Résultat de la recherche
Lithological and tectonic control on groundwater contribution to stream discharge during low-flow conditions
Knowing how stream discharge in an ungauged catchment reacts to dry spells is a major challenge for managing water resources. The role of geology on these dynamics is poorly understood. For the Swiss Molasse basin, we therefore explored how the geology influences the groundwater contribution to stream flow during low-flow conditions. Using existing data from geological reports and maps as well as from deep boreholes, we constructed a basin-wide overview of the hydrogeological quality of the bedrock and investigated five catchments in 3D. We found that catchments with the most permeable sedimentary bedrock are least sensitive to low flows (marine sandstone, K = 10−4 to 10−5 m/s, Peff = 5–10%). In contrast, if bedrock K is low (K < 10−6 m/s), the presence of a productive Quaternary volume becomes decisive for groundwater contribution to stream flow. Limitations exist due to a restricted database for K and Peff values of the Molasse and limited information on continuation of lithologies with depth. This emphasizes the need for more hydrogeologically relevant data for the future management of water resources. Our results highlighting what lithotypes favor groundwater contribution to stream flow are valid also in other regions for the assessment of a catchment’s sensitivity to low flows
Application of carbon-chlorine isotopic analysis to determine the origin and fate of chlorinated ethenes in groundwater
Chlorinated ethenes are ubiquitous groundwater contaminants posing a threat to one of our main drinking water sources. Despite their spill history dating back to more than 40 years ago, these contaminants are still found in groundwater in numerous industrially developed countries due to their persistence, difficult characterisation in the subsurface and resulting challenging remediation. When adequate redox conditions, microbial communities and/or minerals are present, these compounds are known to undergo natural degradation. Applying natural attenuation as a management strategy is thus being increasingly considered as it constitutes a cost-effective environmental friendly approach. Tools enabling to differentiate degradation pathways, predict the fate of contaminants as well as understand the mechanisms underlying their degradation thus constitute the key to a better management of chlorinated ethenes contaminated sites. Methods allowing for contaminant source tracking are also of interest in a legal context where the contamination precursor is searched for. Among the various tools applied to address such questions, compound specific isotopic analysis (CSIA) – which consists in measuring the ratio between light and heavy stable isotopes of one element (i.e. isotopic composition) of a compound – is being increasingly applied.
This thesis was aimed at exploring the benefits and limits of applying CSIA to substantiate the origin and fate of chlorinated ethenes in groundwater. For this purpose, a first field study aimed at investigating the performance of dual Carbon-Chlorine (C-Cl) isotopic analysis for contaminant source discrimination was carried out. Laboratory experiments were then performed in view of getting more insight in the reaction mechanisms underlying tetrachloroethylene (PCE) reductive dechlorination and to explore the potential of dual C-Cl isotopic analysis to differentiate degradation pathways. A mathematical model was further developed to comprehensively simulate chlorinated ethenes C and Cl isotopic evolution during sequential dechlorination. Simulations were compared to experimental data in order to evaluate this model in its ability to reproduce and thus predict real data. Finally, the contribution of C and Cl isotopic analysis to identify changes in redox processes further affecting chlorinated ethenes in groundwater was challenged when assessing the effect of source thermal remediation by steam injection on a chlorinated ethenes plume.
For regulatory reasons, determining the contamination perpetrator is often of interest. As the isotopic signature of solvents produced from different manufacturers showed a large variability, CSIA was suggested as a method to discriminate the origin of contamination between different suspected sources by comparing their isotopic signatures. Such application however relies on the assumption that isotopic signatures will also differ in the field. Our first goal was thus to determine the source isotopic variability of PCE at a country scale. For this purpose, the C and Cl isotopic composition of PCE found in groundwater underlying 10 contaminated sites located in Switzerland was compared to the so far reported isotopic signatures of PCE produced by different manufacturers. It was shown that such variability was less important between the 10 sites than between PCE from different manufacturers (i.e. -26.0 to -23.7 ‰ for C and -0.5 to 0.6 ‰ for Cl in Switzerland and -37.4 to -23.2 ‰ for C and -4.4 to 1.2 ‰ for Cl in PCE from manufacturers). Additionally, some sites could be differentiated based on their isotopic signatures while others could not. The successful application of CSIA therefore largely depends on cases.
