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

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.

Remediation of chlorinated ethenes in fractured sandstone by natural and enhanced biotic and abiotic processes: A crushed rock microcosm study.

Biostimulation was identified as a potential technology to treat a fractured sandstone aquifer contaminated with trichloroethene (TCE) and cis-1,2-dichloroethene (cis-DCE). Most of the mass of TCE and cis-DCE resides within the rock matrix and strategies to restore groundwater to pre-existing conditions are severely limited by back diffusion. A microcosm study using crushed rock and groundwater from the site was performed to assess biostimulation and natural attenuation. Lactate, hydrogen release compound® (HRC), and emulsified vegetable oil (EVO) significantly increased the rate of TCE reduction to cis-DCE. Lactate also stimulated dechlorination of cis-DCE to vinyl chloride (VC) and ethene, suggesting the presence of indigenous Dehalococcoides. Illumina sequencing and qPCR analyses suggest that reductive dechlorination of TCE to cis-DCE is mediated by Geobacter spp. while Dehalococcoides spp. perform reduction of cis-DCE to VC and ethene. The rate of VC reduction to ethene was much slower than the reduction of TCE to cis-DCE and cis-DCE to VC, indicating the indigenous Dehalococcoides perform the final step co-metabolically. This was confirmed in enrichment cultures fed with only VC. Consequently, biostimulation may create an elevated risk due to transient accumulation of VC. Abiotic transformation of TCE and cis-DCE was observed based on accumulation of 14C-labeled products from 14C-TCE and 14C-cis-DCE, as well as enrichment in δ13C-cis-DCE in the absence of reductive dechlorination. Based on accumulation rates for 14C-products in unamended microcosms, pseudo-first-order rates for abiotic transformation were 0.038 yr-1 for TCE and 0.044 yr-1 for cis-DCE. These rates within the rock matrix may be sufficient to support natural attenuation in this diffusion controlled system.

Anaerobic abiotic transformations of cis-1,2-dichloroethene in fractured sandstone.

A fractured sandstone aquifer at an industrial site is contaminated with trichloroethene to depths greater than 244 m. Field data indicate that trichloroethene is undergoing reduction to cis-1,2-dichloroethene (cDCE); vinyl chloride and ethene are present at much lower concentrations. Transformation of cDCE by pathways other than reductive dechlorination (abiotic and/or biotic) is of interest. Pyrite, which has been linked to abiotic transformation of chlorinated ethenes, is present at varying levels in the sandstone. To evaluate the possible role of pyrite in transforming cDCE, microcosms were prepared with groundwater, ~40 mg L(-1) cDCE+[(14)C]cDCE, and crushed solids (pure pyrite, pyrite-rich sandstone, or typical sandstone). During 120 d of incubation, the highest level of cDCE transformation occurred with typical sandstone (11-14% (14)CO(2), 1-3% (14)C-soluble products), followed by pyrite-rich sandstone (2-4% (14)CO(2), 1% (14)C-soluble products) and even lesser amounts with pure pyrite. These results indicate pyrite is not likely the mineral involved in transforming cDCE. A separate experiment using only typical sandstone compared the rate of cDCE transformation in non-sterilized, autoclaved, and propylene-oxide sterilized treatments, with pseudo-first order rate constants of 8.7, 5.4, and 1.0 yr(-1), respectively; however, transformation stopped after several months of incubation. Autoclaving increased the volume of pores, adsorption pore diameter, and surface area in comparison to non-sterilized typical sandstone. Nevertheless, autoclaving was less disruptive than chemical sterilization. The results provide definitive experimental evidence that cDCE undergoes anaerobic abiotic and biotic transformation in typical sandstone, with formation of CO(2) and soluble products.