Joël Rodet

Assessment of direct transfer and resuspension of particles during turbid floods at a karstic spring

Turbid water can be the source of important sanitary problems in karstic regions. It is the case of the Pays de Caux, in Haute Normandie, where the main resource in drinking water is provided by the chalk aquifer. In the case of the typical binary karst of the Pays de Caux, turbidity results from the input in sinkholes of turbid surface water induced by erosion on the plateaus. At some spring tappings, water may be very turbid in period of intense rainfall. The turbidity observed at a karstic spring is a complex signal which contains a part of direct transfer and a part of resuspension of the particles being transported. The aim of this study is turbidigraph separation, which would permit to distinguish the direct transfer and resuspension components of the turbidigraph. These two components are separated by comparing the elementary surface storm-derived water fluxes and elementary turbidity signals at the spring. The procedure takes place in three phases: (i) spring hydrograph separation by means of a two components mixing model (surface water and karstic groundwater) using specific electrical conductivity, (ii) decomposition of storm-derived water flux and turbidity thanks to the second-derivative method, (iii) comparison of the transfer times (< modal times) of the elementary turbidity and surface water flux signals, respectively. The mass corresponding to direct transfer, computed after signal decomposition, is then used to re-calculate a particle recovery rate, which passes so from 51 ^ 4 to 37 ^ 3%. Relations between particle flux and hydrodynamics show that resuspension can be either the fact of the dynamics of the introduction system, or that of the chalk karstic aquifer in general (case of resuspension not associated to surface water flux). In this sense, evolution of particle flux (and consequently of turbidity) can be also a marker of the karst structure. q 2003 Elsevier Science B.V. All rights reserved.

Polyphase karst system in Cretaceous chalk and calcarenite of the Belgian-Dutch border

Along the Belgian-Dutch border, underground and surface quarries dug in Cretaceous calcarenite and chalk intersect many karst features as well as deep large nodes of weathered rock. Their observation allows the reconstruction of the genesis of an original karst system resulting from the merging of initially independent endokarsts and exokarsts. Deep weathering has developed within the Cretaceous formations, creating nodes of weathered chalk and closed cavities. These phenomena are expanded over time and can form interconnected voids. Near the surface, solution pipes are generated under the coarsest deposits of a fluvial terrace capping the Cretaceous formations. These pipes develop vertically and may be related to the progressive lowering of the water table in connection with the incision of the Meuse valley. Some of these phenomena cut up the older endokarsts and organize complex systems of out-flow within the chalk.

Le réseau de fracturation, facteur initial de la karstification des craies dans les collines du Perche: l'exemple du site de la Mansonnière (Bellou-sur-Huisne, Orne, France)

Abstract Tectonics has been known to be the initial factor of karstification for a long time, but this has never been demonstrated because of the progressive nature of karstification, which destroys evidence of the preliminary phases. Because of its dual hydrological quality, chalk limestone has conserved a great variety of specific forms illustrating various steps in evolution, from the initial primokarst to the classic well-developed drain. In the Perche hills, very recent research has shown specific endokarstic galleries, filled up by sands and clays. Physical, chemical and X-ray analyses have shown that such infillings result not from a classical fluviatile deposit process, but on the contrary, from an alteration process, underlining the progress of a weathering front into the limestone mass down to the watertable. The infillings are not sedimentary deposits, but represent a kind of “shadow rock”, a chemical in situ transformation of limestone, without any transport of the solid fraction. The genesis results from the progression of weathering fronts located on the tectonic pattern into the input karst. When the weathering front crosses the watertable, the resulting water mixing produces a chemical reaction capable of opening the original joint. This results in infilled galleries, similar to classical karst drains, which have never known fluviatile drainage. This illustrates the first step in normal karstic evolution, just before a water flux drains the gallery, resulting in a new karstification step: the drained passage. The specificity of the region of the Perche hills is this karst evolution stopped in the first step, illustrating the “primorkarst” modelling by the researcher, which has never been described before. The conditions for this genesis are a well-developed tectonic pattern, an absence of thick superficial layers, a high soil permeability, and an absence of superficial drainage (lack of sinkholes).

