Jean-Michel Vouillamoz

Managed Aquifer Recharge (MAR) offers a practical pathway to secure groundwater resources in hard-rock basement settings, but the relative influence of aquifer and basin parameters on recharge performance remains insufficiently quantified. This study combines finite-element simulations in saturated and variably saturated domains with Hantush’s analytical solution to evaluate how hydrogeological (hydraulic conductivity, storage, aquifer and unsaturated-zone thicknesses, regional gradient) and operational (infiltration mode, injection rate, duration, basin size/shape) factors control hydraulic head rise (ΔH), relaxation time, and spatial footprint. Results show that hydraulic head variations are primarily governed by hydraulic diffusivity, aquifer thickness, and unsaturated zone thickness, while injection rate and basin dimensions dominate operational control. Relaxation dynamics depend jointly on storage, aquifer and unsaturated-zone thicknesses, injection duration, and flow rate. The spatial footprint scales with ΔH and injection time, but decays rapidly with distance; natural gradients exert minor near-field effects. Most induced hydraulic head rise develops within 0.5 days of injection, with dissipation occurring on similar timescales. Numerical modelling and analytical solutions agree well for thin unsaturated zones and within validity ranges (K = 10-4-10-6 m/s; S = 10-3-10-5; aquifer thickness 30-70 m; infiltration 0.5-3 m/d). Practical recommendations include: (i) prioritizing sites with aquifer thickness >40 m and hydraulic conductivity between 10-5-10-6 m/s; (ii) adopting square basins dimensions of 10-20 m with injection durations of 1-3 days; (iii) avoiding prolonged injection in low-storage aquifers; and (iv) implementing basin networks (cross-shaped layout) rather than isolated structures to extend the recharge footprint. This study defines MAR design criteria and operational guidance for basement aquifers in semi-arid West Africa.

Extensive groundwater development is a cornerstone of universal access to drinking water, especially in tropical monsoon systems with seasonal surface water. Groundwater buffers climate variability where storage is sufficient and withdrawals remain limited. In Africa, half the population lives over low-storage hard-rock aquifers and high demographic and urbanization rates may limit this buffer capacity. To explore the limits of groundwater development in hard rock areas, we analysed groundwater storage distribution at the scale of West-Africa, using a database from 28004 boreholes and 98 nuclear magnetic resonance soundings. We compared this storage with current and projected domestic water needs, and to non-withdrawal groundwater discharge to evapotranspiration, streams and deeper flows. We found that groundwater storage in hard rocks of West Africa averages 578 ± 243mm. On average, the storage is two-third of the annual rainfall and less than 0.12% is pumped annually for domestic purpose, while up to 15% sustain non-withdrawal groundwater discharge. The average buffer capacity of groundwater in face of yearly pumping and non-withdrawal groundwater discharge is 6.6 yr (98% CI: [2.7 yr, 15.2 yr]) at the scale of country districts, and remains of 6.3 yr (98% CI: [2.7 yr, 14.8 yr]) if both population and per-capita needs double by 2050. While our results confirm the buffering capacity of groundwater, local heterogeneity remain: in dense urban areas (above around 28600 person.km-2 for 50L.person-1.day-1 needs scenario or 14100 person.km-2 for 100L.person-1.day-1 scenario), urban groundwater storage is insufficient to meet population needs between two seasonal recharge periods, calling for adaptive planning.

