Coupled Diffusion and Abiotic Reaction of Trichlorethene in Minimally Disturbed Rock Matrices

Abstract

Laboratory experiments were performed using minimally disturbed sedimentary rocks to measure the coupled diffusion and abiotic reaction of trichloroethene (TCE) through rock core samples. Results showed that, for all rock types studied, TCE dechlorination occurred, as evidenced by generation of acetylene, ethene, and/or ethane daughter products. First-order bulk reaction rate constants for TCE degradation ranged from 8.3 × 10(-10) to 4.2 × 10(-8) s(-1). Observed reaction rate constants showed a general correlation to the available ferrous iron content of the rock, which was determined by evaluating the spatial distribution of ferrous iron relative to that of the rock porosity. For some rock types, exposure to TCE resulted in a decrease in the effective diffusivity. Scanning electron microscopy (SEM) indicated that the decrease in the effective diffusivity was due to a decrease in the porosity that occurred after exposure to TCE. Overall, these coupled diffusion and reaction results suggest that diffusion of TCE into rock matrices as well as the rate and extent of back-diffusion may be substantially mitigated in rocks that contain ferrous iron or other naturally occurring reactive metals, thereby lessening the impacts of matrix diffusion on sustaining dissolved contaminant plumes in bedrock aquifers.

Abiotic dechlorination in rock matrices impacted by long-term exposure to TCE

“…Each bedding unit consists of several water-bearing zones, alternating with confining zones. It is noted that, in our previous work (Schaefer et al, 2013), the bedrock samples were collected at NAWC from outside the historic TCE plume. Thus, collection of rocks from within the plume for this study was intended to provide a comparison by which the effects of long-term exposure on abiotic reactions within the rock matrix could be assessed.…”

“…Although the studies noted above clearly indicate that abiotic degradation of TCE can occur in rock matrices, it has hitherto not been demonstrated that these reactions are ongoing at bedrock sites that have been exposed to TCE for decades, as our previous work (Schaefer et al, 2013) used rocks that had not been exposed to TCE (i.e., rocks collected from a location where TCE contamination was not present). The impact of long-term exposure to TCE on the observed first-order TCE degradation rate constant is unknown.…”

“…Recently, we performed studies on minimally disturbed rock cores under anoxic conditions (Schaefer et al, 2013). Measuring the coupled diffusion and reaction of TCE through the rock, first order TCE degradation rate constants ranging from 8.3 Â 10 À10 to 4.2 Â 10 À8 s À1 were observed, with no measureable decrease in reactivity observed through 10 months.…”

“…Thus, this study demonstrated that abiotic dechlorination reactions occurred between TCE and ferrous rock minerals within the rock matrix. Differences in results between Schaefer et al (2013) and Darlington et al (2013) may be due to the contaminant (TCE vs. DCE), the nature of the rock tested, and/or the nature of the experiment (e.g., crushed rock vs. intact rock, attainment of anoxic conditions).…”

“…Such confirmation is critical, as determination as to whether such abiotic reactions are short lived (i.e., months to years) due to oxidation of ferrous iron minerals, or are long-lived (i.e., decades), has important implications for inclusion of such reactions in natural attenuation modeling of bedrock sites. The objective of this study was to address these important questions, which involved measurement of the TCE dechlorination rate in rock matrices historically exposed to TCE and the subsequent comparison to our previous work (Schaefer et al, 2013) that measured TCE reaction rates in non-TCE exposed rock matrices, thereby providing improved insight into the longterm fate and transport processes at many chlorinated solvent impacted bedrock sites.…”