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amec— <br /> • <br /> Transport processes, and <br /> • Design considerations (well spacing, etc.). <br /> These are discussed in Sections 2.1 through 2.4. <br /> 2.1 TREATMENT PROCESSES <br /> The primary contaminant removal mechanism is the same for biostimulation and <br /> bioaugmentation of VOCs such as trichloroethene (TCE) and is commonly referred to as <br /> reductive dechlorination, also referred to as dehalorespiration. During dehalorespi ration, TCE <br /> is dechlorinated in a stepwise manner to ethene by specialized microbes that use dissolved <br /> hydrogen (H2) as a reductant, as follows: <br /> TCE + H2 ----> cDCE + Cl- + H+ <br /> cDCE + H2 —> vinyl chloride + Cl- + H+ <br /> vinyl chloride + H2 —> ethene + Cl- + H+ <br /> where cDCE=cis-1,2-dichloroethene, CI-=chloride ion, H+=hydrogen ion (proton). <br /> The H2 used for dehalorespiration can be derived from the fermentation of naturally occurring <br /> or injected organic matter, which also creates the required strongly reducing conditions. <br /> Dehalorespi ration can be a viable process after dissolved oxygen, nitrate and sulfate are <br /> depleted in the aquifer system due to an excess of dissolved organic carbon (Bennett et al., <br /> 2007). The presence of cDCE and vinyl chloride in groundwater samples from monitoring wells <br /> near the WWTP is an indication that dehalorespi ration reactions are already occurring. <br /> However, the primary concern is that the final dechlorination step may be too slow, based on <br /> increased concentrations of vinyl chloride relative to TCE and cDCE. For this reason, the <br /> feasibility of aquifer bioaugmentation with microbes known to rapidly degrade vinyl chloride is <br /> under consideration. <br /> 2.2 SIDE-EFFECTS OF TREATMENT ON GROUNDWATER CHEMISTRY <br /> The following geochemical changes in the groundwater system may be caused by the addition <br /> of organic carbon and nutrients during bioaugmentation: <br /> • An increase in dissolved organic carbon (which is an increase in biochemical oxygen <br /> demand). <br /> • A decrease in sulfate and increase in sulfide. Sulfate, naturally present in groundwater, <br /> is typically reduced to sulfide when organic carbon is added because of enhanced <br /> rates of sulfate reduction. <br /> AMEC Geomatrix, Inc. <br /> \\oad-fs1\doc_safe\9000s\9837.006\4000 REGULATORYTS Assessment_Apx B_01 2711\Attachment B.2\Attach B-2.doc 132-2 <br />