Linda Luquot

The study of the evolution of petrophysical properties and alteration of host rock in an active fault system is essential for understanding the mechanisms of deformation localization. The distribution of alterations is closely linked to fluid flow paths, while the formation of new deformation structures depends on the mechanical contrasts induced by these alterations. Our study focuses on granodiorite samples from a borehole drilled in 1996 at Hirabayashi by the Geological Survey of Japan, one year after the Nanbu-Kobe earthquake. Crossing the active Nojima fault, this borehole intersects the fault core at 625 m. The analyses include imaging of samples using neutron and X-ray microtomography (ILL – NeXT) and thin sections, as well as mineralogical quantification by X-ray diffraction (XRD). X-ray diffractograms on oriented slides and Rietveld analyses of XRD data acquired on disoriented powders reveal the presence of secondary mineral phases (e.g., montmorillonite, kaolinite, laumontite, siderite, ankerite), representative of different fluid-rock interaction conditions during the exhumation of the massif. Their proportions, which increase as they approach the fault, reach more than 30% of the volume of a sample at 625 m. Whereas X-ray µ-tomography imaging allows us to observe density contrasts within the samples (e.g., mineral phases and fracture network). On the other hand, neutron imaging allows us to observe the distribution of hydrated mineral phases due to the high neutron absorption coefficient of hydrogen (e.g. for 25 meV neutrons, hydrogen attenuation is 3.44 vs 0.17 and 0.11 for oxygen and silicon, respectively). Neutron and X-ray image registration in the same reference frame allows us to perform joint image segmentation, using gaussian-mixture-model to quantify uncertainties, based on the neutron and X-ray coefficients of absorption of the pre-identified mineral phases. The volumes segmented in this way enable us to (i) quantify in a non-destructive way the volume of secondary mineral phases present in the samples along the fault damage profile and (ii) obtain their spatial distribution and assess the anisotropies of distribution in relation to the deformation structures. This work will subsequently enable us to understand the impact of both the distribution of secondary mineral phases and the network of microfractures on the evolution of the petrophysical and mechanical properties of a seismogenic fault.

This study investigates the promising use of geoelectrical methods for monitoring groundwater contamination and mineral reactivity. Geoelectrical methods are mostly used as qualitative detection tools for static subsurface characterization. However, we show that geoelectrical signals are complementary tools for the quantitative characterization of chemical species transport and reaction in the porous matrix by developing a coupled mechanistic model. We examine calcite dissolution as an effective proof-of-concept since calcite dissolution is a chemical process occurring ubiquitously in the Earth’s subsurface. Our investigation focuses on the impact of the reactive zone’s position, extent, and intensity of geoelectrical signals under various inlet conditions generating contrasted dissolution regimes. We conducted five experiments on flow-through columns filled with calcite grains and equipped with geoelectrical monitoring on sequential channels along the column. Three experiments explore the self-potential method and two others monitor the complex electrical conductivity from the spectral induced polarization method. pH in the column at two locations is also monitored. Additionally, the outlet fluid is sampled to monitor major ion concentrations, pH, and electrical conductivity. Thus, the study presents a unique dataset that combines traditional physicochemical monitoring of water samples with geoelectrical acquisition on multiple channels along the column. The quantitative analysis of the geoelectrical signals is achieved through their prediction using a 1D numerical workflow that combines reactive transport simulation with petrophysical modeling based on the evolution of the pore space and the geochemistry. Reactive transport is simulated by developing a CrunchFlow code, which well-reproduces the outlet pore water concentrations and pH. The comparison of the predicted geoelectrical signals with the experimental data clearly shows the characterization of the spatial and temporal distributions of the reaction rates, whatever the reaction rate and the reactive zone extent. Self-potential monitoring allows the spatialization of the reactive zone from the electrodiffusive coupling and enables the detection of a low dissolution regime contrary to what is observed from the outlet water electrical conductivity monitoring. The complex electrical conductivity shows significant variations during the intense dissolution regime. Water electrical conductivity, porosity, and the real electrical conductivity of the sample are successfully retrieved from petrophysical computation. This innovative study in which geophysical and geochemical methods are intrinsically intertwined paves the way to broader and more interdisciplinary studies of solute transport and reactivity in porous media and in a more general perspective, the presented methodology applies to contaminant transport.

