Résultat de la recherche
Noble gases as tracers of surface water – groundwater interactions: insights from novel field and modelling approaches
Understanding the interactions between surface water (SW) and groundwater (GW) in alluvial aquifer systems is of critical importance for the sustainable management of water resources. Advances in realtime and continuous measurement of a range of hydrological tracers provide new opportunities for the characterization of SW-GW dynamics at unprecedented spatiotemporal resolutions. Amongst several promising tracer methods, noble gases are particularly well-adapted to the study of SW-GW interactions, and provide an integrated signal on the flow paths and travel times of water. Capitalizing on the insights offered by novel measurement technologies requires tracer interpretation methods that appropriately capture tracer transport processes in dynamic environmental conditions. However, recourse to highly-simplified tracer interpretation methods, conceptually detached from the complexity of natural systems, is still widespread. In such cases, the potentially rich information content of tracer measurements may be underutilized. This thesis aims at investigating how established and emerging noble gas tracer methods can be optimally used - and when they should be avoided - for the study of SW-GW interactions in alluvial aquifer systems. To this end, a range of novel laboratory, field, and modelling approaches are employed to systematically assess the applicability, limitations, and potential of some gas tracer methods toward informing SW-GW exchange processes. The first gas tracer method examined is the 222Rn apparent age model, which is widely used to estimate the ages of very young GW (days to weeks). High-resolution measurements of the spatial distribution of 222Rn emanation rates in an alluvial aquifer reveal significant spatial heterogeneity in 222Rn production. The explicit simulation of 222Rn in synthetic mass-transport models shows that this level of heterogeneity, combined with mixing of GW, can result in strongly biased estimates of GW age, effectively limiting the applicability of the 222Rn apparent age method to a limited range of field conditions. Although temporal changes in 222Rn concentrations may reveal insights into GW age dynamics, the information content of 222Rn measurements may be best extracted through the integration of 222Rn observations in the calibration process of physics-based flow and transport models. Indeed, the second part of this thesis is devoted to exploring how the explicit simulation of tracer concentrations and the assimilation of untransformed tracer data in highly parameterized, physicsbased models may inform model parameters and ultimately predictions of management interest. Within this framework, the joint assimilation of hydraulic and noble gas data (222Rn and helium), acquired over the course of a novel tracer injection experiment, is shown to strongly inform model parameters and reduce predictive uncertainty of several important water management quantities, such as GW age, SW-GW mixing ratios, and SW infiltration fluxes, far beyond what is achieved with “traditional” hydraulic data alone. These results build upon mounting evidence as to the benefits of explicitly simulating and assimilating diverse observation types with physically-based flow and transport models, avoiding the layer of conceptual simplification and potential bias accrued with simplified tracer interpretation models. Finally, the successful combination of novel gas injection methods, developed over the course of this project, and the assimilation of high-resolution gas tracer measurements in an explicit tracer simulation framework strongly support further developments of (noble) gas tracer methods and tracer-numerical model synergies.
Infiltration under snow cover: Modeling approaches and predictive uncertainty
Groundwater recharge from snowmelt represents a temporal redistribution of precipitation. This is extremely important because the rate and timing of snowpack drainage has substantial consequences to aquifer recharge patterns, which in turn affect groundwater availability throughout the rest of the year. The modeling methods developed to estimate drainage from a snowpack, which typically rely on temporally-dense point-measurements or temporally-limited spatially-dispersed calibration data, range in complexity from the simple degree-day method to more complex and physically-based energy balance approaches. While the gamut of snowmelt models are routinely used to aid in water resource management, a comparison of snowmelt models’ predictive uncertainties had previously not been done. Therefore, we established a snowmelt model calibration dataset that is both temporally dense and represents the integrated snowmelt infiltration signal for the Vers Chez le Brandt research catchment, which functions as a rather unique natural lysimeter. We then evaluated the uncertainty associated with the degree-day, a modified degree-day and energy balance snowmelt model predictions using the null-space Monte Carlo approach. All three melt models underestimate total snowpack drainage, underestimate the rate of early and midwinter drainage and overestimate spring snowmelt rates. The actual rate of snowpack water loss is more constant over the course of the entire winter season than the snowmelt models would imply, indicating that mid-winter melt can contribute as significantly as springtime snowmelt to groundwater recharge in low alpine settings. Further, actual groundwater recharge could be between 2 and 31% greater than snowmelt models suggest, over the total winter season. This study shows that snowmelt model predictions can have considerable uncertainty, which may be reduced by the inclusion of more data that allows for the use of more complex approaches such as the energy balance method. Further, our study demonstrated that an uncertainty analysis of model predictions is easily accomplished due to the low computational demand of the models and efficient calibration software and is absolutely worth the additional investment. Lastly, development of a systematic instrumentation that evaluates the distributed, temporal evolution of snowpack drainage is vital for optimal understanding and management of cold-climate hydrologic systems.
