Halloran, Landon J.S.
Halloran, Landon J.S.
landon.james szasz firstname.lastname@example.org
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- PublicationAccès libreControls on the persistence of aqueous-phase groundwater contaminants in the presence of reactive back-diffusion(2020-3)
;The persistence of groundwater contaminants is influenced by several interacting processes. Physical, physico-chemical, and (bio-)chemical processes all influence the transport of contaminants in the subsurface. However,for a given hydrogeologicalsystem, itisgenerally unclear to whichdegree each of these phenomena acts as a con-trol on plume behaviour. Here, we present a comprehensive investigation of these processes and their influenceson plume behaviour and persistence in layered sedimentary systems. We investigate different scenarios that rep-resentfundamental configurationsof common contaminantsituations. A confined aquifer over- and underlain byaquitard layers is investigated in a source-removal scenario and a constant-source equilibrium scenario. Addi-tionally, an aquitard overlain and underlain by high permeability units is investigated in a source-removal sce-nario. In these investigations, we vary layer thickness, as well as parameters governing advection, (back-)diffusion, sorption, and degradation. Extensive analysis of these results enables quantification of the influenceof these parameters on maximum down-gradient concentration, plume persistence duration, and plume spatialextent. Finally, parameterspace dimensionality reductionisused to establishtrends and regimes inwhichcertainprocesses dominate as controls. A lower limit to plume extent as a function of a novel constructed parameter isalso determined. These results provide valuable quantitative information for the assessment of the fate ofgroundwater contaminants and are applicable to a wide range of aqueous-phase solutes.
- PublicationAccès libreTracking chlorinated contaminants in the subsurface usingcompound-specific chlorine isotope analysis: A review of principles,current challenges and applications(2019-11)
;Zimmermann, Jeremy ;Many chlorinated hydrocarbons have gained notoriety as persistent organic pollutants in the environ-ment. Engineered and natural remediation efforts require a monitoring tool to track the progress ofdegradation processes. Compound-specific isotope analysis (CSIA) is a robust method to evaluate theorigin and fate of contaminants in the environment and does not rely on concentration measurements.While carbon CSIA has established itself in the routine assessment of contaminated sites, studiesincorporating chlorine isotopes have only recently become more common. Although some aspects ofchlorine isotope analysis are more challenging than carbon isotope analysis, having additional isotopicdata yields valuable information for contaminated site management. This review provides an overview ofchlorine isotope fractionation of chlorinated contaminants in the subsurface by different processes andpresents analytical techniques and unresolved challenges in chlorine isotope analysis. A summary ofsuccessfulfield applications illustrates the potential of using chlorine isotope data. Finally, approaches inmodelling chlorine isotope fractionation due to degradation, diffusion, and sorption processes arediscussed.
- PublicationAccès libreModelling of C/Cl isotopic behaviour during chloroethene biotic reductive dechlorination: Capabilities and limitations of simplified and comprehensive models(2018-8)
; ;Braun, Fabian ; ;Maillard, JulienPredicting the fate of chloroethenes in groundwater is essential when evaluating remediation strategies. Such predictions are expected to be more accurate when incorporating isotopic parameters. Although secondary chlorine isotope effects have been observed during reductive dechlorination of chloroethenes, development of modelling frameworks and simulation has thus far been limited. We have developed a novel mathematical framework to simulate the C/Cl isotopic fractionation during reductive dechlorination of chloroethenes. This framework differs from the existing state of the art by incorporating secondary isotopic effects and considering both C and Cl isotopes simultaneously. A comprehensive general model (GM), which is expected to be the closest representation of reality thus far investigated, was implemented. A less computationally intensive simplified model (SM), with the potential for use in modelling of complex reactive transport scenarios, was subsequently validated based on its comparison to GM. The approach of GM considers all isotopocules (i.e. molecules differing in number and position of heavy and light isotopes) of each chloroethene as individual species, of which each is degraded at a different rate. Both models GM and SM simulated plausible C/Cl isotopic compositions of tetrachloroethene (PCE), trichloroethene (TCE) and cis-1,2-dichloroethene (cDCE) during sequential dechlorination when using experimentally relevant kinetic and isotopic parameters. The only major difference occurred in the case where different secondary isotopic effects occur at the different non-reacting positions when PCE is dechlorinated down to cDCE. This observed discrepancy stems from the unequal Cl isotope distribution in TCE that arises due to the occurrence of differential secondary Cl isotopic effects during transformation of PCE to TCE. Additionally, these models are shown to accurately reproduce experimental data obtained during reductive dechlorination by bacterial enrichments harbouring Sulfurospirillum spp. where secondary isotope effects are known to have occurred. These findings underscore a promising future for the development of reactive transport models that incorporate isotopic parameters.
- PublicationAccès libreCOMPEST, a PEST-COMSOL interface for inverse multiphysics modelling: Development and application to isotopic fractionation of groundwater contaminants(2019-5)
; ;In the geosciences, inverse problems, wherein observations corresponding to model outputs are known and model parameters are unknown, are commonplace. Many of these problems involve coupled physical, chemical, and other processes that can be modelled using forward finite-element models. Here, we present a novel interface, COMPEST, that connects the parameter estimation and uncertainty analysis package, PEST, with the finite-element modelling package, COMSOL Multiphysics. To demonstrate some of the capabilities of this approach, we also develop and present a novel model for the degradation and transport of chlorohydrocarbons in low-permeability units. This model integrates isotopic fractionation arising from degradation and diffusion. Three implementations of this model with increasing complexity are used to demonstrate the functionality of the developed interface. This linkage provides a means for parameter estimation, uncertainty analysis, and singular value decomposition to gain insight into the behaviour, identifiability, and interdependence of the various parameters in the model. COMPEST is written so as to be suited to a wide range of scientific and engineering applications and thus can be used to link any COMSOL model with PEST. This enables the use of advanced inverse modelling techniques previously unavailable to COMSOL users.