Laboratory evaluation of chlorinated ethene transformation processes in fractured sandstone

A fractured sandstone aquifer at an industrial site in southern California is contaminated with trichloroethene (TCE) 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 overall 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 during this part of the study. 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% 14CO 2 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 non-strippable residue, or NSR). Characterization of the NSR by high performance liquid and ion chromatography identified glycolate, acetate and formate as significant components.; The site contains minerals typical of what is found in sandstone for this region (i.e., iron sulfides, pyrite, fougerite (green rust), magnetite, biotite, vermiculite, and quartz) suggesting that these minerals may play a role in the abiotic transformations observed during the microcosm study. In order to evaluate the role of sandstone composition on the rate and extent of cis-DCE transformation, further experiments were conducted with autoclaved typical and pyrite rich sandstone from the site, as well as with pure pyrite. Since reductive dechlorination was a predominant transformation pathway in the microcosms for TCE but not for cis-DCE, these additional studies were conducted with [14C]cis-DCE. The results suggest that pyrite is not responsible for abiotic transformation of cis-DCE. By contrast, the autoclaved typical sandstone was able to transform as much as 16% of the [14C]cis -DCE to [14C]NSR and [14C]CO2 .; During the microcosm study, the extent of cis-DCE transformation to NSR and CO2 appeared to be greater in the autoclaved versus live treatments. Subsequent experiments were conducted to further test the effect of sterilization method on the rate and extent of abiotic transformation of cis-DCE in the presence of typical sandstone and groundwater. Autoclaving was compared to use of propylene oxide as a method of sterilization, both of which were compared to live microcosms. Formation of [14C]NSR and 14CO2 from [14C]cis-DCE was confirmed. Surface area normalized first order rates of cis-DCE transformation for typical sandstone were 1.08E-05+/-1.1E-06 L/m2d for the autoclaved treatment, 1.31E-05+/-2.8E-06 L/m2d for the live treatment, and 1.00E-06+/-3E-09 for the propylene oxide treatment. Based on XPS analysis of sandstone from the different treatments, autoclaving appears to have increased the availability of magnetite and goethite for the abiotic transformations. In the live sandstone, ferrous (FeO) and ferric oxides (Fe2O3) were present and may be responsible for at least a part of the cis-DCE transformation. Sterilizing with propylene oxide appeared to inhibit transformation of cis-DCE to NSR and CO2. Minor amounts of VC, acetylene, ethene and ethane were formed in both the live and propylene oxide treated sandstone. Detection...