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URS
<br /> tMs.Kathy Amaru Page 4 of 9
<br /> Regional Water Quality Control Board—Central Valley Region
<br /> ' April 30,2014
<br /> ' Emulsified vegetable oil injections were performed in spring 2011 to provide carbon and hydrogen (an
<br /> electron donor), leading to reducing conditions conducive for growth of anaerobic bacterial populations
<br /> capable of degrading chlorinated volatile organic compounds (CVOCs) such as PCE and its daughter
<br /> ' products. Several geochemical indicators are used to evaluate presence of reducing conditions. Levels
<br /> of some geochemical indicators decrease under reducing conditions.These indicators include DO, ORP,
<br /> sulfate, and nitrate. pH also may decrease due to biological activity, including generation of bacterial
<br /> ' waste and formation of organic and inorganic acids. Levels of other geochemical indicators for
<br /> example, Fe", increase under reducing conditions. TOC also increases if a carbon substrate, such as
<br /> vegetable oil, is added. Therefore, evaluating the changes in the concentrations of these indicators
<br /> ' provides a way to monitor the direct effects of the vegetable oil injections. However, due to the
<br /> potential for oxygen entrainment, some field parameter measurements, such as DO,ORP, and to a lesser
<br /> extent, Fe 21,
<br /> can have a relatively high degree of associated uncertainty and should be evaluated in
<br /> ' conjunction with other results.
<br /> Other changes in groundwater quality are less direct or immediate and can be a result of bacterial
<br /> activity or groundwater migration from other upgradient areas. Changes resulting primarily from
<br /> ' bacterial activity include a lowering in pH, as discussed above, and an increase in carbon dioxide. Other
<br /> dissolved gases, including ethene, ethane, and methane, can form as a result of reductive dechlorination
<br /> of chlorinated ethenes and bacterial metabolism(e.g., of carbon dioxide). Some metals, such as arsenic
<br /> ' and manganese, can be mobilized under reducing conditions,provided pH is in a range that is conducive
<br /> to mobilization. If the reducing conditions are effectively established and there is adequate carbon and
<br /> hydrogen to support the growth of dechlormating bacteria, concentrations of PCE and its daughter
<br /> products should decrease and eventually disappear.
<br /> Monitoring wells located in the immediate oil injection area are expected to fully demonstrate the
<br /> reducing characteristics listed above. MW-6-BP is the only performance monitoring well for the EOS
<br /> ' injection pilot study. Current and previous results from this well indicate reducing conditions have been
<br /> established. Although as of September 2013 and continuing to February 2014, TOC in MW-6-BP
<br /> decreased from its high of 1,200 milligrams per liter (mg/L) post-injection to approximately 10 mg/L,
<br /> indicating the carbon source was being depleted. For reference, the baseline (pre-injection) TOC in the
<br /> well was only 2.6 mg/L. TOC levels of 10 mg/L or higher have not been observed in any other wells,
<br /> indicating the injected oil, or its degradation products, likely did not reach the other wells at a
<br /> ' significant level. The observation that TOC in MW-6-BP has remained at 10 mg/L since September
<br /> 2013 indicates that either bacterial metabolism has slowed down, or there remains a continuing source
<br /> of TOC (e.g., EOS degradation products) that is replenishing the organic carbon above baseline levels.
<br /> ' The first explanation does not appear to be true because carbon dioxide concentrations have increased
<br /> and remained at high levels in the well, indicating robust biological activity. Therefore, it can be
<br /> surmised that EOS, and/or its degradation products, continue to persist in the treatment zone.
<br /> ' Field Parameters and CVOC Concentration Trends
<br /> Field parameters for all site wells are plotted in Appendix C, Figures C-1 through C-6. DO levels in all
<br /> ' wells are higher than would be expected under reducing conditions. By contrast, ORP measurements
<br /> have remained negative during the last several sampling events in MW-2-BP, MW-6-BP, and
<br /> MW-24-13P, indicating reducing conditions have been established and persist in these wells.
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