S16

The Montagnes District is a metamorphic rocks area from Western Côte d’Ivoire covering an area of about 31,000 km². Its capital is the city of Man. It is home to the largest industrial gold mine in Côte d'Ivoire, at Ity, with an annual production of 6 to 7 tons. Artisanal mining activities are also carried out in several places in the region. These activities can threaten groundwater, both in terms of quantity and quality. Therefore, a good understanding of the structure and functioning of aquifers in these mining areas is needed to prevent these impacts and to develop mining activities that are as environmentally friendly as possible.

This problem raises more generally the question of the validity of a hydrodynamic model explaining the origin of the permeability of hard rock aquifers in order to optimize groundwater resource management methods and better understand their vulnerability. To address this question, this work was able to rely on a statistical analysis of a database of 1654 boreholes.

Results shows that the structure of the aquifer is similar to that observed in several other hard rock areas in the world: it developed due to weathering processes, comprises the capacitive saprolite, 10–20 m thick on average, and an underlying transmissive fractured layer, overlying the unweathered impermeable hard rock. The fractured layer is about 80 m thick, the first 40 to 45 metres being its most productive zone, with a 11 m³/h median productivity.

The characterization of groundwater flow has led to the proposal of a new approach in the context of scarce piezometric data.

Finally, this research shows that the impacts of mining activities are local, limited mainly to the downstream part of the topographic watersheds where the mines are sited. There, groundwater quality and quantity may be affected, as well as the streams that drain these aquifers. In fact, surface water drains groundwater and therefore collects possible contaminants, downstream of the mines. A new approach to assessing the vulnerability of hard rock aquifers to contamination from mining has therefore been proposed.

How to cite: Aoulou, K. A., Pistre, S., Oga, Y. M. S., Dewandel, B., and Lachassagne, P.: Application of a new hydrogeological conceptual model of basement aquifers: Structuring and distribution of hydrodynamic properties. Critique and revision of piezometric mapping methods in a sub-basin of the Cavally River in western Côte d'Ivoire, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-46, https://doi.org/10.5194/iahs2022-46, 2022.

A reliable description of aquifer heterogeneities is crucial for solute transport modelling. Inverse methods, a field of research that has been developing greatly in recent decades, can provide the horizontal structure of heterogeneities, with the collection of piezometric data as a cornerstone. But the latter are very little sensitive to the vertical structure of the aquifer, leaving its estimation dependent on complex and expensive field methods (electrical resistivity and radar tomography, self-potential methods, cross-hole testing, hydraulic tomography) or lab analysis (grain-size analysis from core sample, secondary permeability tests).

In order to take advantage of the possibilities offered by the inversion techniques, while sidestepping the inconvenience of geophysical and field approach, a method is proposed using 2D inversion of flow (solely reliant on piezometric series) as parameterization constraints for a 3D hydrogeological model (see Figure 1).

The methodology is tested via a synthetic example, ensuring full knowledge and control of the aquifer's structure. It is composed of 5 lithofacies, distributed according to a sedimentary pattern, the level of heterogeneity for hydraulic conductivity spans 3 orders of magnitude and groundwater is unconfined. This synthetic example provides both the piezometric chronicles used to inverse 2D flow parameter fields and the lithological logs used to interpolate the 3D lithological model. Finally, the parameters of each facies are obtained through an optimization loop, minimizing the difference between the 2D and the 3D transmissivity (and specific yield).

The method results in the estimation of parameters very close to the known parameters, even with a scarce piezometric and lithological data sampling. The maximal discrepancy is 61% of the initial value for the permeability and 16% for the specific yield (mean error being respectively 18 and 4%). Although the methodology does not prevent interpolation error, it succeeds in reconstructing flow and transport dynamics very close to the synthetic control data. Due to the inherent limitations of the 2D inversion approach, the method only applies to the saturated zone at this point.

How to cite: Rambourg, D. and Ackerer, P.: 3D hydrogeological parametrization using scarce piezometric data, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-112, https://doi.org/10.5194/iahs2022-112, 2022.

The hydrodynamics and hydrochemistry of magmatic and metamorphic rocks, fissured, altered and with low relief, such as the Armorican massif (France), and this only for such conditions, leads to a redox organization of groundwater, which results in beyond 30-50 m deep, depending on the organization of the hydrogeosystems, there is a quasi-generalized reducing level of nitrates. This is due to the organization of underground hydrodynamic flows, the transport conditions and the redox stability range of nitrates.