Once chlorinated ethenes have been detected in groundwater, it may be of interest to determine whether they are being naturally degraded or not, as this will influence the site management choice (e.g. application of monitored natural attenuation). Chlorinated ethenes are typically known to undergo sequential biotic reductive dechlorination in strictly anoxic conditions (i.e. PCE → trichloroethylene (TCE) → cis-dichloroethylene (cDCE) → vinyl chloride (VC) → ethene). However, the exact reaction mechanism underlying each step of reductive dechlorination remains at the stage of hypothesis where three different reaction mechanisms have so far been proposed.
As molecules containing light isotopes are generally degraded faster than molecules containing heavy isotopes due to energetic reasons, the isotopic composition of chlorinated ethenes is bound to vary during their sequential degradation. CSIA has thus naturally been proposed as a tool to track the biochemical reactions affecting chlorinated ethenes during their degradation as different processes differently affect their isotopic composition. More specifically, rate-limiting steps control the extent of isotopic enrichment during the course of biotransformation. Rate-limiting steps occurring during substrate-enzyme interactions are yet expected to equally affect both elements since such interactions are not bond-specific contrary to the purely chemical degradation reaction which involves a bond breakage. It was hence suggested that simultaneously considering the isotopic composition of two elements of a compound undergoing degradation via the dual C-Cl isotope slope associated with this compound strictly reflected the chemical reaction underlying this compound degradation contrary to single element isotopic data.
In view of getting more insight into the reaction mechanisms underlying reductive dechlorination of chlorinated ethenes, we studied the C and Cl isotopic evolution of PCE and TCE during their reductive dechlorination by two bacterial consortia (SL2-PCEc and SL2-PCEb) harbouring members of Sulfurospirillum spp. These consortia were specific in that they showed a different dechlorination pattern: SL2-PCEb was able to dechlorinate PCE or TCE until cDCE whereas SL2-PCEc only dechlorinated until TCE. Contrary to what was expected, two significantly different dual C-Cl isotope slopes of 2.7 ± 0.3 and 0.7 ± 0.2 associated with PCE reductive dechlorination were determined depending on the bacterial consortia. Such variability was attributed to the existence of two different reaction mechanisms underlying this reaction, under the assumption that dual C-Cl isotope slopes strictly reflect the chemical reaction. Two dual C-Cl isotope slopes associated with PCE reductive dechlorination in two field sites where each slope corresponded to one experimentally determined slope were also determined. This further supported the existence of two unique slopes and constituted another argument in favour of their corresponding to two different reaction mechanisms. It was moreover demonstrated that phylogenetically close bacteria could yield different C-Cl isotope slopes. The apparent kinetic isotope effect (AKIE) reflects the difference in reaction rates involving molecules containing light versus heavy isotopes of one element after correcting for non-reacting positions. Primary isotopic effects affect atoms located in reacting position as opposed to secondary isotopic effects which affect atoms located in non-reacting positions. Based on AKIEs calculations where secondary Cl isotopic effects were neglected, we furthermore suggested that one consortium (SL2-PCEc) more likely involved an electron-transfer or nucleophilic substitution as a first step of reaction mechanism than a nucleophilic addition. Comparing calculated AKIEs to the maximum theoretical kinetic isotope effects (or “semiclassical Streitwieser limits”) associated with C-Cl bond breakage suggested that either the primary Cl isotope effect was larger than the kinetic isotope effect given by the Streitwieser limit, or that a secondary Cl isotope effect occurred.
The Cl isotopic composition of TCE produced by PCE reductive dechlorination was further studied in order to explore in more details the possibility that secondary Cl isotope effects occur. A 1.4 ± 0.2 ‰ to 3.1 ± 0.6 ‰ lighter TCE than PCE at the beginning of the reaction indicated the presence of an inverse secondary effect or at least a difference of -10.6 ± 1 ‰ to -15.9 ± 2.8 ‰ between primary and secondary Cl isotopic effects.