Gravettian painting and associated activity at Le Moulin de Laguenay (Lissac-sur-Couze, Corrèze)

Presented here is the so far unique discovery and interpretation of an occupation area directly associated with Upper Palaeolithic cave paintings. The paintings, of red spots and hand stencils, overlook two hearths with selected flints. There were also fragments of stalactite, deduced by analysis and experiment to be waste products from the manufacture of beads. The authors deduce that the hearths and their assemblage complement the ritual nature of the paintings.

Microgranulometric approach to a chalk karst, western Paris Basin, France

Abstract By definition, karst is the result of dissolution, and if the rock is not completely soluble, residues will remain (“acquired” particles). These insoluble residues provide a history of karstic activity and can be found in the outflows after a possible storage period in the endokarst. A direct connection, even if temporary, between the endokarst and the surface is reflected by the contribution of particles, which are referred to as “inherited”. We have studied the chalk karsts of Haut Normandie by comparing microgranulometric spectra of suspended matter (SM), in subterranean waters and in solutions of the main surface formations (Clay-with-Flints Complex (CFC) and loess) and the Chalk dissolution products of all local stratigraphic levels. Based on these microgranulometric spectra, we propose a conceptual model for processes occurring in chalk karsts and a classification scheme for karstified systems according to how such systems deal with particles. In a “closed” karst, the suspended matter tends to come from the Chalk itself, while in the case of an “open” karst, the majority of suspended particles comes from surface formations. This notion of “openness” differs from the currently used categorization into allogenic and autogenic systems, which depends upon an impermeable cover concentrating the infiltration.

Karst and Underground Landscapes in the Cretaceous Chalk and Calcarenite of the Belgian-Dutch Border—The Montagne Saint-Pierre

The eastern part of the Hesbaye plateau, which extends into the Dutch Limburg, was excavated by many underground quarries that mainly exploited the Cretaceous chalk of the Maastrichtian. In this area, the Meuse and its tributaries cut into the shallow dipping formations. Thus, entries of quarries usually are on hillsides. Most galleries sink horizontally along calcarenite layers, a coarse chalk also called «Tuffeau of Maastricht». The dimensions and extensions of these networks create real underground landscapes. These galleries intersect thousands of karst features and allow a privileged observation in three dimensions. These karst phenomena have dual characteristics: they mainly grow in a very porous lithology and outside fractures, so a context unfavourable to, a priori, a necessary concentrated weathering. Once dewatered, they cease to function, thus fossilizing a karst system in one of the rarely seen early genetic phases.

Dual-Flow Phenomena and Longitudinal Dispersivity Interpretation in a Karstic Aquifer

In situ tracer tests constitute one of the best means to study flow dynamics and transport properties of karst aquifers. In this paper, we attempted to interpret karst-conduit flow and transport parameters by using two different models: the linear graphical method and the Qtracer2 program. The latter program permits us to quantify the karst-conduit parameters (morphological type, flow patterns). The linear graphical method allows us to account for two elementary curves (two-layers flow model) that fit the entire breakthrough curve, notably when strong tailing effects occur. The flow parameters previously calculated by Qtracer2 show that flow in the conduit is dual, with a turbulent-core flow and a pseudo-laminar flow against the wall of the conduit within the so-called viscous sublayer. The lower the flow-rate, the higher the viscous sublayer. This latter result is in agreement with the decrease of the second elementary curve contribution to the total area of the breakthrough curve with increasing flow-rate. We agree that partitioning of flow would also induce a partitioning of solute transport (via the turbulent flow and the viscous sublayer). From a geomorphological view, the karst hydrosystem studied appears to be well established around a main conduit that carries the most important part of the flow. (1) PhD, Lecturer, UMR CNRS 6143, Universite de Rouen, 76821 Mont-SaintAignan Cedex, France, Nicolas.Massei@univ-rouen.fr (2) PhD, Professor, Laboratoire de Mecanique, Physique et Geosciences, Universite du Havre, 76058 Le Havre, France (3) PhD, Research Hydrogeologist, National Center for Environmental Assessment (8623D), Office of Research and Development, U.S. Environmental Protection Agency, 1200 Pennsylvania Ave., NW, Washington DC 20460-0001, USA (4) PhD, Professor, UMR CNRS 6143, Universite de Rouen, 76821 Mont-SaintAignan Cedex, France (5) PhD, CNRS Researcher, UMR CNRS 6143, Universite de Rouen, 76821 MontSaint-Aignan Cedex, France