Does sorption influence isotope ratios of chlorinated hydrocarbons under field conditions
Author(s)
Parker, Beth L.
Chapman, Steven W.
Aravena, Ramon
Date issued
July 2017
In
Applied Geochemistry
No
84
From page
348
To page
359
Reviewed by peer
1
Abstract
This 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.
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.
Publication type
journal article
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