In order reliably predict a chlorinated ethenes plume fate based on a modelling approach considering isotopic data, isotopic effects should be incorporated in a more comprehensive way than in the models so far proposed. A mathematical model aimed at simulating the evolution of C and Cl isotopic composition during sequential reductive dechlorination was thus developed where secondary isotopic effects were taken into account. So that the model reflects effectively occurring processes, Monod kinetics instead of first order kinetics were additionally considered. The rationale behind the approach consisted in considering all isotopocules (i.e. molecules differing in number and position of heavy and light isotopes) of each chlorinated ethene as individual species which were each degraded at different speed depending on the number and position of heavy and light isotopes in the isotopocule. Such difference in degradation rate between isotopocules was described by a matrix containing kinetic isotopocule fractionation factors. The definition of the latter is similar to that of the commonly used kinetic fractionation factor α which corresponds to the ratio between the degradation rate of heavy and light isotopes of a compound. More specifically, one comprehensive model (GM) considering C and Cl isotopes simultaneously was distinguished from a simplified model (SM) where C and Cl were considered separately. Both models almost identically simulated realistic C and Cl isotopic compositions of PCE, TCE and cDCE during sequential dechlorination when using experimentally plausible kinetic and isotopic parameters. They could additionally accurately reproduce our experimental data, leaving a promising future for the development of an integrative reactive transport model incorporating isotopic parameters. It also documented the slight impact of having different Cl secondary isotopic effects as well as the small effect induced by an unequal Cl isotopes distribution between positions of an asymmetric molecule on the produced compound Cl isotopic composition.
Finally, field investigations were performed at a site located in Denmark which was explored in a previous work, prior to source thermal remediation. C and Cl isotopic analysis of chlorinated ethenes from groundwater samples taken along the plume centreline were used to verify and improve the interpretation of redox and chlorinated ethenes concentration data. Dual C-Cl isotope slopes associated with PCE and TCE in the first part of the plume were similar to experimentally determined slopes during biotic reductive dechlorination. Based on the assumption that dual C-Cl isotope slopes directly reflect degradation pathways, it was suggested that steam injection enhanced PCE and TCE biotic reductive dechlorination in the first part of the plume. This was in agreement with the occurrence of more reducing conditions resulting from the release of organic matter likely triggered by the thermal remediation. On the other hand, we suggested based on the dual isotope slope approach that cDCE was probably primarily abiotically degraded by pyrite in the downstream part of the plume before and after the remediation event. This differed from the original postulation which suggested the occurrence of either cDCE anaerobic oxidation or complete reductive dechlorination prior to remediation. Such different conclusion could be drawn based on newly reported dual isotope slopes associated with cDCE abiotic degradation which were not available at the time of the study preceding source remediation. In the middle of the plume, a cDCE C isotopic composition lighter than the estimated initial one for PCE C documented the occurrence of further cDCE degradation despite the very low VC concentrations. On the contrary, a cDCE C isotopic composition equal to that of the initially released PCE indicated the absence of or only little further cDCE degradation at the plume front. Such conclusion was in agreement with the observed plume expansion documented by larger concentration contours in the second campaign than in the first.
To sum up, this thesis reveals that dual C-Cl isotopic analysis should be applied with caution for pathway and source differentiation in the field. It yet demonstrates that such analysis constitutes a valuable complementary tool to explore biochemical processes affecting chlorinated ethenes in groundwater, provided that it is applied at sites where the hydrogeological and biogeochemical contexts are well characterised. The performed studies additionally put more light on C and Cl isotopic effects occurring during PCE and TCE biotic reductive dechlorination even though the specific kinetic processes controlling isotopic behaviours remain unclear. Finally, this work proposes a mathematical model which opens the door to a better incorporation of isotope data when evaluating a plume fate based on a modelling approach.
Hydrogeological and topographical controls on catchment dynamics and their implications for low flows
Même dans des régions relativement humides comme la Suisse, des périodes de sécheresses plus intenses et prolongées sont attendues à cause des changements climatiques. Afin d’appréhender la sensibilité des ressources en eaux aux sécheresses et d’identifier les régions à risque, une compréhension profonde des mécanismes gouvernant la dynamique des bassins versant en l’absence de précipitation est cruciale. Pendant les périodes de sécheresses, les rivières sont principalement alimentées par l’eau souterraine. Leur débit reflète donc la capacité du bassin versant à libérer de l’eau stockée lors de précédents événements pluvieux. Les caractéristiques des bassins versants qui influencent les processus hydrogéologiques sont ainsi inhérentes à leur dynamique de basses eaux. La sensibilité des bassins versants doit donc être évaluée d’une perspective hydrogéologique.
Afin de développer des outils pour l’identification des ressources en eaux sensibles aux sécheresses, cette thèse de doctorat explore les influences des propriétés physiographiques sur la dynamique des bassins versants, en mettant l’accent sur leur comportement de basses eaux et sur le rôle de l’hydrogéologie. Les précédentes études consacrées au lien entre les propriétés physiques des bassins versant et leur dynamique négligent souvent leurs caractéristiques souterraines. De plus, l’identification des effets de chaque propriété physique sur le comportement hydrologique des bassins reste complexe. Afin de contrer ces limites, deux approches sont développées : (1) l’utilisation de modèles hydrogéologiques synthétiques permettant d’évaluer systématiquement l’influence des paramètres hydrogéologiques et topographiques sur les basses eaux, et (2) l’étude de la dynamique hydrologique de 22 bassins versants suisses avec la prise en considération détaillée de leurs caractéristiques géologiques et hydrogéologiques (roche en place ou cohérente – “bedrock” en anglais -- et dépôts quaternaires).