Also, the modification of the distribution of nitrogen inputs in the areas to be protected as a priority (perimeters of protection of catchments along rivers, coastal watersheds, etc.) or more generally, provides a solution to the nitrate pollution of water. This consists in the interdiction or the strict control of nitrogen inputs in the parts of the catchment basins close to the rivers, where the deep denitrification of the nitrated groundwater cannot be done, because the hydraulic current lines reaching to rivers remain in the superficial oxidizing zone. The manuring of inputs would then only be admissible far enough from rivers so that the passage of streamlines through the deep denitrifying zone is inevitable (Fig. 1).

An experimentation or the practice should make it possible to best approach the delimitation of these preferential manuring zones. Numerical simulations show that significant improvements are possible in less than 5 years, the polluting nitrate stocks being mainly located in the oxidizing zone close to rivers, the best renewed part of the water tables (Fig. 2).

Finally, the constraints generated by this approach and the management measures intended to facilitate the deep infiltration of water and to prevent surface runoff as much as possible, such as plowing perpendicular to the slopes of the hillsides, the removal of drains in sewage fields, etc., would be minimal and have no significant impact on agricultural production. This should facilitate their application, especially since they would be accompanied by other types of measures relating to the modification and the amelioration of agricultural practices.

How to cite: Faillat, J.-P.: The use of redox and hydrodynamic characteristics of groundwater in the control of nitrates., IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-282, https://doi.org/10.5194/iahs2022-282, 2022.

More than 80 Mm³per year are pumped into the Roussillon plain coastal aquifer located between the Pyrenean massif to the South and West, and the Mediterranean to the East. This is a 350 m thick Pliocene multilayered aquifer, with sandy layers embedded in low-permeability clayey material and topped by Quaternary alluvial deposits. The groundwater resource is primarily used for drinking water and irrigation. For more than 40 years, this aquifer has been undergoing a piezometric level decline due to pumping, while water demand is expected to increase with ongoing climate change, sea level rise and increasing demand in water use. Consequently, the Roussillon aquifer is likely to suffer from sea water intrusion and marine submersion in the near future.

As part of the Dem'Eaux Roussillon project, a set of downhole geophysical profiles was recorded at three drill sites, in Saint Cyprien and Barcares along the Mediterranean shore and at Pollestres, 14 km inland. Downhole petrophysical data (NMR porosity and permeability, acoustic velocities, electrical resistivity and spectral gamma natural radioactivity) contribute to better define the penetrated structure. Core petrophysical measurements were also made to support and calibrate these analyses and, in particular, to provide a dm-scale description of the subsurface pore fluid electrical conductivity along the length of each hole. A combined analysis of the latter with porosity and permeability points at incipient and m-scale intrusion processes along discreate horizons.

Repeated downhole measurements overtime and fluid sampling provide a means to follow the dynamics of these intrusion processes found to be more acute at the Barcarès site to the North than close to the Pyrenean, at Saint Cyprien. While high-resolution permanent downhole geophysical observatories have been installed to measure at high frequency both formation electrical resistivity and temperature, a real time management of groundwater resources should contribute to improve aquifer water quality in the future. These observatories datasets will be analyzed and modeled on the basis of smaller scale petrophysical data, providing both an assessment of water quality evolution in terms of salinity from resistivity and quantity in terms of flow rate from temperature.

How to cite: Pezard, P., Lofi, J., LeBer, E., Henry, G., Brun, L., Geeraert, M., Hamel, C., Gee, R., Brillouet, N., Neyens, D., and Caballero, Y.: Petrophysical characterization of a clastic coastal aquifer with implications for saltwater intrusion and the evolution of groundwater resources. The DEM'EAUX ROUSSILLON project, Occitanie, France. , IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-536, https://doi.org/10.5194/iahs2022-536, 2022.