Commemorating the 50th anniversary of the Freeze and Harlan (1969) Blueprint for a physically-based, digitally-simulated hydrologic response model
The year 2019 marks the 50th anniversary of a pioneering publication in hydrology. Allan Freeze and Richard Harlan published their Blueprint for a physically-based, digitally-simulated hydrologic response model (Freeze and Harlan, 1969) in this journal. Their vision was for a futuristic model that would integrate key processes and compartments in the hydrologic cycle: precipitation, evapotranspiration, overland runoff, infiltration and groundwater exchange (into and out of) surface water bodies, such as rivers and lakes. Today, the original Blueprint is a reality. Our paper commemorates the 50 year anniversary of the original Blueprint paper. Through personal communications with Allan Freeze, we document the history and genesis of this paper for the first time. We reflect on the uptake of the Blueprint into modern hydrology, the development of numerical models that enabled this, and the range of challenges being tackled by these models. Finally, we consider challenges and opportunities for the future of this area of modelling and hydrologic science.
Recharge quantification and continental freshwater lens dynamics in arid regions: application to the Merti aquifer (eastern Kenya)
Avec une population de près de 400'000 personnes, le camp de réfugiés de Dadaab est le plus grand au monde. Il est situé dans la région semi-aride de l’est du Kenya, proche de la frontière somalienne. L’unique ressource permanente en eau potable pour les réfugiés et les communautés locales provient de l’aquifère de Merti, qui consiste en une large lentille d’eau douce souterraine de 250 km sur 50 km, et qui est entourée par de l’eau salée. L’augmentation des volumes pompés, des signes d’augmentation de la salinité de l’eau et l’incertitude sur la diminution du niveau de l’eau souterraine ont fait prendre conscience du risque d’épuisement de la ressource. Ces observations ont abouti à la nécessité de mieux caractériser la recharge de l’aquifère ainsi que la dynamique entre la lentille d’eau douce et l’eau salée environnante.
Dans un premier temps, une nouvelle méthodologie a été développée pour quantifier la recharge concentrée à travers un modèle numérique, basé sur la physique des écoulements, couplant l’eau souterraine et l’eau de surface. Ce modèle a eu pour but de reproduire les surfaces inondées durant les crues et a été calibré à l’aide d’images satellites. Deuxièmement, la dynamique de la lentille d’eau douce a été investiguée par une série de modèles numériques synthétiques. Ces modèles ont permis d’analyser les effets des taux de recharge et leurs mécanismes (pluie, recharge concentrée) sur la géométrie de la lentille et de comparer ces géométries simulées avec la géométrie réelle de la lentille de l’aquifère de Merti. Ces deux approches ont été contre-validées à l’aide d’un réseau de monitoring de l’eau souterraine composé de vingt stations de mesures à haute résolution temporelle (15 min), installé sur toute la lentille en septembre 2013. Finalement, les résultats de ces trois axes de recherche ont été combinés dans un modèle numérique régional.
