Nicolas Massei

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.

Transport of particulate material and dissolved tracer in a highly permeable porous medium: comparison of the transfer parameters.

We are experimentally studying, by means of short-pulse injection, the transport and deposition kinetics of suspended particles (silts of the order of 10 microm) in a highly permeable medium consisting of a column of gravel. In our experiments, the breakthrough curves (BTCs) are well described by analytical solutions of a convection/dispersion model with first-order deposition kinetics. All the transport parameters calculated by the model for both particles and dissolved tracer depend on the flow rate. We demonstrate the existence of a critical flow rate, determined experimentally, beyond which the transfer time for the particles is longer than that for the tracer. This phenomenon is unusual in comparison with the results available in the literature. The increase in transfer time of particles in comparison to tracer leads us to assume a purely mechanical phenomenon, that is, collision between particles and grains of the medium with instantaneous reset in motion when the flow rate is sufficient to avoid settling. Thanks to the polydispersivity of the injected suspension and the control of grain size at the outlet, it can also be determined that the coarser particles are recovered before the finer particles, as expected when one considers the size-exclusion effect.

Transport of suspended solids from a karstic to an alluvial aquifer: the role of the karst/alluvium interface

This study focuses on the coupled transport of dissolved constituents and particulates, from their infiltration on a karst plateau to their discharge from a karst spring and their arrival at a well in an alluvial plain. Particulate markers were identified and the transport of solids was characterised in situ in porous and karstic media, based on particle size analyses, SEM, and traces. Transport from the sinkhole to the spring appeared to be dominated by flow through karst: particulate transport was apparently conservative between the two sites, and there was little difference in the overall character of the particle size distribution of the particulates infiltrating the sinkhole and of those discharging from the spring. Qualitatively, the mineralogy of the infiltrating and discharging material was similar, although at the spring an autochthonous contribution from the aquifer was noted (chalk particles eroded from the parent rock by weathering). In contrast, transport between the spring and the well appears to be affected by the overlying alluvium: particles in the water from the well, showed evidence of considerable size-sorting. Additionally, SEM images of the well samples showed the presence of particles originating from the overlying alluvial system; these particles were not found in samples from the sinkhole or the spring. The differences between the particulates discharging from the spring and the well indicate that the water pumped from the alluvial plain is coming from the karst aquifer via the very transmissive, complex geologic interface between the underlying chalk formation and the gravel at the base of the overlying alluvial system.

Nutrient dynamics as indicators of karst processes: Comparison of the Chalk aquifer (Normandy, France) and the Edwards aquifer (Texas, U.S.A.)

Karst aquifers display a range of geologic and geomorphic characteristics in a wide range of climatic and land-use settings; identification of transport dynamics representative of karst aquifers in general could help advance our understanding of these complex systems. To this end, nutrient, turbidity, and major ion dynamics in response to storms were compared at multiple sites in two karst aquifers with contrasting characteristics and settings: the Chalk aquifer (Eure Department, Normandy, France) and the Barton Springs segment of the Edwards Aquifer (Texas, U.S.A.). The Chalk aquifer is typified by high matrix porosity, thick surficial deposits (up to 30 m thick), and agricultural land use; the Barton Springs segment is typified by low matrix porosity, outcropping limestone, and urban land use. Following one to three storms, from 5 to 16 samples from springs and wells were analyzed for major ions, and specific conductance and turbidity were monitored continuously. Comparison of the chemographs indicated some generalized responses, including an increase in turbidity and potassium concentrations and a decrease in major ion and nitrate concentrations with infiltrating storm runoff. Factor analysis of major ions and turbidity revealed strikingly similar behavior of the chemical variables for the two aquifers: The first two factors, explaining more than 75% of the variability, illustrate that dynamics of most major ions (including nitrate) are opposed to those of turbidity and of potassium. The results demonstrate that potassium and nitrate are effective tracers of infiltrating storm runoff and resident ground water, respectively, and the similar results for these two highly contrasting aquifers suggest that the dynamics identified might be applicable to karst systems in general.

Polycyclic aromatic hydrocarbon transport processes on the scale of a flood event in the rural watershed of Le Bebec, France

Transport processes of polycyclic aromatic hydrocarbons (PAHs) were investigated in a rural watershed in France, which led to new quantitative results on the scale of a flood event. For that purpose, principal components analysis (PCA) combined with signal decomposition of PAH Breakthrough curves were used. The results obtained by PCA enabled us to distinguish (i) the transport of low molecular weight PAHs in dissolved form and (ii) the transport of high molecular weight PAHs in particulate form. The intermediate molecular weight PAHs are distributed in these two clusters. The application of a signal decomposition method confirms the results of PCA and makes it possible to quantify the mass PAH resulting, respectively, from resuspension (13.3mg, i.e. 77% of the total mass) and from direct transfer (2.8mg=16% of the total mass). For the resuspension phase, desorption of PAH (38% of the total exported mass) could be also distinguished from sediment resuspension (39%).

Transport des particules en milieu poreux : détermination des paramètres hydrodispersifs et du coefficient de dépôt

Abstract We present a chromatographic short-pulse technique for studying suspended particle transport and deposition kinetics in a porous medium. A mathematical method is proposed to determine hydrodispersive characteristics and deposition rates from the breakthrough curves. The results of breakthrough data interpretation demonstrate that all the parameters obtained (fraction of particles recovered, deposition rate, effective porosity and dispersivity) vary with the flow rate. The relationships between these parameters and Darcy’s flow velocity can be given by power laws.