Dans le cadre de la première approche, les propriétés hydrogéologiques et topographiques des bassins (conductivité hydraulique de la roche en place et de l'aquifère alluvial, pente des versants et de la rivière) sont variées systématiquement avec HydroGeoSphere. Ce modèle numérique et distribué simule de manière couplée et simultanée les flux souterrains et de surface. Ainsi, les processus hydrogéologiques sont considérés explicitement et l’impact de chaque propriété physique sur la dynamique des bassins versants peut être quantifié. Ces modèles synthétiques bénéficient grandement à la caractérisation: du lien entre dynamiques de basses eaux et de l’eau souterraine, de l’importance relative de la roche en place et des dépôts alluviaux, et de l’influence combinée de la conductivité hydraulique et de la topographie. En outre, le rôle de propriétés difficilement mesurables sur le terrain, comme la perméabilité de la roche en place (p.ex. la Molasse en Suisse), peut être étudié. Cette caractéristique est d’ailleurs la seule à exercer un effet global sur les basses eaux de tous les bassins synthétiques. Une conductivité relativement haute (p.ex. 10-4 to 10-5 m/s) de la roche en place garantit des débits de basses eaux importants. En fonction de cette valeur, la contribution de la roche en place aux basses eaux peut être favorisée par des versants raides ou diminuée par un relief limité. Lorsque la capacité de la roche en place à subvenir aux bas débits est limitée (quantifiée par le bedrock productivity index BPI), la contribution relative de l’aquifère alluvial peut devenir significative.
Dans la seconde approche, les propriétés physiques des 22 bassins versants suisses sélectionnés (utilisation et types de sol, topographie, géologie et paramètres météorologiques) sont comparées à une multitude d’indicateurs hydrologiques décrivant toutes les gammes de débits sur 20 ans de mesure. Des indicateurs de débits absolus (p.ex. Q95 à Q5) ainsi que des indicateurs relatifs (p.ex. Q95 divisé par le débit moyen) sont utilisés. La normalisation des indicateurs de débit permet de filtrer l'effet des précipitations et donc de se concentrer sur l'influence des propriétés physiques du bassin sur sa dynamique. Ainsi, les effets de la précipitation et des paramètres physiques sur le comportement hydrologique deviennent distinguables. Les indicateurs absolus de débit, à part les bas débits, dépendent principalement de la météorologie. Les indicateurs relatifs, décrivant tout autant les bas que les hauts débits relatifs, sont en revanche uniquement corrélés aux paramètres géologiques et hydrogéologiques des bassins (% de grès, % de dépôts quaternaires productifs). La capacité d’un bassin versant d’”amortir” le signal de la précipitation peut donc être attribuée à ses caractéristiques géologiques et hydrogéologiques. Les résultats suggèrent que ce potentiel de “stabilisation” des débits, quantifié par exemple par le ratio Q95/Qmean, est favorisé par la présence de grès dans le bassin. De plus, des dépôts quaternaires importants semblent également exercer un effet positif sur les bas débits normalisés.
Les deux approches sont complémentaires et permettent d’identifier des processus similaires, cruciaux pour la caractérisation de la dynamique générale et de basses eaux des bassins versants. Selon les deux lignes de recherche, une roche en place relativement perméable (p.ex. 10-5 m/s, du grès) est un prérequis pour des débits soutenus lors de périodes sèches. L’influence de dépôts productifs locaux sur la dynamique des bassins est soulignée par les deux approches. Sur la base de ces résultats, deux aides à l’évaluation de la sensibilité des rivières et des aquifères alluviaux aux sécheresses sont développées. Les méthodes dépendent du type et de la qualité des données disponibles. Si celles-ci sont suffisantes, l’estimation de la sensibilité peut être quantitative, alors qu’elle a une valeur qualitative si les données de débits ou de hauteurs piézométriques sont rares. Dans le second cas, des stratégies de surveillance des ressources en eaux peuvent notamment être établies sur la base des lignes directives proposées. En outre, celles-ci proposent un cadre de comparaison du comportement des bassins versants en période sèche., Periods with scarce precipitation will likely occur more frequently and last longer under changing climatic conditions, even in relatively humid regions like Switzerland. To assess the sensitivity of water resources to dry spells and to identify regions that might experience water scarcity issues, a thorough understanding of the mechanisms governing catchment dynamics in the absence of rain is essential. During dry periods, streamflow is mainly fed by groundwater reservoirs and thus reflects the ability of the catchment to release water that has been previously stored during precipitation events. Catchment characteristics that govern groundwater processes are consequently inherent to low-flow dynamics. The sensitivity of catchments to dry periods thus has to be assessed from a hydrogeological perspective.
