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| Titel |
I{ Relationship between source clean up and mass flux of chlorinated solvents in low permeability settings with fractures} |
| VerfasserIn |
P. L. Bjerg, J. C. Chambon, C. M. Christiansen, M. M. Broholm, P. J. Binning |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250021364
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| Zusammenfassung |
| Groundwater contamination by chlorinated solvents, such as perchloroethylene (PCE), often
occurs via leaching from complex sources located in low permeability sediments such as
clayey tills overlying aquifers. Clayey tills are mostly fractured, and contamination migrating
through the fractures spreads to the low permeability matrix by diffusion. This results in a
long term source of contamination due to back-diffusion. Leaching from such sources is
further complicated by microbial degradation under anaerobic conditions to sequentially
form the daughter products trichloroethylene, cis-dichloroethylene (cis-DCE), vinyl
chloride (VC) and ethene. This process can be enhanced by addition of electron
donors and/or bioaugmentation and is termed Enhanced Reductive Dechlorination
(ERD).
This work aims to improve our understanding of the physical, chemical and microbial
processes governing source behaviour under natural and enhanced conditions. That
understanding is applied to risk assessment, and to determine the relationship and time
frames of source clean up and plume response. To meet that aim, field and laboratory
observations are coupled to state of the art models incorporating new insights of contaminant
behaviour.
The long term leaching of chlorinated ethenes from clay aquitards is currently being
monitored at a number of Danish sites. The observed data is simulated using a
coupled fracture flow and clay matrix diffusion model. Sequential degradation is
represented by modified Monod kinetics accounting for competitive inhibition between
the chlorinated ethenes. The model is constructed using Comsol Multiphysics, a
generic finite- element partial differential equation solver. The model is applied
at well characterised field sites with respect to hydrogeology, fracture network,
contaminant distribution and microbial processes (lab and field experiments). At
one of the study sites (Sortebrovej), the source areas are situated in a clayey till
with fractures and interbedded sand lenses. The site is highly contaminated with
chlorinated ethenes which impact the underlying sand aquifer. Full scale remediation
using ERD was implemented at Sortebrovej in 2006. Anaerobic dechlorination
is taking place, and cis-DCE and VC have been found in significant amounts in
monitoring wells and to some degree in sediment cores representing the the clayey till
matrix.
Model results reveal several interesting findings. The physical processes of
matrix diffusion and advection in the fractures seem to be more important than the
microbial degradation processes for estimation of the time frames and the distance
between fractures is amongst the most sensitive model parameters. However, the
inclusion of sequential degradation is crucial to determining the composition of
contamination leaching into the underlying aquifer. Degradation products like VC
will peak at an earlier stage compared to the mother compound due to a higher
mobility. These model results are supported by actual findings at the Sortebrovej
site.
The findings highlight a need for improved characterization of low permeability aquitards
lying above aquifers used for water supply. The fracture network in aquitards is currently
poorly described at larger depths (below 5-8 m) and the effect of sand lenses on
leaching behaviour is not well understood. The microbial processes are assumed to
be taking place in the fracture system, but the interaction with and processes in
the matrix need to be further explored. Development of new methods for field site
characterisation and integrated field and model expertise are crucial for the design of
remedial actions and for risk assessment of contaminated sites in low permeability settings. |
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