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<br /> ' Mr. Durin Linderholm Page 4 of 10
<br /> Regional Water Quality Control Board—Central Valley Region
<br /> ' October 30, 2014
<br /> ' EOS injections were performed in spring 2011 to provide carbon and hydrogen (an electron donor),
<br /> leading to reducing conditions conducive for growth of anaerobic bacterial populations capable of
<br /> degrading chlorinated volatile organic compounds (CVOCs) such as PCE and its daughter products.
<br /> Several geochemical indicators are used to evaluate presence of reducing conditions. Levels of some
<br /> geochemical indicators decrease under reducing conditions. These indicators include DO, ORP, sulfate,
<br /> and nitrate. pH also may decrease due to biological activity, including generation of bacterial waste and
<br /> ' formation of organic and inorganic acids. Levels of other geochemical indicators, for example, Fez
<br /> +,
<br /> increase under reducing conditions. TOC also increases when a carbon substrate, such as vegetable oil,
<br /> is added. Therefore, evaluating the changes in the concentrations of these indicators provides a way to
<br /> ' monitor the direct effects of the vegetable oil injections. However, due to the potential for oxygen
<br /> entrainment, some field parameter measurements, such as DO, ORP, and to a lesser extent, Fez', can
<br /> have a relatively high degree of associated uncertainty and should be evaluated in conjunction with
<br /> 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). In addition to iron, a relatively
<br /> ' small subset of metals, such as arsenic and manganese, can be mobilized under reducing conditions,
<br /> provided pH is in a range that is conducive to mobilization. If the reducing conditions are effectively
<br /> established and there is adequate carbon and hydrogen to support the growth of dechlorinating bacteria,
<br /> ' concentrations of PCE and its daughter 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 designated performance monitoring well for
<br /> ' the EOS injection pilot study. Post-injection results from this well indicated reducing conditions were
<br /> established in the well vicinity. Post-injection TOC in MW-6-BP increased to a high of
<br /> 1,200 milligrams per liter (mg/L). By September 2013, TOC levels in the well had decreased to
<br /> ' 10 mg/L but remained at that level for three quarters of sampling, through February 2014. However, the
<br /> May 2014 (2Q14) sampling results indicated TOC had decreased to 2.4 mg/L, almost identical to the
<br /> baseline (pre-injection) TOC of 2.6 mg/L. This return to baseline levels indicated that EOS, and its
<br /> ' degradation products, had likely been consumed. Consistent with the TOC levels, carbon dioxide
<br /> concentrations also appeared to have decreased, indicating a decrease in biological activity. However,
<br /> the August 2014 results indicate an unexpected surge in biological activity in this and other site wells.
<br /> ' For example, TOC levels in MW-6-BP rebounded to 6.4 mg/L and carbon dioxide concentrations
<br /> rebounded to levels similar to February 2014. Concurrent with this increase in biological activity, there
<br /> was a return to reducing conditions in nearly all site wells—with DO and ORP decreasing to levels that
<br /> are representative of anaerobic conditions.
<br /> tField Parameters and CVOC Concentration Trends
<br /> ' Field parameters for all site wells are plotted in Appendix C, Figures C-1 through C-6. Since injection,
<br /> DO levels in all wells were typically much higher than would be expected under reducing conditions. In
<br /> 3Q14, however, DO levels decreased unexpectedly to historically low levels in all site wells. The DO
<br /> ' concentration was zero mg/L in MW-24-BP and MW-25-BP, less than 1.0 in MW-6-BP and less than
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