decreased with depth <br /> throughout the upper and intermediate aquifer zones (Figure B.4-3). As shown in Figure B.4-3, <br /> nitrate and boron concentrations appear to be positively correlated in B-zone groundwater <br /> samples. This suggests a common source for both of these constituents, and also suggests <br /> that nitrate is not being removed from the system relative to boron (i.e. it is not being reduced). <br /> The correlation of nitrate with boron in the upper aquifer wells suggests surficial anthropogenic <br /> impacts to groundwater, because nitrate is associated with agricultural practices (fertilizers) <br /> and wastewater discharge. Of the four B-zone monitoring wells sampled, the highest nitrate <br /> concentrations were detected at M-613 (52 mg/L as nitrogen; N), followed by M-1 B (30 mg/L as <br /> N), M-813 (19 mg/L as N) and M-1 7B (7.2 mg/L as N). Municipal wastewater impacts do not <br /> appear to be the source for high nitrate and boron concentrations in B-zone groundwater <br /> samples because a positive correlation between boron and chloride would also be expected, <br /> and this was not observed (Figure B.4-3). Other anthropogenic sources of nitrate and boron <br /> such as fertilizer may be the source for nitrate and boron impacts to the B-aquifer zone. Nitrate <br /> concentrations were much lower (non-detect to 7 mg/L) in intermediate and lower aquifer <br /> groundwater samples and were not correlated with boron or chloride (Figure B.4-3) and were <br /> lowest in samples from intermediate aquifer monitoring wells closer to the WWTP (TH-10, M- <br /> 31 C1, M-20C1, M-1 7C1) compared to those farther away (M-10C1, M-6C1, and M-25C1). As <br /> discussed in Section 3.2, this may be due to redox conditions near the WWTP. <br /> 3.2 OVERVIEW OF POTENTIAL INTRINSIC REMEDIATION PROCESSES <br /> Indirect evidence for intrinsic bioremediation can be obtained by comparing predominant <br /> groundwater redox conditions with those that are associated with microbiologically-mediated <br /> COC destruction processes. Abiotic destruction reactions for TCE, cDCE and vinyl chloride <br /> are generally controlled by the mineralogy of aquifer sediments. Section 3.2.1 focuses on <br /> redox conditions suitable for biodegradation of CDCs; Section 3.2.2 discusses abiotic COC <br /> removal processes. <br /> 3.2.1 Redox Conditions and Biodegradation <br /> Research has shown that chlorinated VOCs such as TCE, cDCE and vinyl chloride are <br /> biodegradable under a wide range of geochemical conditions typical of those encountered <br /> under most freshwater aquifer environments (Bradley and Chapelle, 2010). The degradation <br /> pathways vary with redox conditions and the oxidation state of each chlorinated VOC. Bradley <br /> and Chapelle (2010) conclude that: <br /> 1. TCE is resistant to biodegradation where dissolved oxygen (DO) is greater than 1 <br /> mg/L (aerobic or oxic conditions) although co-metabolic aerobic degradation of TCE <br /> can be important if a sufficient amount of available co-substrate (e.g. hydrocarbons or <br /> other organics) is present in the groundwater system. <br /> AMEC Geomatrix, Inc. <br /> \\oad-fs1\doc_safe\9000s\9837.006\4000 REGULATORYTS Assessment_Apx B_012711\Attachment B.4\Attach B-4.docx 134-7 <br />