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. 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 cis-dichloroethene (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. One site was characterized as a complex hydrological and geochemical system at the groundwater-surface water interface where a tetrachloroethene- (PCE) plume discharged through the streambed. Another site was located under relatively stable hydrological conditions and characterized by a 2-km long plume 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 same approach was employed at a field site to determine the fate of cDCE. Numerical studies evaluated the field applicability of isotope-based quantification, according to the Rayleigh isotope fractionation model, to estimate the extent of degradation and the first-order reaction rate 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 evaluated with a series of 3D analytical solutions to accommodate sequential reactions.The studies demonstrated that stable carbon analysis is a robust and sensitive tool to identify the progress of reductive dechlorination of chlorinated ethenes as well as 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.

Thèse de doctorat : Université de Neuchâtel, 2007 ; Th.1970