Limestone aquifers constitute important drinking water resources but are vulnerable to groundwater contaminations. They are also known to be often heterogeneous. However, the hydraulic properties of their entire vadose zone and the relationship of these hydraulic properties with their mineralogical and geochemical characteristics remain rarely studied. The hydraulic properties of soft (soil, powdery limestone, and calcareous sand) and hard (limestone rock) materials sampled throughout the entire vadose zone profile of the Beauce limestone aquifer (France) were determined with
A series of laponites and synthetic OH- and fluorinated hectorites prepared from hydrothermal and melting experiments at both industrial and laboratory scale were examined with XRD and FTIR (MIR and NIR) to determine their mineralogical composition and possible compositional heterogeneity. The end materials contained both Li- and Na-bearing phases. The industrial hydrothermal OH-smectites prepared at low temperatures consist of random mixed layer hectorite-stevensite-kerolite with about 40–50% hectorite layers, the remaining being stevensite and kerolite at roughly equal proportions. The FTIR spectra of these smectites contain, besides the main Mg3OH stretching/overtone bands at 3695–3690 and 7225–7214 cm−1, respectively, additional OH overtone bands at ~3716 and 7265 cm−1 (hydrated state). These bands might be linked to Mg2LiOH stretching modes. The melt-derived smectites are kerolite-free but still contain stevensite layers, although the preparation methods involved heating in the excess of 1000 °C. In these smectites Li might be partitioned to both octahedral and interlayer sites. Subsequent annealing of the melt-derived Mg-Li smectites caused migration of the exchangeable Li to the vacant octahedral due to the Hofmann-Klemen effect and thus decrease of the layer charge, as was indicated by the νO-D method. Hydrothermal synthesis of Mg-Li smectites at high temperature (400 °C) and pressure (1 kbar), yielded pure hectorite without stevensite or kerolite domains.
<p>Water retention and hydraulic conductivity are the most important properties governing water flow and solute transport in unsaturated porous media. However, transport processes in the vadose zone (VZ) are still not completely understood, in spite of their importance for the preservation and management of aquifers, especially in the geographic zones under intensive agriculture. This study has been carried out as part of the construction of the O-ZNS platform (Observatory of transfers in the vadose zone). This platform aims to integrate observations over a wide range of spatial and temporal scales thanks to a large access well (depth&#8211;20 m & diameter&#8211;4m) surrounded by several boreholes in order to combine broad characterization and focused monitoring techniques.</p><p>Three cored boreholes have been drilled in Spring 2017. Structural and mineralogical analyses were carried out for four types of materials sampled throughout the entire VZ profile (20 m depth) including soft sediments (soil, marl and sand) and fractured limestone rock. Hydraulic properties (q(h) and K(h)) were measured on representative core samples by means of a triaxial system used by applying the multistep outflow method. Simulations were then made using HYDRUS-1D to simulate water flow and bromide (conservative tracer) transport over 50 years using meteorological and water table level data.</p><p>The results brought valuable information about factors contributing to the heterogeneity of hydraulic properties within the VZ. For the applied matric heads (from 0 to -1000&#160;cm), the water content and hydraulic conductivity of (i) the soft materials (9 samples) ranged from 0.173 to 0.485&#160;cm<sup>3</sup>/cm<sup>3</sup> and from 1.26.10<sup>-5</sup> to 2.41 cm/d, respectively ; (ii) the hard materials (5 samples) ranged from 0.063 to 0.340&#160;cm<sup>3</sup>/cm<sup>3</sup> and from 8.54.10<sup>-5</sup> to 1.82&#160;cm/d, respectively. The shape of the water retention and hydraulic conductivity curves obtained for the soft sediments is strongly related to the physical properties of the material but also to the proportion and the nature of clay minerals. The soil material displayed the largest average water retention capacity due to the presence of smectite and kaolinite, indicating weathering and matrix transformation. The water retention capacity of the marl and sand materials was lower due to higher content in palygorskyte and calcite. The limestone rock materials displayed an important heterogeneity in their hydraulic properties. Mineralogical analysis helped understanding water flow pathways within the limestone aquifer. The non-altered matrix, that seemed impermeable at first sight, presented few thin microfractures where water probably accumulates. The altered matrix showed microfractures where water has circulated and calcite has been replaced by phyllosilicates, thus increasing the water retention capacity. Natura macrofractures observed at dm-scale showed the presence of iron oxides which highlighted an exposure to high water flow. Simulations made using HYDRUS-1D allowed a first estimation of water and solutes travel time through this highly heterogeneous vadose zone. The results highlighted transfer time of between 25 to 35 years for the bromide to reach water table. The differences observed between the three cored boreholes were mainly due to the heterogeneity of the marl materials located between 1 and 7 m deep.</p>
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