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- PublicationAccès libreAssessing the effect of chlorinated hydrocarbon degradation in aquitards on plume persistence due to back-diffusion(2018-4)
; ;Parker, Beth L.This modeling study aims to investigate how reactive processes in aquitards impact plume persistence in adjacent aquifers. For that purpose themigration of a trichloroethene (TCE) plume in an aquifer originating from dense nonaqueous phase liquid (DNAPL) source dissolution and back-diffusion from an underlying reactive aquitardwas simulated in a 2D-numericalmodel. Two aquitard degradation scenarios were modeled considering one-step degradation from TCE to cis-dichloroethene (cDCE): a uniform (constant degradation with aquitard depth) and a nonuniform scenario (decreasing degradation with aquitard depth) and were compared with a no-degradation scenario. In the no-degradation scenario, a long-term TCE tailing above the Maximum Contaminant Level (MCL) caused by back-diffusion after source removal was observed. In contrast, in the aquitard degradation scenarios, TCE backdiffusion periods were shorter, whereby the extent of back-diffusion reduction depended on the aquitard degradation depth and the rate. For high degradation rates (half-life: 30–80 days), an aquitard degradation depth greater than 65 cm prevented TCE plume persistence after source removal but generated a long-term tailing above the MCL for the produced cDCE. For slow degradation rates (half-life: b200 days), TCE was only partially degraded after source removal, independent of the aquitard degradation depth, leading to a long-term dual contamination of the aquifer by cDCE and TCE. A sudden enrichment of 13C in TCE and cDCE was observed after source removal in the uniform and non-uniform degradation scenarios that was distinct from δ13C patterns observedwhen aquifer degradation occurs (continuous enrichment of 13C along the plume axis) and forwhen there is absence of degradation (no change of isotope ratios). This demonstrates that δ13C measurements in the aquifer can be used as a diagnostic tool to demonstrate aquitard degradation, which simplifies the identification of reactive processes in aquitards, as aquifers are usually easier to monitor than aquitards.
- PublicationAccès libreDoes sorption influence isotope ratios of chlorinated hydrocarbons under field conditions(2017-7)
; ;Parker, Beth L. ;Chapman, Steven W. ;Aravena, RamonThis study aims to investigate the effect of sorption on isotope ratios of chlorinated hydrocarbons migrating through the subsurface. For this purpose concentration and isotope ratio profiles (carbon and chlorine) were determined in saturated low permeability sediments below two DNAPL sources (1,2- Dichloroethane (1,2-DCA) and Dichloromethane (DCM)). The sources had been emplaced artificially as part of a long-term, emplaced source field experiment 15.5 years (5673 days) ago. Low permeable sediments are well-suited for investigating sorption-induced isotope fractionation under field conditions. The advancing concentration front, where isotope fractionation due to sorption is expected, can be localized precisely and sampled at a high spatial resolution. Along a concentration profile below the 1,2- DCA and DCM DNAPL sources, opposite isotope trends were observed with an enrichment of heavy carbon isotopes (Dd13C ¼ 1.9‰for 1,2-DCA and 2.4‰for DCM) and a depletion of heavy chlorine isotopes (Dd37Cl ¼ 1.3‰ for 1,2-DCA). For field data interpretation laboratory experiments were conducted to determine sorption and diffusion-induced isotope fractionation factors for 1,2-DCA and DCM and included in a numerical model. When considering only diffusive isotope fractionation, numerical simulation failed to reproduce the opposite isotope trends. In contrast when sorption-induced isotope fractionation was also included, the model reproduced the data well. Hence, the observed isotope trends reflect a superposition between competing isotope effects due to sorption and diffusion. For chlorine the diffusive isotope effect is larger than for carbon due to the mass difference of two between the stable isotopes overruling the sorption effect, while for carbon the sorption effect dominates. The observed shifts of isotope ratios due to sorption are in the range of the 2‰ threshold value, which is often used for identifying reactive processes. Numerical modelling showed that under specific conditions (strong sorption behavior, early transient diffusion) even higher shifts of isotope ratios can occur. Hence, when shifts of isotope ratios in the range of 2‰ are observed under field conditions where sorption prevails, their attribution to reactive processes should be made with caution. This is especially crucial if a reactive process is slow and associated with a small isotope fractionation factor.
- PublicationAccès libreIsotope fractionation due to aqueous phase diffusion e What do diffusion models and experiments tell? e A review(2018-12)
;For the interpretation of stable isotope ratio trends in saturated geochemical systems, the magnitude of aqueous phase diffusion-induced isotope fractionation needs to be known. This study reviews how five diffusion models (Fick, Maxwell-Stefan, Einstein, Langevin, Mode-Coupling Theory Analysis (MCTA) of diffusion) predict isotope fractionation due to aqueous phase diffusion and compares them with experimental results. The reviewed diffusion models were not consistent regarding the prediction of the mass (m) dependency of the aqueous phase diffusion coefficient (D). The predictions range from a square root power law (D f m0.5) to an opposite mass dependency of D (D f mb). Experimental studies exhibited consistently a weak power law mass dependency of the diffusion coefficient (D f mb with b < 0.5) for the vast majority of dissolved species and a larger diffusion-induced isotope effect for low weight noble gases (D f m0.5). The weak power law mass dependency of D for the species other than low weight noble gases is consistent with the MCTA of diffusion. The MCTA suggests that the weak power law mass dependency of D originates from interplays between strongly mass dependent short-term and mass independent long-term solute-solvent interactions. The larger isotope fractionation for low weight noble gases could be attributed to quantum isotope effects significantly magnifying the aqueous phase diffusion-induced isotope fractionation. Our review shows, that except for low weight noble gases a weak power law mass dependency of D is likely the most adequate assumption for aqueous phase diffusioninduced isotope fractionation in geochemical systems.