Groundwater from weathered basement aquifers (WBAs) is a strategic water resource, but its potential for resource development remains poorly characterized. Meanwhile, groundwater abstraction from WBAs is growing to meet water and food security needs in most sub-Saharan African countries. In this study, the impact of the instructions given to water borehole drillers on the characterization of WBA properties was assessed by modelling WBA short-term productivity using a novel numerically based, stochastic modeling approach. The numerical modeling of 10,000 synthetic WBAs reveals systematic biases in borehole databases due to instructions given to water borehole drillers, such as the discharge target ( Q _target ) together with the maximum allowed borehole depth ( Z _max ) and the minimum allowed borehole depth ( Z _min ). Insufficient drilling depth below the base of the saprolite ( Z ≤ 35 m) leads to undersampling of deeper water-bearing fracture, causing an underestimation of aquifer productivity and fractured-layer thickness (> 10%). These biases persist across discharge targets (0.5–10 m ³ /h) and are exacerbated by shallow drilling. Moreover, traditional borehole-database-processing methodologies, fail to account for instructions given to water borehole drillers, misrepresenting useful aquifer thickness (Lu) by up to 100%. To enhance accurate use of drilling databases, several approaches could be considered: (1) classify boreholes by exploration depth and instantaneous discharge, excluding shallow borehole data (≤ 35 m below the base of saprolite); (2) archive drilling instructions ( Q _target , Z _max and Z _min ) to contextualize data limitations; and (3) adopt robust indicators like specific capacity and depth-dependent yield instead of instantaneous discharge.

In borehole siting within the basement rock aquifers of West Africa, lineament analysis has traditionally been the primary hydrogeological target, where lineaments may indicate the presence of deep geological structures. However, the limited success of groundwater borehole drilling projects suggests limitations to this approach, calling for a reassessment of its effectiveness. This study examines the role of alternative hydrogeological targets in relation to borehole productivity, focusing on weathering layers and lineaments. A comprehensive database of 2347 boreholes in Burkina Faso was developed for this purpose, revealing a strong correlation between weathered bedrock aquifer geometry and borehole productivity. The analysis indicates that the thickness of regolith and saprock strongly influence drilling success rates, regardless of whether the target yield is for hand-operated or motorized pumps. Conversely, the presence, orientation, and length of lineaments show no significant correlation with borehole productivity. The study emphasizes the need to shift focus from lineaments to weathering properties in basement aquifer exploration and highlights the dominance of a landform-regolith conceptual model over a deep geological structure model. These findings have implications for achieving universal access to drinking water and food security in West Africa’s bedrock regions.

In recent years, there has been a growing interest in utilizing the groundwater response to Earth tides as a means to estimate subsurface properties. However, existing analytical models have been insufficient in accurately capturing realistic physical conditions. This study presents a new analytical solution to calculate groundwater response to Earth tide strains, including storage and compressibility of the aquitard, borehole storage and skin effects. We investigate the effects of aquifer and aquitard parameters on well water response to Earth tides at two dominant frequencies (O₁ and M₂) and compare our results with hydraulic parameters obtained from a pumping test. Inversion of the six hydro-geomechanical parameters from amplitude response and phase shift of both semi-diurnal and diurnal tides provides relevant information about aquifer transmissivity, storativity, well skin effect, aquitard hydraulic conductivity and diffusivity. The new model is able to reproduce previously unexplained observations of the amplitude and frequency responses. We emphasize the usefulness in developing relevant methodology to use the groundwater response to natural drivers for characterizing hydrogeological systems.

Estimating subsurface hydraulic properties using the groundwater (GW) response to Earth tides (ET) and atmospheric pressure is an alternative approach to pumping or slug tests (passive vs. active methods). Yet testing the applicability of models under different subsurface conditions and comparing results with traditional hydraulic methods are lacking. We first review the assumptions of analytical models used to evaluate the GW response to ET and their applicability in unconfined to confined conditions. Second, we develop a robust approach to select the right model based on amplitude and phase pattern of the diurnal and semi‐diurnal tides. Third, based on earlier works we develop an approach to derive the hydraulic conductivity of the screened interval using the GW response to atmospheric tides, here named “atmospheric slug test” (AST). We estimate transmissivity and storativity at three shallow aquifers in Cambodia and compare the results with subsurface properties derived from pumping tests (PT). Transmissivity values from AST and PT are in good agreement. However, we show that storativity values derived from ET show large discrepancies if borehole skin effects are ignored. Further, the GW response to ET exhibits a strong decay in amplitude with frequency while maintaining close to zero and positive phase shifts. When supplemented with the calculated transmissivity values, none of the analytical models was able to reproduce this frequency dependent behavior. Our work emphasizes the need for evaluating passive methods robustness under different subsurface conditions. Further work is required to understand the frequency dependent GW response to natural or artificial forces.

