Use of stable isotope analyses to assess natural attenuation of chlorinated ethenes in groundwater

Chlorinated ethenes are among the most frequently detected groundwater contaminants in industrially developed countries. Although their degradation pathways have been understood, assessing the progress of natural attenuation of chlorinated ethenes still remains a challenge. Chlorinated ethenes undergo sequential reductive dechlorination under anoxic conditions; hence, the presence of less chlorinated transformation intermediates indicates the transformation of more chlorinated parent compound. However, the transformation intermediates can be degraded by other pathways such as aerobic and anaerobic oxidation and abiotic reduction which make it difficult to identify the fate of these intermediates. This thesis explored the possibilities and limits of the use of compound-specific stable isotope analysis as a field investigation tool to document and to quantify the progress of natural attenuation of chlorinated ethenes. The thesis focused particularly on the fate of reductive dechlorination intermediates such as cisdichloroethene (cDCE) and vinyl chloride (VC) as the success of natural attenuation of chlorinated ethenes depends principally on the fate of these compounds. Two field investigations were carried out to demonstrate the use of stable isotope analysis to characterize the progress of natural attenuation of chlorinated ethenes. Laboratory studies evaluated the use of combined carbon and chlorine isotope analysis to distinguish different degradation pathways of cDCE and VC, and the approach was employed at a field site to determine the fate of cDCE. Numerical studies evaluated the field applicability of isotopebased quantification to estimate the extent of degradation and the first-order reaction rate based on the Rayleigh equation under various flow conditions. In addition, a scheme to quantify the reductive dechlorination rates of chlorinated ethenes based on field concentration and isotope data at one of the field sites was proposed and studied numerically. Field investigations took place at a site where a tetrachloroethene(PCE) plume discharged through the streambed accompanied by high levels of hydrological and geochemical variability (Angus site) and at another site characterized by a 2-km long plume of chlorinated ethenes (Rødekro site). At the Angus site, sampling was carried out at the discharging area in the streambed. Spatially detailed sampling allowed the identification of various redox and hydrological conditions. The progress of reductive dechlorination of chlorinated ethenes at each sampling location was determined by carbon isotope analysis. The approach is based on the principle that the carbon isotope ratio of an accumulating intermediate progressively approaches that of the parent compound because all carbons in the ethene backbone are preserved during the course of reductive dechlorination. The carbon isotope analysis suggested that the site consisted of locations with various degrees of reductive dechlorination ranging from insignificant to complete dechlorination. At locations where the accumulation of an intermediate was observed, the carbon isotope ratio of the accumulating intermediate remained stable at that of the parent compound; thus, the observed concentration decrease was caused solely by the effect of dispersion. At some locations, the carbon isotope ratio of ethene, the final product of reductive dechlorination, became more enriched than that of the original PCE indicating further transformation of ethene likely to ethane. The conclusions drawn from carbon isotope analysis were compared to the information provided by other field indicators for complete reductive dechlorination (strong redox conditions and the presence of Dehalococcoides). Complete reductive dechlorination was observed under sulfate-reducing and methanogenic conditions. However under sulfate-reducing conditions cDCE accumulation also took place at different locations. Different progress of reductive dechlorination under sulfate-reducing conditions was caused primarily by the difference in sediment compositions. Relatively permeable streambed gives rise to a faster discharge rate, hence a shorter residence time of the water which resulted in incomplete reductive dechlorination to cDCE. On the other hand, complete reductive dechlorination was observed at a location with relatively impermeable streambed. Therefore, the progress of reductive dechlorination is not only driven by the local redox conditions, but also by the local hydrological conditions. The presence of Dehalococcoides based on taxon-specific PCR analysis was confirmed at locations under methanogenic conditions which carried out complete reductive dechlorination and the further reduction of ethene to ethane, but Dehalococcoides was not detected at a location under sulfate-reducing conditions with ethene as a final degradation product. At the Rødekro site, sampling was carried out along the plume centerline at multiple depths. The concentration analysis of chlorinated ethenes suggested cDCE accumulation. However, under the observed redox conditions (manganese-reducing), both reductive dechlorination and anaerobic oxidation of cDCE could take place. The conservation of carbons in the ethene backbone during reductive dechlorination implies that the combined carbon isotope ratios of all chlorinated ethenes and ethene remain constant as long as reductive dechlorination is the only active transformation pathway. Therefore, carbon isotope analysis of chlorinated ethenes can provide valuable information to indicate whether reductive dechlorination is the only degradation pathway or the intermediates are degraded by other degradation pathways such as oxidative degradation which involves with the loss of carbons, hence enrichment in combined carbon isotope ratios. At the Rødekro site, the combined carbon isotope ratio remained constant at the majority of the plume, suggesting that reductive dechlorination was the only transformation pathway. However