Biodegradation of Chlorinated Ethenes at a Karst Site in Middle Tennessee

Table 4. Geochemical indicators of terminal electron acceptor processes measured during this study



Remarks concerning interpretation

Dissolved oxygen (DO)

If DO is greater than 1 mg/L (aerobic conditions), reductive dechlorination is unlikely; however, cometabolism and direct oxidation of partially dechlorinated solvents are possible.

Nitrate (NO3-)

Reductive dechlorination of tetrachloroethylene (PCE) and trichloroethylene (TCE) to cis-1,2-dichloroethylene (cDCE) can occur under these mildly reducing conditions. Direct oxidation of partially dechlorinated solvents also may occur under NO3--reducing conditions.

Ammonia (NH3)

Elevated NH3 concentrations may indicate NO3--reduction if DO is less than 1 mg/L. If NH3 is present at 0.5 mg/L or greater and DO levels are greater than 1.0 mg/L, then partially dechlorinated solvents may be cometabolized through the NH3-monooxygenase pathway.

Soluble manganese (Mn2+) and ferrous iron (Fe2+).

Increased concentrations of Mn2+ and Fe2+, coinciding with low DO concentrations, are indicators that insoluble manganese and iron (Mn4+ and Fe3+) are serving as electron acceptors. Reductive dechlorination of PCE and TCE to cDCE can occur under mildly reducing conditions. Mn4+- and Fe3+-reducing bacteria are capable of consuming vinyl chloride (VC) and cDCE.

Sulfate (SO42-)

Reductive dechlorination of TCE to cDCE, then to VC and ethene can occur under SO42--reducing conditions. The chlorinated solvents compete directly with SO42- to serve as electron acceptors.

Sulfide (S2-)

Elevated S2- concentrations may indicate SO42--reduction and conditions favorable for reductive dechlorination of chlorinated solvents.


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