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Hunkeler, Daniel

Nom
Hunkeler, Daniel
Affiliation principale
Fonction
Professeur ordinaire
Email
daniel.hunkeler@unine.ch
Identifiants
https://libra.unine.ch/handle/123456789/142

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  • Publication
    Accès libre
    Multiphase Transport of Tritium in Unsaturated Porous Media—Bare and Vegetated Soils
    (2012)
    Jiménez-Martínez, J
    ;
    Tamoh, K
    ;
    Candela, L
    ;
    Elorza, F.J
    ;
    Hunkeler, Daniel 
    Tritium is a short-lived radioactive isotope (T1/2 = 12.33 yr) produced naturally in the atmosphere by cosmic radiation but also released into the atmosphere and hydrosphere by nuclear activities (nuclear power stations, radioactive waste disposal). Tritium of natural or anthropogenic origin may end up in soils through tritiated rain, and may eventually appear in groundwater. Tritium in groundwater can be re-emitted to the atmosphere through the vadose zone. The tritium concentration in soil varies sharply close to the ground surface and is very sensitive to many interrelated factors like rainfall amount, evapotranspiration rate, rooting depth and water table position, rendering the modeling a rather complex task. Among many existing codes, SOLVEG is a one-dimensional numerical model to simulate multiphase transport through the unsaturated zone. Processes include tritium diffusion in both, gas and liquid phase, advection and dispersion for tritium in liquid phase, radioactive decay and equilibrium partitioning between liquid and gas phase. For its application with bare or vegetated (perennial vegetation or crops) soil surfaces and shallow or deep groundwater levels (contaminated or non-contaminated aquifer) the model has been adapted in order to include ground cover, root growth and root water uptake. The current work describes the approach and results of the modeling of a tracer test with tritiated water (7.3 × 108 Bq m−3) in a cultivated soil with an underlying 14 m deep unsaturated zone (non-contaminated). According to the simulation results, the soil’s natural attenuation process is governed by evapotranspiration and tritium reemission. The latter process is due to a tritium concentration gradient between soil air and an atmospheric boundary layer at the soil surface. Re-emission generally occurs during night time, since at day time it is coupled with the evaporation process.Evapotranspiration and re-emission removed considerable quantities of tritium and limited penetration of surface-applied tritiated water in the vadose zone to no more than ∼1–2 m. After a period of 15 months tritium background concentration in soil was attained.
  • Publication
    Accès libre
    Intrinsic biodegradation potential of aromatic hydrocarbons in an alluvial aquifer - Potentials and limits of signature metabolite analysis and two stable isotope-based techniques
    (2011)
    Morasch, Barbara
    ;
    Hunkeler, Daniel 
    ;
    Zopfi, Jakob 
    ;
    Temime, B
    ;
    Höhener, Patrick
    Three independent techniques were used to assess the biodegradation of monoaromatic hydrocarbons and low-molecular weight polyaromatic hydrocarbons in the alluvial aquifer at the site of a former cokery (Flémalle, Belgium).
    Firstly, a stable carbon isotope-based field method allowed quantifying biodegradation of monoaromatic compounds in situ and confirmed the degradation of naphthalene. No evidence could be deduced from stable isotope shifts for the intrinsic biodegradation of larger molecules such as methylnaphthalenes or acenaphthene. Secondly, using signature metabolite analysis, various intermediates of the anaerobic degradation of (poly-) aromatic and heterocyclic compounds were identified. The discovery of a novel metabolite of acenaphthene in groundwater samples permitted deeper insights into the anaerobic biodegradation of almost persistent environmental contaminants. A third method, microcosm incubations with 13C-labeled compounds under in situ-like conditions, complemented techniques one and two by providing quantitative information on contaminant biodegradation independent of molecule size and sorption properties. Thanks to stable isotope labels, the sensitivity of this method was much higher compared to classical microcosm studies. The 13C-microcosm approach allowed the determination of first-order rate constants for 13C-labeled benzene, naphthalene, or acenaphthene even in cases when degradation activities were only small. The plausibility of the third method was checked by comparing 13C-microcosm-derived rates to field-derived rates of the first approach. Further advantage of the use of 13C-labels in microcosms is that novel metabolites can be linked more easily to specific mother compounds even in complex systems. This was achieved using alluvial sediments where 13C-acenaphthyl methylsuccinate was identified as transformation product of the anaerobic degradation of acenaphthene.
  • Publication
    Accès libre
    Analytical modelling of stable isotope fractionation of volatile organic compounds in the unsaturated zone
    (Elsevier, 2011)
    Bouchard, Daniel 
    ;
    Cornaton, Fabien 
    ;
    Höhener, P.
