Biodegradation of Chlorinated Ethenes at a Karst Site in Middle Tennessee

Constituent

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 (SO₄^2-)

Reductive dechlorination of TCE to cDCE, then to VC and ethene can occur under SO₄^2--reducing conditions. The chlorinated solvents compete directly with SO₄^2- to serve as electron acceptors.

Sulfide (S^2-)

Elevated S^2- concentrations may indicate SO₄^2--reduction and conditions favorable for reductive dechlorination of chlorinated solvents.