Abiotic dechlorination of tetrachloroethene (PCE) and trichloroethene (TCE) by naturally present or biogeochemically augmented iron minerals (e.g., mackinawite and green rust) is an important process during the natural attenuation (NA) of chlorinated ethenes, but evidence supporting abiotic degradation in field studies has been difficult to establish. Acetylene is a major product of PCE and TCE reduction by iron minerals and is widely considered an indicator molecule of abiotic degradation. However, the potentially slow rate of acetylene generation and its relatively high biodegradability make it hard to detect and quantify at sites undergoing NA. This proof-of-concept study will focus on developing a quantitative tool to assess the rates of long-term abiotic transformation occurring at sites contaminated by common chlorinated solvents by developing a sensitive and robust device that can selectively accumulate acetylene over a field relevant time-frame. The acetylene collected can be reliably and accurately quantified using standard electrochemical means. The mass of acetylene collected over time may be further used to generate reasonable estimates of abiotic dechlorination rates.
The project involves four major tasks: i) evaluation of acetylene sensing by commercial electrodes in simple aqueous media; ii) design of protective covers for removing constituents in groundwater that interfere with acetylene sensing or cause electrode poisoning; iii) evaluation of acetylene sensing performance in batch solutions; and iv) evaluation of the prototype device in a laboratory flow-through column to simulate aquifers with varying permeability properties.
If successful, the device and methodology are expected to provide an independent line of evidence for the occurrence of abiotic degradation of chlorinated ethenes and to generate quantitative estimates of abiotic degradation rates and the long-term effectiveness of NA. Compared to direct water or soil gas sampling, this approach is particularly attractive for sites with low but nontrivial abiotic reduction activity (i.e., slow acetylene generation rates) and for characterizing abiotic attenuation in low-permeability media that are difficult to sample and investigate in the past. The outcome will fill in critical data gaps in measuring the effectiveness of remedial actions and support long-term site management decisions. (Anticipated Project Completion - 2023)