Assessing the effect of chlorinated hydrocarbon degradation in aquitards on plume persistence due to back-diffusion
Author(s)
Date issued
April 2018
In
Science of the Total Environment
No
633
From page
1602
To page
1612
Reviewed by peer
1
Subjects
Aquifer-aquitard system Chlorinated hydrocarbons Plume persistence Back-diffusion Degradation Stable carbon isotopes
Abstract
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.
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.
Publication type
journal article
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