L’approche développée dans cette étude a permis de quantifier une recharge concentrée entre 195 et 329 x 106 m3/a. La recharge diffuse sur la lentille d’eau douce est quant à elle estimée entre 12 et 62 x 106 m3/a. Comparée à ces taux de recharge, l’extraction courante de l’eau souterraine (environ 4.8 x 106 m3/a) est considérée comme durable à l’échelle de l’aquifère. Cependant, la recharge totale est environ 50 à 100 fois plus grande que l’inféroflux traversant le niveau exploité, calculé à l’aide du gradient hydraulique de l’aquifère et des valeurs de transmissivité issues des essais de pompage. Cet écart a mené à postuler la présence d’un aquifère multi-couche considérablement plus épais qu’uniquement l’horizon couramment exploité, mais avec une importante incertitude sur son épaisseur et sur le gradient vertical de salinité entre la lentille d’eau douce et l’eau salée sous-jacente.
Les modèles numériques ont révélé la très grande inertie de l’aquifère et ont également confirmé que la recharge a principalement lieu de manière concentrée lors des évènements de crue, juste en amont de la lentille d’eau douce. La grande inertie de l’aquifère est cohérente avec les faibles variations des niveaux d’eau et de la conductivité électrique (salinité) observées avec le réseau de monitoring.
En conclusion, ce travail ouvre de nouvelles perspectives pour la quantification de la recharge en milieu aride à semi-aride lors d’évènements de crue. Il a également permis de mettre en avant la nécessité de poursuivre le monitoring de l’aquifère et de mener de nouvelles investigations sur l’épaisseur de l’aquifère afin de confirmer les résultats de la recharge., The Dadaab refugee camp, the largest refugee camp in the world with a population of approximately 400’000 persons, is located in the arid to semi-arid eastern Kenya, close to the Somali border. The only permanent water resource for the refugees and the host communities comes from the Merti aquifer which consists in a large continental freshwater lens of 250 km by 50 km surrounded by salty water. The increasing groundwater abstractions as well as signs of increasing salinity and uncertainty on the water level depletion led to the necessity to better characterise the aquifer recharge and the dynamics between the freshwater lens and the surrounding salty water.
Firstly, a new methodology was developed for quantifying the concentrated groundwater recharge through a physically-based coupled surface/groundwater numerical model reproducing inundated surfaces during flood events which is calibrated with inundated surfaces derived from satellite images. Secondly, the dynamics of freshwater lenses are investigated with a series of synthetic numerical models. These models aim to analyse the effect of the recharge rates and mechanisms (rainfall, concentrated recharge) on the freshwater lens geometries and to compare these geometries with the observed geometry of the freshwater lens of the Merti aquifer. These two approaches are cross-validated owing to a groundwater monitoring network of twenty high time-resolution devices installed over the whole freshwater lens in September 2013. Finally, results from these three axes are combined in a regional numerical model.
The approaches developed in this study allowed to quantify a concentrated groundwater recharge to be between 195 and 329 x 106 m3/y. Diffuse recharge contributing to the freshwater lens is estimated to be between 12 and 62 x 106 m3/y. Compared to these recharge rates, the current groundwater extraction (about 4.8 x 106 m3/y), is considered as sustainable on the regional scale. However, this recharge is about 50 to 100 higher than the axial flow estimated with the gradient and the transmissivities obtained with pumping tests. This discrepancy led to postulate the presence of a multi-layer aquifer much thicker than the currently exploited horizon but with uncertainties on its thickness and the vertical salinity gradient.
The synthetic numerical model revealed a very high inertia of the Merti aquifer and confirmed that the recharge of the aquifer is mainly controlled by concentrated recharge on flood plains in the upstream area of the freshwater lens. The high inertia of the aquifer is consistent with the very small groundwater level and electrical conductivity variations observed with the monitoring network.
As conclusion, this work opens new perspectives for the quantification of groundwater recharge in arid to semi-arid areas occurring during large scale flood events. However, it also showed the necessity to continue the monitoring of the aquifer and to carry out further investigations on the aquifer thickness if further exploitations are foreseen.
