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Sédimentologie --- Somme (bassin) --- Alluvions

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The region of Spa has been well-known for the quality of its waters for a long time. This region is still a vast resource for groundwater nowadays. Prospection for water resource has been guided by local geological knowledge and the use of drillings to identify suitable spots for water production. Geophysical methods were implemented in order as a time-saving and cost-efficient method I order to guide the location of future drillings. 
This work focuses on the implementation, processing and interpretation of the electrical resistivity tomography (ERT) geophysical method. This method measures the underground response to an electrical current passing through. This response, known as electrical resistivity, is essentially linked to the abundance of clay material in the underground, the interconnectivity of the solid grains within the rocks, and, most remarkably, to the amount of free water.
This study takes place in the cambro-ordovician context of the Stavelot-Venn Massif. The field of investigation is divided into two distinct geological regions: on the north-east lies the first district, called Area 1, characterized by quartzite-rich environment; on the south-west lies the second area, called Area 2, where clay-rich geological materials such as clay and phyllite are abundant. Both regions have been affected by two orogenetic episodes and carry the scars of such events. These take the form of faults and folds, as well as various degree of fracturing within rock layers. Features related to faults and intense fracturing are key to the water production prospect. In this region, faults act as lifts for deep waters, and fractured layers may form suitable water reservoirs.
ERT is particularly appropriate in this context for it shows strong contrasts of between massive rock layers and free waters. Faults and highly fractured area bearing water appear clearly on profiles. Some ambiguities may still appear due to occasional artifacts and loss of resolution, particularly at depth.
The method was implemented in four distinct regions. The first region, PleinFays, revealed the presence of two faults in a transition area between the clay-rich Area 1 and a quartz-rich area in the south-east. The second region, Américains, revealed the presence of a fault deep enough to be explored for water production. The third region, Belle Hé, revealed a deep area that could correspond to a water reservoir, but suffers from ill-resolution. The fourth area, Fontaines, revealed the presence of a fault that could be exploited for water production as well.
Eventually, ERT proved to be an efficient tool to identify geological structures related to water production. Further research could be carried out in order to map the horizontal and vertical expansion of identified faults.

Geophysics, Aquifers, Geology --- Ingénierie, informatique & technologie > Géologie, ingénierie du pétrole & des mines

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Le système terre est en train de basculer d’un état vers un autre, entraînant avec lui la modification du cycle hydrologique et par conséquent, des stratégies de gestion des ressources en eau. Mais qu’en est-il pour les Ardennes belges, où les précipitations sont particulièrement abondantes comparées au reste du pays ? Le déclin des réserves en eau est-il à craindre pour cette région? Dans le but d’évaluer les impacts du changement climatique sur les ressources en eau de Spa et de Bru, une détection de tendance sur les composantes du bilan hydrogéologique ainsi que sur d’autres variables dérivées, comme des indices d’aridité, fût menée au moyen de tests statistiques (régressions, tests de Mann-Kendall et de Pettitt). Les données utilisées s’étendent entre 1971 et 2100. Elles proviennent d’observations ainsi que de simulations. Ces dernières sont issues du Modèle Atmosphérique Régional (MAR) et du modèle sol et zone vadose EPICgrid, forcé par MAR de 2020 à 2050 en incluant différents scénarios climatiques. Il en résulte que l’ensemble des variables va dans la même direction, à savoir vers un tarissement de la ressource sur le long terme, même si toutes les tendances ne sont pas significatives. Ce déclin peut être expliqué par la combinaison de l’intensification de l’évapotranspiration, qui induit des sécheresses de plus en plus sévères, avec les changements de précipitations, notamment au travers de leur quantité annuelle et de leur saisonnalité. La manière dont la future végétation et le relief particulier devraient influencer l’évapotranspiration a également été étudiée et montre qu’une végétation plus ouverte composée d’arbustes devrait diminuer l’évapotranspiration réelle, tandis que le relief ne montre pas d’effet global sur les rayonnements solaires interceptés en surface. Les recommandations de cette étude visent à établir une stratégie d’adaptation pour l’avenir afin de concilier les ambitions de captage avec le renouvellement de la ressource, impactée par le changement climatique comme démontré par ce mémoire. Climate change is expected to profoundly affect hydrological fluxes and therefore alter water resources management. In the Belgian Ardennes, where rainfall is abundant and water scarcity might not represent a major concern, global warming consequences must though be evaluated. In order to assess these potential effects on the Spa and the Bru springs, a trend detection was carried out using statistical tests (regressions, Mann-Kendall and Pettitt tests) on water cycle components along with other variables such as aridity indices. The data used range from 1971 to 2100 and include both observations and simulations. The Regional Atmospheric Model (MAR) and the EPICgrid soil and vadose zone model, forced by MAR from 2020 to 2050 and including different climatic scenarios, provided the simulated data. Even if trends are not all significant, they all point in the same direction, towards a long term resources decline. The depletion is due to intensified evapotranspiration leading to increasingly severe droughts and likely decreasing total annual precipitation as well as changes in their seasonality. In addition, the way in which future vegetation and the particular topography should influence evapotranspiration was also investigated. It showed that open shrublands should decrease the actual evapotranspiration while the topography shows no overall effect on the solar radiation intercepted at the surface. The recommendation of this study is to establish a strong adaptation strategy for the future in order to meet the catchment ambitions with the renewal of the resources, impacted by climate change as demonstrated by this thesis.

Changement climatique --- gestion des ressources en eau --- recharge des aquifères --- précipitations --- évapotranspiration --- aridité --- changement de végétation --- Spa --- climate change --- water resources management --- groundwater recharge --- precipitation --- evapotranspiration --- aridity index --- vegetation changes --- Spa --- Sciences du vivant > Sciences de l'environnement & écologie --- Physique, chimie, mathématiques & sciences de la terre > Sciences de la terre & géographie physique