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  • Publication
    Accès libre
    Assessing 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.