Abstract The implementation of new groundwater tracer tests to the chalky karst plateau of the northwestern end of the Paris Basin, combined with a critical review of previous tracer test investigations, makes it possible to characterize the role of karsts in relation to many structural features. The impact of tectonic structures and lithology on the development and evolution of the karst networks is analysed. The consequences for drinking-water supplies and its protection are examined.

Abstract Water resource management is a major concern in Marquenterre, a maritime plain located in the western part of the Somme department. Water management is particularly indispensable for protecting wetlands in Marquenterre, regulating water usage and avoiding saline intrusion into aquifers. Various approaches including geological modelling and hydrogeological and hydrogeochemical studies were used to prepare a conceptual model of the Marquenterre hydrosystem and to provide better water management in this sector. The conceptual model shows that the chalk aquifer and the various Quaternary deposit aquifers are in hydrogeological continuity. No seawater intrusion has been discovered in groundwater. Salinization discovered at depth is a result of the most recent marine transgressions. Finally, wetlands are primarily supplied by the chalk groundwater or by rainfall. The study results are used to direct policies for surface and subsurface water resource management. The proposed conceptual model may be useful for other coastal aquifers along the English Channel in order to address challenges of managing the chalk groundwater and backshore swamps.

Abstract The study of the temperature of two rivers in Normandy (France), the Orne and the Touques, between 2013 and 2018 allowed the main controlling factors regulating their thermal regime to be determined. The analysis was conducted by coupling different statistical treatments: linear regression between water and air temperatures, independent component analysis, principal component analysis and a multiple linear regression model. The temperature of the two rivers is mainly controlled by climatic factors but secondary regulation factors are demonstrated to play important roles: runoff for the two rivers and groundwater for the Touques. The influence of the chalk aquifer on river temperature appears to vary seasonally throughout the year, reaching its maximum in the early spring and increasing from upstream to downstream. The coupled use of the different statistical complex methods showed its validity in understanding both the temporal and spatial variations in water temperature and its correlation with the secondary factors, that could not be inferred from a simpler approach based on linear regression. These techniques could be valuable in other areas with rivers sufficiently monitored to determine the controlling water temperature factors and thus their sensitivity to climate change.

Abstract When assessing a rock formation for its suitability as a potential host rock for the disposal of radioactive waste, knowledge about its pore water chemical composition is essential. When the HADES underground research laboratory became operational in the early 1980s, it offered the possibility of extracting pore water from Boom Clay. At the time, however, there was almost no experience of sampling pore water from deep clay formations. The low hydraulic conductivity of clays makes it difficult to extract pore water and the sampling process itself can induce changes in the observed chemical characteristics due to oxidation and re-equilibration with the ambient air. In the past decades, significant progress has been made in the techniques and protocols to sample and monitor pore water. The use of nitrogen instead of compressed air to drill boreholes and installing piezometers limited the disturbances induced by oxidation of the clay. Furthermore, an advanced system was developed to simultaneously sample pore water and dissolved gases and measure some key geochemical parameters such as pH, pCO 2 and redox potential under in situ conditions. This has resulted in a more reliable characterization of the Boom Clay pore water and a better understanding of perturbing processes such as oxidation.

Abstract Since the 1990s, SCK CEN, EIG EURIDICE and ONDRAF/NIRAS have been investigating the impact of gas generation on the Boom Clay and the engineered barriers. Several experiments have been performed to study gas transport in Boom Clay at laboratory scale and in the HADES URL. This paper gives an overview of these experiments. The transition from the laboratory to the in-situ scale is still a challenging task. It is our ambition to address these issues for Boom Clay, starting with the diffusive transport of dissolved gas. A large set of gas diffusion coefficients in Boom Clay from small-scale lab experiments (centimetre scale) is already available, and in order to validate these for use on a larger (metre) scale, an in-situ diffusion experiment with dissolved gas will be performed in the HADES URL, using the existing boreholes. In this new experiment, called NEMESIS, dissolved neon gas will be injected in one filter, and its diffusion will be monitored by three other filters. By re-using existing boreholes dating from the 1990s, the NEMESIS experiment will continue to provide new diffusion data for the next five years.

Abstract The Enhanced Sealing Project (ESP) is monitoring the thermal–hydraulic–mechanical (THM) responses of a full-scale shaft seal at Canadian Nuclear Laboratories (CNL) former Underground Research Laboratory (URL) site. The evolution of such a full-scale construction has direct relevance to closure design of repository shafts (and tunnels) in a range of host rock types. The shaft seal is installed across a water-bearing fracture (fracture zone (FZ2) at c . 270 m depth), marking the interface between deeper, more saline groundwater and nearer-surface freshwater environments. Intended to demonstrate an ability to install a sealing structure that can limit the movement of water between the two hydrological regimes, the main shaft seal consists of a 40% bentonite clay: 60% fine aggregate by dry mass component ( c . 6 m thickness) sandwiched between two 3 m-thick concrete segments in the approximately 5 m-diameter main shaft. The clay and concrete components provide the primary hydraulic sealing and mechanical constraint to the sealing material, respectively. This paper presents the monitoring results of the ESP from its installation in 2009 through to mid-2017. At present, the seal is still not completely water-saturated, pressures within it are still developing and flooding of the upper shaft is continuing. The ESP provides a comprehensive long-term dataset that will assist in calibrating numerical models describing the performance of placement room/shaft/tunnel seals in a Deep Geological Repository.

Abstract Research in Cretaceous shales from West Africa has demonstrated that significant permeability can develop within shales at shallow depths (<100 m), equivalent to a permeability of >1 m day −1 . Much of the variation in permeability is related to the degree of burial metamorphism, with shales that have been altered and that approach the anchizone having the highest permeability and those that are largely unaltered (early diagenetic zone) having the lowest permeability. However, further research targeting largely unaltered shales dominated by smectite clay has shown that the presence of small igneous intrusions can radically alter the hydrogeology. Twenty-four exploratory boreholes were drilled into smectite-dominated shale and nine of these boreholes were targeted to small dolerite intrusions within the shale. The dolerite was intensely fractured at the intrusion edge, with significant zeolite growth along the fracture surfaces. The permeability in the fractured dolerite was the highest measured in any shale borehole, with transmissivities of up to 60 m 2 day −1 measured from pumping tests. Fracturing was less where dolerite was intruded into sandstones, however, and the measured transmissivity was lower (<0.5 m 2 day −1 ). We postulate that the low permeability and high water content of the shale enabled high pressures to develop during intrusion, facilitating the development of fractures along the intrusion contact zone.