This PhD thesis, with the global aim of providing tools for the identification of catchments sensitive to dry conditions, explores the physiographic controls on catchment dynamics with emphasis on low flows and on the role of hydrogeological factors. Previous studies dedicated to the relationship between catchment properties and streamflow dynamics often disregard the subsurface characteristics. Moreover, unravelling the various physical controls on hydrological signatures is complex based on observed data. To cope with these limitations, two approaches are developed: (1) the use of hydrogeological synthetic models, which allow the systematic assessment of topographical and hydrogeological influence on low flows and groundwater storage, and (2) an investigation of streamflow dynamics of 22 Swiss catchments with the consideration of detailed geological and hydrogeological descriptors of both the general geological environment (bedrock lithologies) and alluvial quaternary aquifers.
In the first approach, catchment hydrogeological and topographical features (bedrock and alluvial hydraulic conductivity, hillslope and river slope) are systematically varied using the numerical model HydroGeoSphere. This software simulates surface and subsurface flow in a fully coupled, distributed way. It thus allows the explicit consideration of groundwater processes and the quantification of the impact of each physical property on catchment dynamics. The synthetic models provide great insights on the relationship between low flows and groundwater processes, on the relative importance of the bedrock and the alluvial aquifer, and on the combined impact of hydraulic conductivity and slope gradients. Moreover, the role of catchment properties whose observation in the field is bound to high uncertainties, such as the hydraulic conductivity of the bedrock, can be explored with the synthetic models. The only catchment property exerting an overall impact on low flows is indeed the hydraulic conductivity of the bedrock. Relatively high hydraulic conductivities (e.g. 10-4 to 10-5 m/s) of the bedrock guarantee sustained low flows. Depending on this value, the contribution of the bedrock to low flows can be increased respectively diminished by steep respectively flat hillslopes. When the capacity of the bedrock to sustain the stream (quantified by the proposed bedrock productivity index BPI) is limited, the relative contribution of the alluvial aquifer can become significant.
In the second approach, the catchment properties of the 22 selected catchments, encompassing land use, soil, topography and geology, as well as precipitation characteristics, are compared to numerous streamflow indicators describing the entire range of dynamics over 20 years. Absolute (e.g. Q95 to Q5) as well as relative indicators (e.g. Q95 divided by mean discharge) are used. The normalisation of the discharge indicators filters the influence of precipitation, which allows focusing on the impact of catchment properties on discharge dynamics. The meteorological and the catchment controls on hydrological signatures thus become distinguishable. The impact of precipitation is consequent on the absolute discharge indicators except for the low-flow range. The relative indicators, which describe both high and low normalised discharges, are however only correlated to the geological properties of the catchments (% of sandstone and % of productive quaternary deposits). The ability of the catchment to “buffer” the precipitation signal can thus be attributed to its geological and hydrogeological characteristics. The results suggest that this “stabilisation” effect on streamflow, quantified for instance by Q95/Qmean, is sustained by the presence of sandstone in the catchment. Moreover, productive quaternary deposits with a large extent or volume also seem to have a favourable effect on normalised low flows.
The two approaches are complementary and enable to identify similar processes and governing mechanisms, which are of high relevance for the characterisation of catchment and of low-flow dynamics. According to both approaches, a relatively permeable bedrock (e.g. 10-5 m/s, sandstone) is a prerequisite for sustained streamflow during dry periods. The influence of local productive deposits on catchment dynamics is also highlighted by both methods. Based on these findings, two guidelines are developed to assess the sensitivity of rivers and alluvial aquifers to dry periods. The assessment can be quantitative if adequate time series and data describing the resource exist, whereas it has a qualitative value if scarce discharge or groundwater head data are available. In the latter case, monitoring strategies can for instance be established on the basis of this guideline. Furthermore, it provides a framework for catchment inter-comparison with regards to their behaviour under dry conditions.