Estimation of hydraulic parameters is essential to understand the interaction between groundwater flow and seawater intrusion. Though several studies have addressed hydraulic parameter estimation, based on pumping tests as well as geophysical methods, not many studies have addressed the problem with clayey formations being present. In this study, a methodology is proposed to estimate anisotropic hydraulic conductivity and porosity values for the coastal aquifer with unconsolidated formations. For this purpose, the one‐dimensional resistivity of the aquifer and the groundwater conductivity data are used to estimate porosity at discrete points. The hydraulic conductivity values are estimated by its mutual dependence with porosity and petrophysical parameters. From these estimated values, the bilinear relationship between hydraulic conductivity and aquifer resistivity is established based on the clay content of the sampled formation. The methodology is applied on a coastal aquifer along with the coastal Karnataka, India, which has significant clayey formations embedded in unconsolidated rock. The estimation of hydraulic conductivity values from the established correlations has a correlation coefficient of 0.83 with pumping test data, indicating good reliability of the methodology. The established correlations also enable the estimation of horizontal hydraulic conductivity on two‐dimensional resistivity sections, which was not addressed by earlier studies. The inventive approach of using the established bilinear correlations at one‐dimensional to two‐dimensional resistivity sections is verified by the comparison method. The horizontal hydraulic conductivity agrees with previous findings from inverse modelling. Additionally, this study provides critical insights into the estimation of vertical hydraulic conductivity and an equation is formulated which relates vertical hydraulic conductivity with horizontal. Based on the approach presented, the anisotropic hydraulic conductivity of any type aquifer with embedded clayey formations can be estimated. The anisotropic hydraulic conductivity has the potential to be used as an important input to the groundwater models.

At the scale of a magnetic resonance sounding (MRS) field setup, the earth’s magnetic field in the subsurface may vary laterally with depth and over time. These variations can be caused by different natural factors and generally cannot be compensated for by accurate tuning of the measuring device. The varying geomagnetic field (GMF) causes nonresonance conditions of excitation that affect the amplitude and phase of the MRS signal. Usually, variations of the GMF do not exceed a few hertz and their effect on the amplitude is relatively small, permitting us to assume near-resonance conditions for inversion. However, in some cases, the results may be erroneous if a varying GMF is not taken into account. Motivated by possible improvements in MRS inversion, we have developed a procedure for measuring and interpreting MRS data that considers a varying GMF. Our results showed that it is relatively easy to take a time-varying GMF into account. As a demonstration, we have developed the inversion of MRS data measured in Benin (West Africa). A depth-varying GMF is a more complex problem, and to consider this, we have developed an algorithm of nonlinear inversion. We have tested this approach on synthetic data, which resulted in an improved inversion. Field validation of this procedure awaits the discovery of a suitable test site with known variations of the earth’s magnetic field in the subsurface.

Many human communities living in coastal areas in Africa and Asia rely on thin freshwater lenses for their domestic supply. Population growth together with change in rainfall patterns and sea level will probably impact these vulnerable groundwater resources. Spatial knowledge of the aquifer properties and creation of a groundwater model are required for achieving a sustainable management of the resource. This paper presents a ready-to-use methodology for estimating the key aquifer properties and the freshwater resource based on the joint use of two non-invasive geophysical tools together with common hydrological measurements. We applied the proposed methodology in an unconfined aquifer of a coastal sandy barrier in South-Western India. We jointly used magnetic resonance and transient electromagnetic soundings and we monitored rainfall, groundwater level and groundwater electrical conductivity. The combined interpretation of geophysical and hydrological results allowed estimating the aquifer properties and mapping the freshwater lens. Depending on the location and season, we estimate the freshwater reserve to range between 400 and 700 L m⁻² of surface area (± 50%). We also estimate the recharge using time lapse geophysical measurements with hydrological monitoring. After a rainy event close to 100% of the rain is reaching the water table, but the net recharge at the end of the monsoon is less than 10% of the rain. Thus, we conclude that a change in rainfall patterns will probably not impact the groundwater resource since most of the rain water recharging the aquifer is flowing towards the sea and the river. However, a change in sea level will impact both the groundwater reserve and net recharge.