Nitrate is a significant contaminant in many aquifers due to the massive use of fertilizers in agriculture and wastewater discharge into streams. Globally, elevated nitrate levels in aquifers pose health concerns due to nitrate ingestion linked to diseases such as methemoglobinemia and blue-baby syndrome in children under six years of age. Furthermore, nitrates can potentially form N-nitroso compounds, known to cause carcinogens, after being transformed into nitrites in the esophagus. Consequently, the European Union and the World Health Organization have set a limit of 50 mg/L for NO₃⁻. Despite these regulations, global nitrate exposure persists, notably in Germany, where 27.1% of groundwater bodies exceed the established limit, indicating an increasing trend. To mitigate this situation, we propose implementing Permeable Reactive Barriers (PRBs) at a pilot site at the Fredersdorfer Weg area in northeastern Germany, about 21 km east of Berlin. The Fredersdorfer Weg is located within the Fredersdorfer Mühlenfließ catchment area, adjacent to the Münchenhofe Wastewater Treatment Plant. To date, Permeable Reactive Barriers (PRBs) have been one of the leading technologies for addressing the in-situ remediation of migrating contaminant plumes in groundwater. However, only a limited number of models simulate both advective-dispersive and reactive transport related to PRBS and microbial growth and decay, which leads to a lack of integration in the growth of microorganisms and changes in permeability because of biomass growth. To address this problem, we present a one-dimensional reactive transport model in MIN3P. Our model replicates the processes of nitrate degradation in heterotrophic conditions, employing different carbon sources. Processes in the model include advective-dispersive and reactive transport, attachment, and detachment of Microbial from the sediment, as well as allowing the dependence of reaction rates on biomass, dissolved organic carbon (DOC), and NO₃⁻ concentrations. To validate our model, we propose highly controlled soil column experiments under heterotrophic denitrification. The proposed experiments involve conducting tests under anaerobic conditions using plexiglass columns of 30 cm in length and 5 cm in diameter. Each soil column contained different carbon sources, such as wood chips, mulch, and soil/sawdust combinations, to determine their effectiveness as reactive materials for nitrate degradation. The bacterium Pseudomonas sp. was selected based on its denitrification capabilities, and four sampling ports were distributed along the columns to obtain better data resolution for DOC and NO3- concentrations. Through these experiments, we aimed to get a detailed and comprehensive dataset for validating kinetic model parameters, such as microbial growth yield, rate constants, and Monod half-saturation constants. Our main objective is to establish the ideal kinetic parameters for heterotrophic denitrification to minimize the migration of nitrates into groundwater through in-situ remediation strategies.

The fate of emerging contaminants from runoff/storm waters in urban areas has aroused widespread concern as it poses a threat to the water managing. Contaminated water can reach, for instance, the river that passes through the area, contributing to the degradation of the aquatic ecosystem, or the aquifer that supplies drinking water to the community. In this framework, one of the objectives of the URBANWAT project is to evaluate the capacity of the Barcelona urban soil to retain the contaminants of emerging concern from runoff waters in order to propose an improvement of tools and criteria for groundwater management in urban areas.To this aim, a set of batch and packed soil column experiments were performed. The soil selected for the study is located in the Barcelona urban area at 20 m depth and composed of 48 % quartz, 28 % albite and 24 % microcline. It is constituted by 91 % sand, 7 % silt and clay and 2 % gravel, being the sand particle size dimension the one selected to perform the column experiments. The resident water in contact with the soil was analysed by ICP-MS and has a conductivity of 723 µS/cm and pH of 7.9. A list of representative emerging contaminants has been selected from diverse pharmaceutical families and UV filters based on the concentration values found in the runoff waters of the Barcelona urban area.The batch experiments were carried out first to verify if the soil already contained the target contaminants and second to know the soil sorption capacity. The target contaminants have different properties (pka, charge) that cause them to be sorbed with variable efficiency by the soil tested (80-100 % sorption for Paroxetine and Venlafaxine, 60-80 % for Cocaine and Caffeine, 40-60 % for Tradadol and Climbazole and low sorption (< 40 %) for a list of more than 15 emerging contaminants). Sorption capacities obtained in the batch experiments were also identified in the percolation tests, obtaining significantly different breakthrough curves for the studied target contaminants. By means of the percolation column experiments, the effect of the flow rate on the soil retention capacity was evaluated and the two main processes involved in the pollutants retention mechanism (sorption and biodegradation) were quantified to verify the specific contribution of each process to the global procedure.

