Identification of runoff generation processes using combined hydrometric,tracer and geophysical methods in a headwater catchment in South Africa / Identification des processus de génération de l'écoulement par combinaison de méthodes hydrométriques,de traçage et géophysiques dans un bassin versant sud-africain

Résumé

Des mesures hydrométriques classiques et l'imagerie 2-D de résistivité électrique (IRE) détaillée ont été combinées avec le prélèvement de traceurs pour identifier les processus hydrologiques en jeu dans un bassin versant semi-aride de la province Cap-Oriental, en Afrique du Sud. L'étude des événements de précipitation et d'écoulement a souligné la forte relation qui existe entre les précipitations et les caractéristiques de la génération des écoulements. L'observation du potentiel hydrique du sol ainsi que du niveau d'eau souterraine a mis en évidence le développement d'une nappe perchée dans le sol. Ces résultats sont confirmés par des séparations d'hydrogrammes à base de traceurs et illustrent le rôle important des écoulements souterrains superficiels. L'étude par IRE a permis de mieux connaître la structure du sous-sol. Finalement, l'étude par IRE, combinée avec des mesures obtenues par réflectométrie en domaine temporel (TDR), a permis d'extrapoler à des mesures sélectives de teneur en eau. Pour récapituler, l'utilisation et la combinaison de différentes méthodes de terrain ont mené au développement d'un modèle conceptuel du fonctionnement hydrologique de ce bassin versant. Le rôle dominant des mécanismes de subsurface a été démontré.     

Analysis of karst hydrodynamics through comparison of dissolved and suspended solids' transport

In karst systems, rain events often result in a decrease of the conductivity (a tracer of dissolved phase transport) and an increase in turbidity (a tracer of suspended solids transport) at wells and springs. This study shows that the comparison of suspended solids and solute transport by the coupled approach of $T\text{–}C$ curves (Turbidity–Conductivity) and autocorrelations gives evidence of the transport processes in the karst network and allows understanding the karst hydrodynamics. To cite this article: D. Valdes et al., C. R. Geoscience 337 (2005).     

Electrochemical properties of solids at the aqueous–solid interface and heterogeneity of surfacePropriétés électrochimiques des solides à l'interface solide–eau et hétérogénéité de surface

In aqueous medium, solid surfaces are in general electrically charged. The induced electrical and chemical properties govern numerous phenomena, such as colloidal stability or transport of pollutants. Numerous industrial processes make use of these properties. The understanding of the underlying mechanisms at molecular level is of high importance in order to predict and master the behaviour of dispersed matter in the environment and in industrial processes. The present paper shows the evolution of theories and experimental methods, their recent developments and applications. To cite this article: F. Thomas et al., C. R. Geoscience 334 (2002) 633–648.     

Hydrogeological investigation in the Mekong Delta around Ho-Chi-Minh City (South Vietnam) by electric tomographyInvestigation hydrogéologique dans le delta du Mékong,autour de la ville de Ho-Chi-Minh (Sud-Vietnam), par tomographie électrique

In the Mekong Delta (South Vietnam), the agglomeration of Ho-Chi-Minh (HCM) City, with more than 5 million inhabitants, is confronted with a dramatic shortage of fresh water supply because of the pollution of several aquifers at different depths. The electric tomography, obtained by concurrent inversion of two complementary geoelectrical methods, the Vertical Electrical Sounding (VES) and the Magneto-Telluric Sounding (MTS), turned out to be very efficient to provide a complete electrical image of the underground from the surface until about 800 m depth. This methodology constitutes a very cheap guide for the evaluation of the quality of the groundwater resources in the vast alluvial plain of the Mekong Delta. To cite this article: V.N. Pham et al., C. R. Geoscience 334 (2002) 733–740.     

Estimation of LNAPL saturation in fine sand using time-domain reflectometry / Estimation de la saturation en LPNAL dans du sable fin grâce à la réflectométrie en domaine temporel

Abstract

Abstract Recently, substantial progress has been made in detection and observation of non-aqueous phase liquids (NAPLs) in the subsurface using different experimental techniques. However, there is still a lack of appropriate direct methods to measure the saturation of NAPL (θ_NAPL). This paper provides a guide for estimating θ_NAPL and water content (θ _w ) in unsaturated and saturated sand based on direct measurements of soil dielectric constant (K_a ) and electrical conductivity (σ _a ) using time domain reflectometry (TDR). The results show that the previously used dielectric mixing model fails to predict θ_NAPL in the case of a four-phase system. A new methodology is suggested and exemplified by showing that the measured K_a gives accurate estimation of θ_NAPL for a three-phase system while in a four-phase system, both θ _a and K_a need to be measured. The results show that using the suggested methodology, accurate predictions of θ _w (R ² = 0.9998) and θ_NAPL lower than 0.20 m³ m^-3 (average R ² = 0.9756) are possible.     

Cartographie de corps stériles sous couverture quaternaire par méthode de résistivités électriques dans le gisement phosphaté de Sidi Chennane (Maroc)Sterile bodies mapping under Quaternary cover using resistivity-sounding method in the phosphatic bearing of Sidi Chennane (Morocco)

In the Ouled Abdoun sedimentary basin (Morocco), the phosphatic series is composed of regular interbedded phosphatic and marly limestone layers. Some phosphatic deposits in this basin show sterile bodies causing two kinds of problems: (1) as they are hard, compact and masked by a Quaternary cover, they disturb the exploitation in some yards and give bad reserve calculations; (2) even the use of wells and mechanical boreholes did not evidence their delimitation. Therefore, electric prospecting method has been used to evidence their geometrical shape. Petrographical and geometrical studies on these sterile bodies allowed us to choose the appropriate geophysical method to map them. The electrical resistivity survey that we used in the Sidi Chennane area shows that this technique is a good tool to contour these sterile bodies. This method is now considered as useful to the mining engineers to get round them during the exploitation. To cite this article: A. Kchikach et al., C. R. Geoscience 334 (2002) 379–386.     

Delineation of groundwater zones using lithology and electric tomography in the Khartoum basin, central Sudan

The three towns of Khartoum, Omdurman and Khartoum are located north of the capital of Sudan. This capital has agglomerated to reach more than 7 million inhabitants. The present situation is confronted with a dramatic shortage in freshwater supply. The treatment of surface water from the Nile became costly, particularly in flood season. Thus the authorities turned towards developing groundwater resources. The electric tomography, obtained by the combination of Vertical Electrical Sounding (VES) and well logs, turned out to be very efficient to provide a complete electrical image of the underground from the surface until about 200-m depth. This methodology constitutes a very cheap guide for the evaluation of the quality of groundwater resources in the Khartoum area. To cite this article: M.T. Hussein, H.S. Awad, C. R. Geoscience 338 (2006).