Investigating the origin of chloroform in soils and groundwater using carbon and chlorine stable isotopes analysis
Chloroform (CHCl3) has been considered for a long time as an anthropogenic contaminant which is known to be possibly carcinogenic to humans. Chloroform is also an important contributor to gaseous chlorine in the atmosphere and may catalyze some chemical reactions in the lower atmosphere. The presence of chloroform in aquatic and terrestrial environments has been widely demonstrated. The frequent detection of chloroform in forest soils, peatland and groundwater in absence of other anthropogenic contaminants suggests that chloroform may be produced naturally by biogeochemical processes. Numerous studies on natural organohalogens have suggested that enzymes such as haloperoxidases (e.g. chloroperoxidase from Caldariomyces fumago) and halogenases excreted by some fungi, plants and bacteria could play an important role in biosynthesis of chlorinated organic compounds in soils. Although the natural formation of chloroform in some forest soils and peatlands is rather well establish, it is difficult to demonstrate a natural origin of chloroform in groundwater as chloroform is widely used as solvent or chemical reagent and can be transported over extended distances from anthropogenic sources. For risk assessment and to implement appropriate remediation measures, there is a need for methods to differentiate between natural and anthropogenic chloroform. Moreover, little is known about the mechanisms leading to the formation of natural chloroform in the terrestrial environment.
This PhD thesis investigates if stable isotope methods can be used to distinguish between different sources of chloroform and provide additional insight into the mechanism of natural chloroform formation. It is expected that anthropogenic and natural chloroform have a different isotope signature due to the differences in the origin of the carbon and formation mechanisms. To evaluate the feasibility of the method, the isotope signature of chloroform was characterized at forested sites where chloroform is likely produced naturally and compared to the isotope signature in underlying groundwater. In addition, the isotope composition of industrial chloroform and chloroform at sites with known anthropogenic contamination was evaluated. Based on previous studies, chloroform formation might proceed via the formation of trichloroacetyl-containing compounds (TCAc) from which chloroform is released by hydrolysis. To confirm this hypothesis, the concentration and isotope ratio of TCAc was quantified as well at the sites where natural chloroform production was observed. The mechanism of chloroform formation and factors that control the isotope ratio of chloroform were investigated in more detailed using laboratory chlorination experiments. In addition to NOM, simple model compounds were used which make it easier to relate observed isotope trends to underlying reaction mechanisms. Chlorination was induced by using chloroperoxidase (CPO) and HOCl, which is expected to be the active compound also during enzymatic chlorination. A first set of studies explored the role of TCAc in chloroform formation in more detail. In these studies, the isotope composition of chloroform and TCAc containing compounds from laboratory incubation studies were compared to field observations and to predictions from an analytical model. In order to evaluate the contribution of different functional groups to chloroform formation, isotope effects associated with chlorination of model compounds with specific function groupds were compared with isotope effects during chlorination of NOM. Finally a series of experiments were carried out that focues on chlorine isotope analysis to explore potential rate-limiting steps in chlorination in more detailed. These experiments also provided insight into the potential chlorine isotope composition of naturally formed chloroform.
In order to demonstrate that carbon isotope analysis can be used to determine the origin of chloroform in groundwater, the δ13C of chloroform was determined in soil gas and groundwater at five different sites including three forested areas, one urban site and one landfill. The δ13C of chloroform at the water table (-22.0‰) of forest sites corresponded well to the δ13C of soil gas chloroform (-22.8 and -26.2‰) demonstrating that chloroform maintains its characteristic isotope signature during transport through the unsaturated zone. At the three forested sites, the δ13C of groundwater δ13C from -22‰ to -27 ‰) was close to the values of soil gas chloroform indicating a natural origin of chloroform. This conclusion is plausible as the three sampling sites were located within spruce and pine forest which are associated with soils that are favorable for chloroform production. In contrast, chloroform from the landfill (-42.1‰) and urban (-47.0‰) sampling sites have clearly an anthropogenic signature (δ13C from -43‰ to -63‰) consistent with a concentration higher than at the three forested sites. Chloroform was detected in groundwater samples as old as 35 years demonstrating that chloroform can persist over extended periods in oxic groundwater. The isotope analysis of groundwater samples collected in spruce forest indicates that the origin of chloroform can still be determined based on isotope composition even if chloroform was transported over an extensive distance. The δ13C of chloroform sampled down gradient of a spruce plantation with an average groundwater age of 30 years had a δ13C of -24‰, which is still within the range of the 13C in soil gas. Thus, the strong difference in δ13C between natural and industrial chloroform makes it possible to unmistakably identify the origin of chloroform even if the some changes of the isotope composition occur during transport.