at the plume front, the combined ratio deviated from that of the source value indicating the possibilities of either anaerobic oxidation of intermediates (cDCE or VC) or further reductive dechlorination to ethene (even though it was not detected at the site). Based on the low concentrations of VC and the isotope mass balance calculations, anaerobic oxidation of VC could not possibly give rise to the observed deviation in the combined carbon ratio. Therefore, it was postulated that either anaerobic oxidation of cDCE or complete reductive dechlorination to a non-detected end product took place at the plume front. Although carbon isotope analysis can indicate the progress of reductive dechlorination and the possibility of alternative degradation pathways, it can not identify the responsible degradation mechanism at field scales. Therefore, the use of combined carbon and chlorine isotope analysis was evaluated to distinguish different degradation pathways. Because different pathways involve with breaking of different bonds, it was expected that the ratio between carbon and chlorine isotope fractionation would be different for different pathways. Laboratory studies revealed that aerobic oxidation of VC was accompanied by significantly smaller chlorine isotope fractionation compared to that for reductive dechlorination of VC and cDCE. This difference is caused by the absence of the cleavage of a carbon-chlorine bond in the initial step of aerobic oxidation of VC. As a result, the ratio between the chlorine and carbon isotope fractionation is smaller for aerobic oxidation compared to reductive dechlorination. The dual isotope approach was tested at the Rødekro site to determine the fate of accumulating cDCE. A strong shift in chlorine isotope ratios (6‰) which were highly correlated with a shift in carbon isotope ratios (14‰) was observed, indicting that cDCE was transformed by reductive dechlorination. Therefore, at the Rødekro site, reductive dechlorination was the responsible degradation pathway. Field isotope data are often used to estimate the extent of degradation and the first-order degradation rate based on the Rayleigh isotope fractionation equation. The premise of the Rayleigh-based quantification method is that groundwater travels at a constant velocity corresponding to a uniform residence time distribution much like plug-flow. However, groundwater residence time varies due to the effect of dispersion caused by the subsurface heterogeneity. In order to evaluate the field applicability of the Rayleigh-based quantification method, the effect of residence time variability was evaluated using an analytical 2D solution to the advection-dispersion equation. Residence time variability was represented by varying the dispersion coefficient. The studies revealed that the Rayleigh-based quantification methods generally underestimate the extent of degradation and the degradation rate. The magnitude of the underestimation increases with the increasing dominance of dispersion over advection in flow conditions. Therefore, the effect of dispersion needs to be considered for the Rayleigh-based quantification methods. If the flow and transport parameters such as groundwater velocity and dispersion coefficients can be estimated at a site of interest, the degree of underestimation can be corrected based on the systematic typecurves presented in the thesis. Due to the simultaneous formation and transformation of intermediate compounds, the Rayleigh-based quantification methods do not apply to reductive dechlorination of chlorinated ethenes. Therefore, a scheme to quantify the dechlorination rate during reductive dechlorination of chlorinated ethenes using a series of 3D analytical solutions which accommodate consecutive reactions was developed. The conservation of carbons in the ethene backbone during reductive dechlorination has an important implication that the summed molar concentrations of all chlorinated ethenes and ethene can be considered as a quasi-conservative tracer as long as reductive dechlorination is the only transformation pathway. Since isotope analyses confirmed that reductive dechlorination is the responsible pathway at the Rødekro site, the concentration and isotope data at the Rødekro site were used to quantify the dechlorination rate of each chlorinated ethene species. Flow and transport model was calibrated to the summed concentrations of chlorinated ethenes. Based on the calibrated transport model, the dechlorination rates were estimated based on the carbon isotope and the concentration data separately for comparison. The concentrationbased quantification method tends to yield greater transformation rates than the isotope-based method especially when the contaminant concentrations are low. However, the difference between the two methods is relatively small by a factor of four, suggesting that once effective flow and transport parameters are obtained, relatively stable transformation rates can be estimated. Parameter sensitivity analyses revealed that the variation in groundwater velocity is directly reflected in the quantified transformation rate; thus, it plays a significant role in quantifying the rate. In conclusion, the studies demonstrated that stable carbon analysis is a robust and sensitive tool to identify the progress of reductive dechlorination of chlorinated ethenes and the possibility of other degradation pathways of intermediate compounds even under geochemically and hydrologically complex conditions and even if only a small level of degradation occurs. Although carbon isotope analysis alone can not conclusively determine the fate of degradation intermediates such as cDCE and VC, the use of combined chlorine and carbon isotope analysis can assist in elucidating their fate at field scales. Furthermore, based on the field isotope data, it is possible to estimate the degradation rate even for a complex reaction series such as sequential reductive dechlorination of chlorinated ethenes. And the accuracy of the rate quantification increases with increased knowledge of local flow conditions.