    ;
    Hunkeler, Daniel 
    Analytical models were developed that simulate stable isotope ratios of volatile organic compounds (VOCs) near a point source contamination in the unsaturated zone. The models describe diffusive transport of VOCs, biodegradation and source ageing. The mass transport is governed by Fick's law for diffusion. The equation for reactive transport of VOCs in the soil gas phase was solved for different source geometries and for different boundary conditions. Model results were compared to experimental data from a one-dimensional laboratory column and a radial-symmetric field experiment. The comparison yielded a satisfying agreement. The model results clearly illustrate the significant isotope fractionation by gas phase diffusion under transient state conditions. This leads to an initial depletion of heavy isotopes with increasing distance from the source. The isotope evolution of the source is governed by the combined effects of isotope fractionation due to vaporisation, diffusion and biodegradation. The net effect can lead to an enrichment or depletion of the heavy isotope in the remaining organic phase, depending on the compound and element considered. Finally, the isotope evolution of molecules migrating away from the source and undergoing degradation is governed by a combined degradation and diffusion isotope effect. This suggests that, in the unsaturated zone, the interpretation of biodegradation of VOC based on isotopic data must always be based on a model combining gas phase diffusion and degradation.
  • Publication
    Accès libre
    Comments on “Analytical modelling of fringe and core biodegradation in groundwater plumes.” by Gutierrez-Neri et al. in J. Contam. Hydrol. 107: 1–9
    (2010)
    Hunkeler, Daniel 
    ;
    Höhener, P
    ;
    Atteia, O
    In this comment, we revisit equations concerning the analytical solutions presented by Gutierrez-Neri and co-workers for reactive transport for a pollutant undergoing core and fringe degradations. We state that a correction needs to be made in Eq. (9) of the work of Gutierrez- Neri et al. in order that the equation follows closely previous work published by J. Bear (in 1-D) and P.A. Domenico (in 3-D). Furthermore we derive alternative solutions for Eqs. (13)–(16) which separate more clearly the first-order reaction and the instantaneous reaction. It is shown that the corrected solution agrees better with the results from the numerical model than the previous solution. An improvement is also made by giving a solution which avoids negative concentrations. Furthermore, the corresponding solution for the electron acceptor reacting with the pollutant is given.
  • Publication
    Accès libre
    Quantification of biodegradation for o-xylene and naphthalene using first order decay models, Michaelis–Menten kinetics and stable carbon isotopes
    (2009)
    Blum, P.
    ;
    Hunkeler, Daniel 
    ;
    Weede, M
    ;
    Beyer, C
    ;
    Grathwohl, P
    ;
    Morasch, Barbara
    At a former wood preservation plant severely contaminated with coal tar oil, in situ bulk attenuation and biodegradation rate constants for several monoaromatic (BTEX) and polyaromatic hydrocarbons (PAH) were determined using (1) classical first order decay models, (2) Michaelis–Menten degradation kinetics (MM), and (3) stable carbon isotopes, for o-xylene and naphthalene. The first order bulk attenuation rate constant for o-xylene was calculated to be 0.0025 d−1 and a novel stable isotope-based first order model, which also accounted for the respective redox conditions, resulted in a slightly smaller biodegradation rate constant of 0.0019 d−1. Based on MM-kinetics, the o-xylene concentration decreased with a maximum rate of kmax=0.1 μg/L/d. The bulk attenuation rate constant of naphthalene retrieved from the classical first order decay model was 0.0038 d− 1. The stable isotope-based biodegradation rate constant of 0.0027 d−1 was smaller in the reduced zone, while residual naphthalene in the oxic part of the plume further downgradient was degraded at a higher rate of 0.0038 d−1. With MM-kinetics a maximum degradation rate of kmax=12 μg/L/d was determined. Although best fits were obtained by MM-kinetics, we consider the carbon stable isotope-based approach more appropriate as it is specific for biodegradation (not overall attenuation) and at the same time accounts for the dominant electron-accepting process. For o-xylene a field based isotope enrichment factor εfield of −1.4 could be determined using the Rayleigh model, which closely matched values from laboratory studies of o-xylene degradation under sulfate-reducing conditions.
  • Publication
    Accès libre
    Groundwater–surface water interaction and its role on TCE groundwater plume attenuation
    (2007)
    Chapman, Steven W.
    ;
    Parker, Beth L.
    ;
    Cherry, John A.
    ;
    Aravena, Ramon
    ;
    Hunkeler, Daniel 
    A field investigation of a TCE plume in a surficial sand aquifer shows that groundwater–surface water interactions strongly influence apparent plume attenuation. At the site, a former industrial facility in Connecticut, depth-discrete monitoring along three cross-sections (transects) perpendicular to groundwater flow shows a persistent VOC plume extending 700 m from the DNAPL source zone to a mid-size river. Maximum TCE concentrations along a transect 280 m from the source were in the 1000s of μg/L with minimal degradation products. Beyond this, the land surface drops abruptly to a lower terrace where a shallow pond and small streams occur. Two transects along the lower terrace, one midway between the facility and river just downgradient of the pond and one along the edge of the river, give the appearance that the plume has strongly attenuated. At the river, maximum TCE concentrations in the 10s of μg/L and similar levels of its degradation product cis-DCE show direct plume discharge from groundwater to the river is negligible. Although degradation plays a role in the strong plume attenuation, the major attenuation factor is partial groundwater plume discharge to surface water (i.e. the pond and small streams), where some mass loss occurs via water–air exchange. Groundwater and stream mass discharge estimates show that more than half of the plume mass discharge crossing the first transect, before surface water interactions occur, reaches the river directly via streamflow, although river concentrations were below detection due to dilution. This study shows that groundwater and surface water concentration measurements together provide greater confidence in identifying and quantifying natural attenuation processes at this site, rather than groundwater measurements alone.
