[1] The distribution of groundwater fluxes in aquifers is strongly influenced by topography, and organized between hillslope and regional scales. The objective of this study is to provide new insights regarding the compartmentalization of aquifers at the regional scale and the partitioning of recharge between shallow/local and deep/regional groundwater transfers. A finite-difference flow model was implemented, and the flow structure was analyzed as a function of recharge (from 20 to 500 mm/yr), at the regional-scale (1400 km²), in three dimensions, and accounting for variable groundwater discharge zones; aspects which are usually not considered simultaneously in previous studies. The model allows visualizing 3-D circulations, as those provided by Tothian models in 2-D, and shows local and regional transfers, with 3-D effects. The probability density function of transit times clearly shows two different parts, interpreted using a two-compartment model, and related to regional groundwater transfers and local groundwater transfers. The role of recharge on the size and nature of the flow regimes, including groundwater pathways, transit time distributions, and volumes associated to the two compartments, have been investigated. Results show that topography control on the water table and groundwater compartmentalization varies with the recharge rate applied. When recharge decreases, the absolute value of flow associated to the regional compartment decreases, whereas its relative value increases. The volume associated to the regional compartment is calculated from the exponential part of the two-compartment model, and is nearly insensitive to the total recharge fluctuations.

Underground pumped storage hydropower (UPSH) is an attractive opportunity to manage the production of electricity from renewable energy sources in flat regions, which will contribute to the expansion of their use and, thus, to mitigating the emissions of greenhouse gasses (GHGs) in the atmosphere. A logical option to construct future UPSH plants consists of taking advantage of existing underground cavities excavated with mining purposes. However, mines are not waterproofed, and there will be an underground water exchange between the surrounding geological medium and the UPSH plants, which can impact their efficiency and the quality of nearby water bodies. Underground water exchanges depend on hydrogeological features, such as the hydrogeological properties and the groundwater characteristics and behavior. In this paper, we numerically investigated how the hydraulic conductivity (K) of the surrounding underground medium and the elevation of the piezometric head determined the underground water exchanges and their associated consequences. The results indicated that the efficiency and environmental impacts on surface water bodies became worse in transmissive geological media with a high elevation of the piezometric head. However, the expected environmental impacts on the underground medium increased as the piezometric head became deeper. This assessment complements previous ones developed in the same field and contributes to the definition of (1) screening strategies for selecting the best places to construct future UPSH plants and (2) design criteria to improve their efficiency and minimize their impacts.

Heat as a tracer in fractured porous aquifers is more sensitive to fracture‐matrix processes than a solute tracer. Temperature evolution as a function of time can be used to differentiate fracture and matrix characteristics. Experimental hot (50 °C) and cold (10 °C) water injections were performed in a weathered and fractured granite aquifer where the natural background temperature is 30 °C. The tailing of the hot and cold breakthrough curves, observed under different hydraulic conditions, was characterized in a log–log plot of time vs. normalized temperature difference, also converted to a residence time distribution (normalized). Dimensionless tail slopes close to 1.5 were observed for hot and cold breakthrough curves, compared to solute tracer tests showing slopes between 2 and 3. This stronger thermal diffusive behavior is explained by heat conduction. Using a process‐based numerical model, the impact of heat conduction toward and from the porous rock matrix on groundwater heat transport was explored. Fracture aperture was adjusted depending on the actual hydraulic conditions. Water density and viscosity were considered temperature dependent. The model simulated the increase or reduction of the energy level in the fracture‐matrix system and satisfactorily reproduced breakthrough curves tail slopes. This study shows the feasibility and utility of cold water tracer tests in hot fractured aquifers to boost and characterize the thermal matrix diffusion from the matrix toward the flowing groundwater in the fractures. This can be used as complementary information to solute tracer tests that are largely influenced by strong advection in the fractures.

In southern Belgium, 23% of abstracted groundwater volumes are from chalk aquifers, representing strategic resources for the region. Due to their specific nature, these chalk aquifers often exhibit singular behaviour and require specific analysis. The quantitative evolution of these groundwater resources is analysed for the Mons Basin and Hesbaye chalk aquifers as a function of past evolution, in the short and long term. Groundwater level time series exhibit decreases when analysed over different periods. This is particularly visible for the Hesbaye chalk aquifer when comparing the 1960–90 and 1990–2020 periods. Such decreases are associated with observed temperature increases and precipitation decreases, inducing a decrease of aquifer recharge, and a probable increase of groundwater abstraction in the adjacent catchment. Past evolution is also discussed considering recent winter and summer drought events. The aquifers exhibit long delays in response to recharge events, particularly where the thickness of the partially saturated zone plays a crucial role in observed delays. Regarding future evolution, simulations of the impact of climate changes using medium–high emission scenarios indicate a probable decrease of the groundwater levels over the Hesbaye chalk aquifer.

Pumped storage hydropower (PSH) enables the temporary storage of energy, including from intermittent renewable sources, and provides answers to the difficulties related to the mismatch between supply and demand of electrical energy over time. Implementing a PSH station requires two reservoirs at different elevations and with large volumes of water. The idea of using old, flooded open-pit quarries as a lower reservoir has recently emerged. However, quarries cannot be considered as impervious reservoirs, and they are connected to the surrounding aquifers. As a result, PSH activities may entail environmental impacts. The alternation of the pumping–discharge cycles generates rapid and periodic hydraulic head fluctuations in the quarry, which propagate into the surrounding rock media forcing the exchange of water and inducing the aeration of groundwater. This aeration can destabilize the chemical balances between groundwater and minerals in the underground rock media. In this study, two numerical groundwater models based on the chalk quarry of Obourg (Belgium) were developed considering realistic pumping–discharge scenarios. The aim of these models was to investigate the hydrodynamic and hydrochemical impact of PSH activities on water inside the quarry and in the surrounding rock media. Results showed that (1) water exchanges between the quarry and the adjacent rock media have a significant influence on the hydraulic head, (2) the frequency of the pump–discharge scenarios influence the potential environmental impacts, and (3), in the case of chalk formations, the expected impact of PSH on the water chemical composition is relatively limited around the quarry. Results highlight that those hydrogeological and hydrochemical concerns should be assessed when developing a project of a PSH installation using a quarry as a lower reservoir, considering all particularities of the proposed sites.