The TCAc in some forest soils are strongly enriched in 13C (-10‰) compared to natural chloroform. Chlorination experiments combined with a mathematical model have revealed that TCAc could play a fundamental role in the formation of chloroform. Indeed, the laboratory experiments have shown that in addition to chloroform 13C-enriched TCAc are also formed during the chlorination of NOM and humic substances. The large isotope fractionations measured experimentally (kinetic isotope effect from 1.014 to 1.018) for the hydrolysis of TCAc were comparable with those observed for field samples and for the hydrolysis of trichloropropanone. The kinetic isotope effect (KIE) corresponds to the difference in reactions due to the presence of a heavy carbon isotope at the position where hydrolysis takes place. The strong enrichment of TCAc in 13C indicates that a fraction of the trichloromethyl groups is released as chloroform by hydrolysis. Using a mathematical model combined with the fractionation factors determined experimentally, it was shown that when the formation of TCAc and hydrolysis reach a steady state, the isotope composition of chloroform is expected to correspond to isotope ratio of NOM while TCAc should be enriched in 13C. This study confirms that TCAc are reaction intermediates which subsequently release chloroform by hydrolysis, and explains why natural chloroform has a similar isotope signature as NOM despite a large carbon isotope fractionation during its release.
In order to assess the role of selected NOM functional groups in the chloroform formation process as function of pH, the isotopic trends of chloroform produced at different pH (4, 7 and 8) by chemical chlorination of model compounds, humic acid and SOM were measured. As phenolic and ketone functional groups are among the most abundant reactive NOM moieties, phenol and 2-propanone were chosen as model compounds. The isotopic trends of chloroform formed by chlorination of humic acid and NOM were compared with those measured for the formation of chloroform from model compounds to explore which functional groups in NOM might contribute to chloroform formation. These chlorination experiments have demonstrated that the apparent kinetic isotope effects for chloroform formation from model compounds representing NOM functional groups are strongly pH-dependent. Chloroform production from phenol displays a normal KIE (KIE>1), at pH 4 and 7, whereas at pH 8 the process gives rise to an inverse KIE (KIE<1). For chloroform production from 2-propanone, an opposite pH-dependence is observed with a normal KIE at higher pH (7 and 8) and an inverse KIE at lower pH (4). These results indicate that for both chloroform precursors the reaction mechanism and/or the rate limiting step in the sequence of reactions leading to chloroform changes as a function of pH. The comparison of KIE associated with the chlorination of model compounds and NOM suggest that phenolic and ketone groups might be responsible for chloroform formation upon chlorination of NOM in forest soils.
Chlorine isotopes analysis of chloroform sources might contribute to better discriminate natural and industrial chloroform in the terrestrial environment. To estimate the expected range of δ37Cl values for chloroform naturally formed in forest soils and to gain a better understanding of the mechanisms involved in the CPO-catalyzed chlorination of humic substances, abiotic and enzymatic chlorination experiments were carried out. The chlorine isotope analysis of chloroform formed by CPO-catalyzed chlorination at different pH suggested that the formation of an HOCl−ferriprotoporphyrin(IX) intermediate is likely rate-limiting in forest soils (KIE between 1.006 and 1.007). Therefore, in the case where natural chloroform would be produced in forest soils by microorganism excreting extracellular iron-containing CPO and with chloride from atmospheric deposition with δ37Cl value between -1‰ and 1‰, the δ37Cl value of natural chloroform should ranges between -5‰ and -8‰. Thus, natural and industrial chloroform sources might be distinguished from the chlorine isotope composition since the δ37Cl values of industrial chloroform range between 0.32 and -5.4‰ while natural chloroform is expected to be more depleted in 37Cl. The analysis of the KIEs associated with the abiotic and enzyme catalyzed chlorination suggests also that the enzymatic chlorination of NOM occurs via the formation of free hypochlorous acid. These results support the hypothesis according to the CPO-catalyzed chlorination occur via chemo-enzymatic pathway where transiently formed hypochlorous acid diffuse out of the enzyme and react with an organic substrate.