This study attempts to give some answers on how the electrical signal is impacted by conduit formation in limestone due to calcite dissolution, and how the electrical properties can be related to evolving structural parameters. Ensuring sustainable strategies to manage water resources in karst reservoirs requires a better understanding of the mechanisms responsible for dissolution features in the rock mass. The dissolution of carbonate core samples caused by CO2 or acid solution injection has already been well studied in the laboratory to understand the formation of conduits and their intricate coupling with transport properties such as permeability and porosity. However, these experiments generally rely on image analysis, an accurate technique that cannot be used in the field. Additionally, in a subsurface context, chemical analysis of the pore water can be quite intrusive, providing only restricted and spatially limited information. Thus, studying large-scale heterogeneities such as karst environments can benefit from the use of non-invasive tools such as the ones proposed in hydrogeophysics. In particular, geoelectrical methods are good candidates to detect the emergence of karstification related to the heterogeneous dissolution of large volumes since they present a high sensitivity to the physical and chemical properties of both porous matrix and interstitial fluids. We monitored the electrical conductivity, porosity, and permeability of two limestone core samples during controlled dissolution experiments driven by acid injection at atmospheric conditions under different flow rates creating preferential conduits. These two samples were also characterized before and after the acid percolation with laboratory methods and CT scan imaging. First, we confront the electrical conductivity variations to the evolution of permeability with time. We show that monitoring electrical properties allows us to sense the impact of dissolution in the porous medium long before the sample is percolated. This result is a key finding to highlight the great interest that electrical properties can represent in monitoring reactive percolation in karst systems. Then, we interpret the monitored electrical conductivity of the acid percolation with a physics-based model. This model describes the porous medium as a fractal cumulative distribution of tortuous capillaries with a sinusoidal variation of their radius. According to the model description, the sample electrical conductivity is interpreted in terms of effective structural parameters which are tortuosity and constrictivity. Confronting the model with the experimental results shows that the electrical signature of calcite dissolution is more impacted by the evolution of constrictivity than by tortuosity, while most of the literature focuses on the tortuosity and even neglects constrictivity when describing the pore space complexity with electrical conductivity measurements. Finally, based on our experimental results and data sets from the literature, we show that the characteristic Johnson length is a valuable structural witness of calcite dissolution impact linking electrical and hydrological properties. This small-scale approach to heterogeneous dissolution is an analog of the natural processes involved in forming conduits by dissolution leading to karstification. The results of this study are transferable to large-scale applications such as the survey of karst formation, CO2 geological storage, and geothermal energy recovery.

We evaluated the feasibility of five sustainable materials (clay, zeolite, biochar, compost, and woodchips) to be included in a reactive barrier intended to improve the removal of pharmaceutical active compounds (PhACs) during soil aquifer treatment (SAT). The potential of the five materials to sorb PhACs was assessed and compared to sand capacity through batch experiments. Materials with high organic carbon content (biochar, compost, and woodchips) exhibited a greater capacity to sorb PhACs than materials with a low organic carbon content (clay and zeolite). The ion speciation of the molecules is a major parameter controlling the fate and transport of PhACs during SAT operations. Anionic species are the ones with the highest risk of reaching the aquifer since they are generally less sorbed compared to cationic and neutral species. Experimental parameters such as contact time, solid:water ratio, and grain size had no a significant effect on the sorption of PhACs in the studied materials. The incorporation of biochar, compost, and/or woodchips into reactive barriers will promote the sorption of PhACs and will boost the ability of the SAT system to improve the quality of the recharged water.

Ensuring sustainable strategies to manage water resources in karst reservoirs requires a better understanding of the mechanisms responsible for conduits formation in the rock mass and the development of detection methods for these hydrological and geochemical processes. In this study, we monitored the electrical conductivity of two limestone core samples during controlled dissolution experiments. We interpret the results with a physics-based model describing the porous medium as effective structural parameters that are tortuosity and constrictivity. We obtain that constrictivity is more affected by calcite dissolution compared to tortuosity. Based on our experimental results and data sets from the literature, we show that the characteristic Johnson length is a valuable structural witness of calcite dissolution impact linking electrical and hydrological properties.