  • Publication
    Accès libre
    Use of stable isotopoe analyses to assess natural attenuation of chlorinated ethenes in groundwater
    (2007)
    Abe, Yumiko
    ;
    Hunkeler, Daniel 
    Chlorinated ethenes are among the most frequently detected groundwater contaminants in industrially developed countries. Although their degradation pathways have been understood, assessing the progress of natural attenuation of chlorinated ethenes still remains a challenge. This thesis explored the possibilities and limits of the use of compound-specific stable isotope analysis as a field investigation tool to document and to quantify the progress of natural attenuation of chlorinated ethenes. The thesis focused particularly on the fate of reductive dechlorination intermediates such as cis-dichloroethene (cDCE) and vinyl chloride (VC) as the success of natural attenuation of chlorinated ethenes depends principally on the fate of these compounds. Two field investigations were carried out to demonstrate the use of stable isotope analysis to characterize the progress of natural attenuation of chlorinated ethenes. One site was characterized as a complex hydrological and geochemical system at the groundwater-surface water interface where a tetrachloroethene- (PCE) plume discharged through the streambed. Another site was located under relatively stable hydrological conditions and characterized by a 2-km long plume of chlorinated ethenes. Laboratory studies evaluated the use of combined carbon and chlorine isotope analysis to distinguish different degradation pathways of cDCE and VC, and the same approach was employed at a field site to determine the fate of cDCE. Numerical studies evaluated the field applicability of isotope-based quantification, according to the Rayleigh isotope fractionation model, to estimate the extent of degradation and the first-order reaction rate under various flow conditions. In addition, a scheme to quantify the reductive dechlorination rates of chlorinated ethenes based on field concentration and isotope data at one of the field sites was proposed and evaluated with a series of 3D analytical solutions to accommodate sequential reactions.The studies demonstrated that stable carbon analysis is a robust and sensitive tool to identify the progress of reductive dechlorination of chlorinated ethenes as well as the possibility of other degradation pathways of intermediate compounds even under geochemically and hydrologically complex conditions and even if only a small level of degradation occurs. Although carbon isotope analysis alone can not conclusively determine the fate of degradation intermediates such as cDCE and VC, the use of combined chlorine and carbon isotope analysis can assist in elucidating their fate at field scales. Furthermore, based on the field isotope data, it is possible to estimate the degradation rate even for a complex reaction series such as sequential reductive dechlorination of chlorinated ethenes. And the accuracy of the rate quantification increases with increased knowledge of local flow conditions.
  • Publication
    Accès libre
    Benzene dispersion and natural attenuation in an alluvial aquifer with strong interactions with surface water
    Batlle-Aguilar, Jordi
    ;
    Brouyère, Serge
    ;
    Dassargues, Alain
    ;
    Morasch, Barbara
    ;
    Hunkeler, Daniel 
    ;
    Höhener, Patrick
    ;
    Diels, Ludo
    ;
    Vanbroekhoven, Karolien
    ;
    Seuntjens, Piet
    ;
    Halend, Henri
    Field and laboratory investigations have been conducted at a former coke plant, in order to assess pollutant attenuation in a contaminated alluvial aquifer, discharging to an adjacent river. Various organic (BTEX, PAHs, mineral oils) and inorganic (As, Zn, Cd) compounds were found in the aquifer in concentrations exceeding regulatory values. Due to redox conditions of the aquifer, heavy metals were almost immobile, thus not posing a major risk of dispersion off-site the brownfield. Field and laboratory investigations demonstrated that benzene, among organic pollutants, presented the major worry for off-site dispersion, mainly due to its mobility and high concentration, i.e. up to 750 mg L-1 in the source zone. However, benzene could never be detected near the river which is about 160 m downgradient the main source. Redox conditions together with benzene concentrations determined in the aquifer have suggested that degradation mainly occurred within 100 m distance from the contaminant source under anoxic conditions, and most probably with sulphate as main oxidant. A numerical groundwater flow and transport model, calibrated under transient conditions, was used to simulate benzene attenuation in the alluvial aquifer towards the Meuse River. The mean benzene degradation rate used in the model was quantified in situ along the groundwater flow path using compound-specific carbon isotope analysis (CSIA). The results of the solute transport simulations confirmed that benzene concentrations decreased almost five orders of magnitude 70 m downgradient the source. Simulated concentrations have been found to be below the detection limit in the zone adjacent to the river and consistent with the absence of benzene in downgradient piezometers located close to the river reported in groundwater sampling campaigns. In a transient model scenario including groundwater–surface water dynamics, benzene concentrations were observed to be inversely correlated to the river water levels, leading to the hypothesis that benzene dispersion is mainly controlled by natural attenuation.