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A fractured sandstone aquifer at an industrial site in southern California is contaminated with trichloroethene (TCE) and cis-1,2-dichloroethene (cis-DCE) to depths in excess of 244 m. Field monitoring data suggest that TCE is undergoing reduction to cis-DCE and that additional attenuation is occurring. However, vinyl chloride (VC) and ethene have not been detected in significant amounts, so that if transformation is occurring, a process other than reductive dechlorination must be responsible. The objective of this study was to evaluate the occurrence of biotic and abiotic transformation processes at this site for TCE, cis-DCE, and VC. Anaerobic microcosms were constructed with site groundwater and sandstone core samples. 14C-labeled compounds were used to detect transformation products (e.g., CO2 and soluble products) that are not readily identifiable by headspace analysis. The microcosms confirmed the occurrence of biotic reduction of TCE to cis-DCE, driven by electron donor in the groundwater and/or sandstone. VC and ethene were not detected. Following incubation periods up to 22 months, the distribution of 14C indicated statistically significant transformation of [14C]TCE and [14C]cis-DCE in live microcosms, to as high as 10% 14CO2 from TCE and 20% 14CO2 from cis-DCE. In autoclaved microcosms, significant transformation of [14C]TCE and [14C]cis-DCE also occurred; although some 14CO2 accumulated, the predominant 14C product was soluble and could not be stripped by N2 from an acidic solution (referred to as nonstrippable residue, or NSR). Characterization of the NSR by high-performance liquid and ion chromatography identified glycolate, acetate, and formate as significant components. These results suggest that a combination of abiotic and biotic transformation processes is responsible for attenuation of TCE and cis-DCE in the fractured sandstone aquifer. Tracking the distribution of 14C during the microcosm study was essential for observing these phenomena.

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.

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