This thesis demonstrates that carbon and chlorine isotope analysis constitute a method of choice to determine the origin of chloroform in soils and groundwater and to study the formation mechanisms. However, the fate of chloroform and related compounds in the studied forest soils and groundwater remains unclear. The biodegradation of anthropogenic chloroform might produces δ13C values in the same range as natural chloroform, underscoring the need to evaluate the isotope signature at contaminated sites with care. Although compound-specific isotope analysis is a valuable approach for source fingerprinting, the δ13C values of chloroform must be interpreted within the detailed hydrogeological and geochemical context of the site. The isotope analysis has also provided valuable information about the role of TCAc in the chloroform formation mechanism and the reactivities of the some functional groups representative of NOM.
Groundwater dynamics and streamflow generation in a mountainous headwater catchment: process understanding from field experiments and modeling studies/
Groundwater systems in mountainous headwater catchments significantly sustain downstream freshwater bodies and therefore play an important role in the regional water cycle. Complex interactions between atmospheric, subsurface and ecological variables occur that determine groundwater quantity and quality as well as streamflow-generation mechanisms at different spatiotemporal scales. An integrated understanding of the hydro(geo)logic processes in such areas is a necessary precursor to develop successful adaption methods in the face of climate change. For this, not only does our mechanistic understanding of groundwater flow in mountainous headwater catchments has to be improved, but also the complex land-atmosphere interactions with groundwater have to be understood. Although there exists a wide breadth of studies on hydrology in mountainous regions, research on groundwater dynamics in these settings still is comparably rare.
In order to close that research gap, an extensive field- and modeling study was carried out within this PhD project. Hydro-climatic data from a dense observation network in the Swiss pre-Alpine upper Rietholzbach Research Catchment (URHB, ~1km2) were used, where the major variables of the water cycle are continuously monitored at high temporal and spatial resolution. Scientifically significant results have been achieved in the four areas covered by this project, which refer to the first-order-controls of groundwater recharge (i.e., climatic forcing and landscape properties) and to the hydrologic responses driven by groundwater discharge (i.e., streamflow generation and solute transport). In the first project phase, six well-established groundwater recharge estimation techniques were evaluated systematically. From the inconsistencies among the applied GR estimation methods first-order controls of GR were identified that helped to better understand GR mechanisms. With the focus on groundwater discharge, a more detailed analysis of groundwater dynamics at the event-time scale was pursued in the second part of this thesis to identify dominant streamflow-generating mechanisms and threshold-responses. It was found that groundwater discharge from the shallow aquifer in the valley bottom of the URHB represents the dominant fraction of peak flow during most rainfall periods. The conceptual description of the hydro(geo)logic system in the URHB was evaluated in the third part of this thesis with an analytical model that consists of two linear reservoirs for event-flow generation and a baseflow storage with relatively constant discharge rates. Here, rainfall-driven event flow is generated in the riparian zones and the adjacent hillslopes, while baseflow was assumed to originate from the deep fractured-rock aquifer and to be rather constant. The model adequately reproduced the observed streamflow signal, however, the performance improved after implementation of the variable contributing area concept. Although the shrinking/expansion of the riparian zones was small compared to the total catchment area (up to 14 %), this process strongly controlled the streamflow hydrograph when wet antecedent moisture conditions coincide with high-intensity rainfall periods.
Overall, this PhD compiles various a practical approaches to analyze and characterize groundwater systems and streamflow-generation mechanisms in mountainous headwater catchments. By focusing on the two first-order controls on groundwater recharge, climate and subsurface properties, an important foundation for future research is provided that deals with potential negative effects of climate change and land use on water quality and quantity in mountainous headwater catchments.
Investigation of virus attenuation mechanisms in a fluvioglacial sand using column experiments
Virus inactivation and virus adsorption, resulting from interactions with minerals, constitute important aspects of an aquifers disinfection capacity. Investigations using a 20 cm. column tilled with medium-grained natural sands demonstrated that the sands can attenuate up to 62% of a pulse of viruses injected. Experiments using repeatedly washed sands had significantly lower attenuation capacity than fresh sands, due to removal of fine-grained (silt and clay-sized) coatings on grain surfaces. X-ray diffraction analyses of the sand, and the associated fine-grained coating indicated that no significant mineralogical differences existed between these two materials. The experimental data suggested that rougher surfaces/crystal edges in the grain coatings reduced repulsive forces between viruses and the sands permitting greater virus adsorption to the column matrix. Soaking all sands with Tryptone solution after testing released adsorbed viruses indicated that short-term viral inactivation due to interactions with the column matrix was a negligible part of the attenuation process. (C) 2004 Federation of European Microbiological Societies. Published by Elsevier B.V. All rights reserved.