Quantification of TCE Co‐Oxidation in Groundwater Using a ¹⁴ C–Assay

Abstract

Abstract Bacteria that degrade natural organic matter in groundwater contain oxygenase enzymes that can co‐oxidize trichloroethene (TCE). This degradation pathway is promising for large dilute plumes, but its evaluation is limited because the density of the bacteria with oxygenase enzymes has not been correlated to field scale rates of degradation. A 14 C–TCE assay was developed to determine pseudo first‐order rate constants for the aerobic co‐oxidation of TCE in groundwater. The assay involved incubating 14 C–TCE in samples of groundwater contained in 160 mL serum bottles, and monitoring the accumulation of radiolabel in degradation products. A first‐order rate constant for co‐oxidation was extracted from the rate of accumulation of 14 C in products, accounting for volumetric changes in the serum bottles due to sampling and subsequent changes to the distribution of TCE between the aqueous and gaseous phases. Of the groundwater samples evaluated from 19 wells at five sites, eight samples at three sites had 14 C product accumulation rates that exceeded the accumulation rate in filter‐sterilized groundwater controls. First‐order rate constants ranged from 2.65 to 0.0066 year −1 , which is equivalent to half‐lives of 0.26 to 105 years. Groundwater samples from a few of the wells in which co‐oxidation occurred had volatile organic contaminants in addition to TCE; their presence may have induced the oxygenase enzymes that are needed for TCE co‐oxidation. 14 CO 2 represented ~37% to 97% of the 14 C products that accumulated; the balance of the products was soluble and non‐volatile.