Thermal conductive heating is an in-situ groundwater remediation technique that can be implemented in weathered or fractured rock. However, the characterization and identification of heat transfer in fractured rock is challenging because of complex hydrogeological and thermodynamic processes, particularly for irregular heat source configurations. As an effective practice, sensitivity analysis has been widely used to screen, rank, and quantify the influential factors in complex systems. In this study, a three-dimensional numerical model was applied to investigate, using global and local sensitivity analyses, the significance of six input factors that influence the heating of fractured rock. The factors include the radius and energy delivery strength of the heat source (which were used to study the scale effect and heating processes), the fracture aperture, fracture spacing, groundwater velocity, and the thermal conductivity of the rock matrix. A discrete Latin Hypercube-One at A Time (LH-OAT) approach is proposed and utilized as an experimental design and data analysis method for the discrete input factors that apply to this study. We also used Machine Learning techniques to enhance the robustness of simulation results with a significant reduction of computational cost. The results show that at all locations within the heating area, the radius of the source and energy delivery strength are the most influential input factors. The scale effect associated with the radius of the heat source is significant for the rock matrix temperature. The groundwater flow related factors jointly determine the temperature variation in the upstream areas. The contribution of the thermal conductivity of the rock matrix to heat dissipation is nearly isotropic in the heating area. A nonlinear trending of sensitivity indices is observed for all factors in the input space. Based on these results, the discrete LH-OAT approach has been demonstrated to efficiently identify influential factors within a discrete input value space. The use of a two-way OAT perturbation approach is suggested to avoid potential errors caused by the one-way perturbation method for parameter ranking.

How to cite: Wu, X., Kueper, B., and Novakowski, K.: Global and local sensitivity analysis of heat transport in fractured rock using a modified LH-OAT method, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-556, https://doi.org/10.5194/iahs2022-556, 2022.

Anthropogenic contamination, such as agricultural or urban activities, has led in less than a hundred years to the degradation of the quality of water resources. To assess the vulnerability of groundwater regarding organic and inorganic pollution (i.e. nitrates, pesticides, and pharmaceutical compounds) isotopic tracers may be used. However, in areas where multiple pollution sources occur, isotopic tracers may not be sufficiently discriminant, making source identification difficult. To overcome this problem, a combination of different tracers, including pharmaceutical compounds, along with the multi-isotopic approach is considered. Here, we present the preliminary results of the interpretation of a multi-tracer approach that show a mixed origin of NO₃^- contamination from agriculture and urban origin as well as the interaction between surface water (Waste-Water Treatment Plant effluent) and groundwater. The multi-tracer approach was applied in the Vistrenque basin area (Gard, France) to characterize NO₃^- sources. A combination of natural and anthropogenic tracers is implemented, including tracers of water origin (major ions, trace elements (Br, Li, and Sr)), nitrogen and oxygen stable isotopes as a tracer of NO₃^- (δ₁₅N/δ₁₈O-NO₃^-), boron stable isotopes (δ₁₁B), water stable isotopes (δ₁₈O/δ₂H-H₂O) and strontium isotopes (⁸⁷Sr/⁸⁶Sr). In addition, a suite of 80 organic molecules including pesticides and pharmaceutical compounds are trialed as tracers of the origin of the contamination. Analyses were carried out on groundwater samples, surface water, and soil samples to characterize the geochemical and isotopic signature of end-members. The dual-isotope approach δ₁₅N/δ₁₈O-NO₃^- for nitrate sources highlights differences between nitrate influenced by nitrification of NH₄⁺ in fertilizer & precipitation origin on one hand, and, nitrates nitrate influenced by manure and septic waste origin on the other hand. However, little contrast between agricultural contamination (i.e. manure) and urban contamination (i.e. sewage) was seen with this method. In parallel, a mixing trend between groundwater and WWTP effluent is evidenced by the combination between δ¹⁵N-NO₃^- and δ₁₁B. Moreover, this trend is confirmed with the occurrence of pharmaceutical compounds and a positive anomaly in Gadolinium (i.e. contrast-agent) in groundwater. As pharmaceuticals are found in several water supply boreholes the potential of pharmaceutic to discriminate the origin of organic nitrate will be investigated.

How to cite: Bonnière, A., Le Gal La Salle, C., Khaska, S., Sebilo, M., Louvat, P., and Verdoux, P.: Groundwater resources vulnerability assessment in an alluvial aquifer subjected to strong anthropogenic contamination: geochemical and multi-isotopic fingerprinting approach, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-736, https://doi.org/10.5194/iahs2022-736, 2022.