Infiltration under snow cover: Modeling approaches and predictive uncertainty
Groundwater recharge from snowmelt represents a temporal redistribution of precipitation. This is extremely important because the rate and timing of snowpack drainage has substantial consequences to aquifer recharge patterns, which in turn affect groundwater availability throughout the rest of the year. The modeling methods developed to estimate drainage from a snowpack, which typically rely on temporallydense point-measurements or temporally-limited spatially-dispersed calibration data, range in complexity from the simple degree-day method to more complex and physically-based energy balance approaches. While the gamut of snowmelt models are routinely used to aid in water resource management, a comparison of snowmelt models’ predictive uncertainties had previously not been done. Therefore, we established a snowmelt model calibration dataset that is both temporally dense and represents the integrated snowmelt infiltration signal for the Vers Chez le Brandt research catchment, which functions as a rather unique natural lysimeter. We then evaluated the uncertainty associated with the degree-day, a modified degree-day and energy balance snowmelt model predictions using the nullspace Monte Carlo approach. All three melt models underestimate total snowpack drainage, underestimate the rate of early and midwinter drainage and overestimate spring snowmelt rates. The actual rate of snowpack water loss is more constant over the course of the entire winter season than the snowmelt models would imply, indicating that mid-winter melt can contribute as significantly as springtime snowmelt to groundwater recharge in low alpine settings. Further, actual groundwater recharge could be between 2 and 31% greater than snowmelt models suggest, over the total winter season. This study shows that snowmelt model predictions can have considerable uncertainty, which may be reduced by the inclusion of more data that allows for the use of more complex approaches such as the energy balance method. Further, our study demonstrated that an uncertainty analysis of model predictions is easily accomplished due to the low computational demand of the models and efficient calibration software and is absolutely worth the additional investment. Lastly, development of a systematic instrumentation that evaluates the distributed, temporal evolution of snowpack drainage is vital for optimal understanding and management of cold-climate hydrologic systems.
Estimation et cartographie de la vulnérabilité des aquifères en milieu forestier
L’étude de la vulnérabilité des aquifères est un outil précieux pour les protéger contre les pollutions. Ainsi préservés, ces derniers peuvent être exploités pour l’approvisionnement en eau potable, sans devoir subir de traitement. C’est pourquoi de nombreuses méthodes de cartographie de la vulnérabilité des eaux souterraines ont été développées, comme EPIK et DRASTIC. Toutefois, ces méthodes ne considèrent pas les aspects spécifiques à l’écosystème forestier. Il s’agit là d’une lacune majeure dans l’optique d’une gestion durable des ressources en eau, car une grande partie de l’eau potable est issue de captages forestiers.
Cette thèse propose de combler cette lacune en développant deux approches innovantes : ForSIG et ForDISK pour caractériser la vulnérabilité des aquifères en milieu forestier. Il s’agit de méthodes semi-quantitatives à paramètres et indices superposables, qui prennent en compte les principaux critères environnementaux intervenant dans les mécanismes de rétention et de transfert des substances polluantes. De la superposition de ces critères est obtenu un degré de vulnérabilité pour chaque zone du bassin d’alimentation étudié. Les approches ForSIG et ForDISK proposent d’évaluer quatre critères qui n’étaient jusqu’alors que peu ou pas considérés. Parmi eux se retrouvent : l’épaisseur du sol sa perméabilité, le pourcentage de résineux et la répartition des âges dans les peuplements forestiers. Les études de cas de Thyez et des bois du Jorat menées durant cette recherche confirment l’importance de ces critères pour l’estimation de la vulnérabilité des aquifères en milieu forestier.
La méthode ForSIG permet la réalisation de cartes de vulnérabilité sur de grandes surfaces. Elle est testée sur la source de l’Eperon dans cette étude. Une comparaison avec les méthodes EPIK et DRASTIC prouve que la méthode ForSIG produit les cartes les plus réalistes quant à la vulnérabilité effective du terrain dans un contexte forestier. Toutefois, un essai de multi-traçages sur la source du Montant montre que l’addition d’un facteur de dilution de l’aquifère est primordial pour déterminer sa vulnérabilité, particulièrement pour les systèmes karstiques. L’introduction de ce facteur de dilution permet d’obtenir des cartes réalistes et fiables sur les terrains étudiés.
