Diffusion-Coupled Degradation of Chlorinated Ethenes in Sandstone: An Intact Core Microcosm Study

Abstract

Matrix diffusion must be considered when assessing natural attenuation and remediation of chlorinated ethenes in fractured porous bedrock aquifers. In this study, intact sandstone rock and groundwater from a trichloroethene (TCE)-contaminated site were used in microcosms (maintained for approximately 600 days) to simulate a single fracture-matrix system with a chamber at the top of the core allowing advection to represent fracture flow. Diffusion-coupled degradation with and without biostimulation were evaluated and compared to crushed-rock, batch microcosms. In the diffusion-transport microcosms, lactate stimulated reductive dechlorination of TCE to cis-1,2-dichloroethene (cDCE) and sulfate reduction. Reduction of TCE to cDCE led to a higher rate of chlorinated ethene removal from the cores, likely due to higher concentration gradients, along with lower sorption and a higher diffusion coefficient for cDCE relative to TCE. Reduction of cDCE to vinyl chloride or ethene did not occur as in crushed rock microcosms, inferring an absence of Dehalococcoides in the intact cores. Abiotic transformation was evident in the core microcosms based on the appearance of acetylene and enrichment in δ13C-TCE and δ13C-cDCE. Core microcosms permit a more realistic representation of the behavior of chlorinated ethenes in water-saturated fractured porous rock by incorporating the combined influence of fracture flow and matrix diffusion on transport and transformation.

Chlorinated Ethene Degradation Rate Coefficients Simulated with Intact Sandstone Core Microcosms

“…Reductive dechlorination of TCE to cDCE but no further occurred in five of the lactate-amended microcosms and one unamended microcosm . However, no reduction to VC or ethene occurred, as in a previous crushed rock study .…”

“…Once per week, 2 mL of site groundwater (containing no detectable TCE or bromide) was injected through one valve, and the displaced groundwater was collected from the other valve into a 12 mL serum bottle, simulating clean fracture flow over the contaminated sandstone matrix. Lactate was added to the head chamber of the lactate-amended microcosms during the exchange of groundwater, at concentrations of 2 mM (days 0-196), 4 mM (days 196-307), and 5 mM (>day 307) . Samples were analyzed weekly for volatile organic compounds, bromide, sulfate, organic acids, and pH.…”

“…Equation was activated after a lag period ( T lag ) that occurred prior to the onset of biotic reductive dechlorination of TCE . An initial estimate of T lag (Table S2) was made based on the behavior of the microcosms.…”

“…However, crushing may enhance the surface area and expose fresh minerals for reactions, resulting in uncertainty when such rates are applied to simulations of real field conditions. The importance of using intact rock specimens for experiments to assess TCE degradation in diffusion dominated sedimentary rock environments has been demonstrated by Schaffer et al ,, Therefore, laboratory experiments were conducted using cylindrical specimens of intact sandstone cores from one of the TCE-contaminated zones at the site . In this paper, a numerical model that captures the effects of diffusion, transformation, and isotope fractionation is used to simulate the intact core microcosms from Yu et al Although previous studies , have determined rate coefficients for abiotic transformation of chlorinated compounds in low permeability matrices, none incorporated the kinetics of both abiotic and biotic transformation coupled with transport phenomena, or considered limitations presented by reductive capacity of the minerals.…”

A Large Undisturbed Column Method to Study Flow and Transport in Macropores and Fractured Media

“…Future research will likely address reactive transport and more complex biogeochemical systems. Further development of the LUC method will include: (1) transport with sorption and biodegradation of organic contaminants (including pesticides, pharmaceuticals, and micropollutants) in macroporous geologic formations (Rosenbom et al 2014;Yu et al 2018); (2) flow and transport in macropores and fractured media under unsaturated conditions (Mortensen et al 2004); (3) transport of inorganic contaminants and colloids (McKay et al 1993;Cohen and Weisbrod 2018;James et al 2018); (4) transport of major cations and anions with electrostatic interactions within the pore water and at the solid/solution interface (Rolle et al 2013b;Muniruzzaman et al 2014); (5) flow and transport of immiscible phases such as chlorinated solvents (Pankow and Cherry 1996;O'Hara et al 2000); (6) impact of chemical and combined physico-chemical heterogeneity (Li et al 2014;Fakhreddine et al 2016;Battistel et al 2018), and (7) microbial metabolism with gas production and determination of efflux gases (Sihota et al 2018). Experimental advances in the LUC setup will also be paralleled by numerical model development.…”