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Original article (peer-reviewed)

Journal Science of the Total Environment
Volume (Issue) 633
Page(s) 1602 - 1612
Title of proceedings Science of the Total Environment
DOI 10.1016/j.scitotenv.2018.03.192

Abstract

This modeling study aims to investigate how reactive processes in aquitards impact plume persistence in adjacent aquifers. For that purpose the migration of a trichloroethene (TCE) plume in an aquifer originating from dense non-aqueous phase liquid (DNAPL) source dissolution and back-diffusion from an underlying reactive aquitard was sim-ulated in a 2D-numerical model. Two aquitard degradation scenarios were modeled considering one-step degrada-tion from TCE to cis-dichloroethene (cDCE): a uniform (constant degradation with aquitard depth) and a non-uniform scenario (decreasing degradation with aquitard depth) and were compared with a no-degradation sce-nario. 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 back-diffusion periods were shorter, whereby the extent of back-diffusion reduction depended on the aquitard degrada-tion 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 δ13Cpatternsobservedwhenaquifer degradation occurs (continuous enrichment of 13C along the plume axis) and for when there is absence of degrada-tion (no change of isotope ratios). This demonstrates that δ13C measurements in the aquifer can be used as a diag-nostic tool to demonstrate aquitard degradation, which simplifies the identification of reactive processes in aquitards, as aquifers are usually easier to monitor than aquitards.
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