La méthode ForDISK permet quant à elle une appréciation simple et rapide de la sensibilité des zones boisées sur lesquelles des interventions sylvicoles sont envisagées. Elle a l’avantage de pouvoir être utilisée par les forestiers directement sur le terrain. Des recommandations relatives à l’exploitation forestière sont données pour éviter que des pollutions accidentelles ne se produisent sur les zones les plus vulnérables des bassins d’alimentation et ne contaminent la ressource en eau. Cette méthode est appliquée avec succès sur le site de Saint-Laurent dans la dernière partie de ce travail. Les résultats obtenus montrent qu’en plus de donner une bonne estimation de la vulnérabilité, la méthode ForDISK reste une méthode des plus utiles pour sensibiliser les forestiers aux problèmes de pollution des captages.
Les méthodes ForSIG et ForDISK s’appliquent toutes deux à l’évaluation de la vulnérabilité des aquifères à des pollutions accidentelles sur des bassins d’alimentation recouverts de 75 % de forêts au moins. Non testées sur des zones autres que tempérés et humides, une extension de leur application à d’autres contextes climatiques ou à d’autres types de pollution doit encore être mise à l’épreuve avant de pouvoir être recommandée. Malgré leurs imperfections, ces deux méthodes offrent un premier outil intéressant pour assurer de manière contrôlée la protection des ressources en eau dans les bassins d’alimentation forestiers., Groundwater vulnerability assessment is a valuable tool to protect groundwater resources against pollution. Through an active protection of vulnerable aquifers, uninterrupted drinking water supply without expensive treatment can be ensured. Many methods for groundwater vulnerability mapping exist, such as EPIK and DRASTIC. However, these are general methods that do not take into account a range of characteristics that are specific to forested catchments. This is a major impediment for sustainable water resources management, because a large part of drinking water originates from forested catchments.
The present research aims to close this methodological gap by developing two new approaches (ForSIG and ForDISK) that allow assessing the vulnerability of groundwater resources in forested catchments. They are semi-quantitative methods, based on a Parametric System Model. This implies a parametric rating and weighting of sensitivity criteria that are controlling the retention of pollutants. Vulnerability degrees are attributed to sub-areas of the catchment, according to the superposition of the sensitivity criteria. As opposed to existing vulnerability approaches, the ForSIG and ForDISK methods consider soil thickness and permeability, as well as forest structure and composition. Based on extensive field research at two sites (Thyez and Jorat), the importance of these parameters has been elaborated and considered in the methodology developed.
The ForSIG method has been developed to map groundwater vulnerability for regional scales. It has been applied to the Eperon spring. A comparison with the existing EPIK and DRASTIC approaches revealed that the ForSIG method produces maps that better reflect the vulnerability. However, extensive testing of the method using multiple tracers (at the Montant study site) clearly showed that additionally the dilution capacity of the aquifer plays a crucial role in the assessment of groundwater vulnerability, especially for karst systems. The introduction of the dilution factor allowed developing realistic and robust maps of vulnerability of the tested areas.
The ForDISK method, on the other hand, was developed to quickly assess the vulnerability of local scale forest plots. The main benefit of this approach is its field applicability for foresters: According to the identified vulnerability, recommendations on forest management can be obtained to avoid contamination due to forest work. The ForDISK methodology has been successfully tested on the Saint-Laurent study site. It provides a good and rapid estimation of the vulnerability on a small scale. Additionally, this approach is very useful to raise awareness among foresters on potential groundwater contamination due to forest harvesting.
The ForSIG and ForDISK methods can be applied only on catchments that are covered by at least 75% of forests. They were tested in humid temperate climate regions. Their applicability to other climatic contexts has to be assessed. Moreover, these methods only consider the intrinsic vulnerability of the aquifer with regard to accidental pollution, located at a single point of the catchment. Even though additional research on the applicability of these methods is required, they provide a first and important step towards the protection of groundwater resources in forested catchments.