Investigation of Transport Processes inside Karst Aquifer by Means of STATIS

Environmental data sets are often multidimensional and consequently display complex structure. This article shows the limitations of principal component analysis (PCA) for the study of such three-dimensional (3D) data sets. These limitations can be resolved by the use of the statistical tool STATIS. The inlet (a swallow hole) and the outlet (a spring) of a karst system of the Western Paris basin were sampled during three rain events of various intensities. These 3D geochemical data sets (variables x sites x dates) for a karst system were analyzed by STATIS method to identify hydrological processes. STATIS proceeds in three steps (interstructure, compromise, and intrastructure), which allows us to focus the analysis of hydrologic systems at different temporal and spatial scales. Compromise plane shows that suspended matter and flood are not simultaneous and highlights a rapid flow, characterized by turbidity and phosphate, which represents a point source contamination, and a ground water flow contaminated by nitrate. Intrastructure plane allows us to compare hydrochemical variations between the swallow hole and the spring lead. By this way, hydrological processes such as direct transfer and resuspension of intrakarstic sediments before and after the flood were identified what cannot be realized by comparison of inlet and outlet breakthrough curves. Finally, results obtained from the same data set by STATIS and a coupled study using PCA and normalized hysteresis curves were compared. This comparison shows the efficiency of STATIS at the identification of transport processes and vulnerability of karst system and its potential for hydrological applications.

Hydrological variability of major French rivers over recent decades, assessed from gauging station and GRACE observations

Abstract This study was carried out in the framework of the Surface Water and Ocean Topography (SWOT) programme of the French National Centre of Space Studies (CNES). Based on discharge measurements and Gravity Recovery and Climate Experiment (GRACE) determination of total water storage (TWS), we have investigated the hydrological variability of the main French drainage basins (Seine, Loire, Garonne and Rhône) using a wavelet approach (continuous wavelet analyses and wavelet coherence analyses). The results of this analysis have shown a coherence ranging between 82% and 90% for TWS and discharge, thus demonstrating the potential use of TWS for characterization of the hydrological variability of French rivers. Strong coherence between the four basin discharges (between 73% and 92%) and between their associated TWS data (from 82% to 98%) suggested a common external influence on hydrological variability. To determine this influence, we investigated the relationship between hydrological variability and the North Atlantic Oscillation (NAO), considered as an index of prevailing climate in Europe. Basin discharges show strong coherence with NAO, ranging between 64% and 72% over the period 1959–2010. The coherence between NAO and TWS was 62% to 67% for 2003–2009. This is similar to the coherence between NAO and basin discharges detected for the same period. According to these results, strong influence of the NAO was clearly observed on the TWS and discharges of the major French river basins. Editor Z.W. Kundzewicz

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.

Hydrological role of karst in the Chalk aquifer of Upper Normandy, France

The role of karst on large-scale groundwater flow is defined for the Chalk aquifer of Upper Normandy (western Paris Basin), France. In the regional context, chalk plateaus occupy the greater part of watersheds and are the main sites of groundwater recharge. Previous studies focused on karstic output systems in the valleys and less on water-level variations in the recharge zones upstream. This study assesses the relevant hydrogeological processes using time-series data (boreholes and springs) recorded along a down-gradient hydrologeological cross-section in two selected watersheds. These hydrological data are interpreted in the framework of previous descriptions of the morphological organization of the study area’s karst network. The results highlight the hydrological role of (1) the input karst (vertical conduits) which drains recharging water, (2) the output karst (sub-horizontal conduits widely developed in the vicinity of valleys in the surface watersheds) which drains the output flows, and (3) the connections between these two (input and output) networks, which control the upstream water levels and allow quick transfer to springs, particularly after strong rainfall events. A conceptual model of the hydrological functioning of this covered karst aquifer is established, which should serve for the structuring and parameterization of a numerical model.RésuméLe rôle du karst sur les écoulements d’eau souterraine à grande échelle est définie pour l’aquifère de la craie de la Haute Normandie (Ouest du Bassin de Paris) en France. Du point de vue du contexte régional, les plateaux crayeux occupent la plus grande partie des bassins versants et constituent les principaux sites de recharge de l’aquifère. Les études précédentes sont ciblées sur les systèmes karstiques d’exhaure dans les vallées et moins sur les variations des eaux souterraines dans les zones de recharge à l’aval. Cette étude évalue les processus hydrogéologiques pertinents à partir de l’analyse des séries temporelles (dans les forages et au niveau des sources) enregistrées selon un gradient hydrogéologique le long d’une section pour deux bassins versants sélectionnés. Ces données hydrologiques sont interprétées considérant les descriptions antérieures de l’organisation morphologique du réseau karstique de la zone d’étude. Les résultats mettent en évidence le rôle hydrologique (1) du karst d’introduction (conduits verticaux) qui assurent le drainage de la recharge, (2) du karst d’exhaure (conduits sub-horizontaux largement développés à proximité des vallées dans les bassins versants) qui draine les eaux vers les sources et (3) les connexions entre ces deux types de réseaux (introduction et exhaure), qui contrôlent les niveaux d’eau souterraine dans les parties amont des bassins versants et permettent le transfert rapide de l’eau vers les sources, en particulier après d’événements pluvieux importants. Un modèle conceptuel du fonctionnement hydrologique de cet aquifère sous couverture a été établi, qui servira à la structure et à la paramétrisation d’un modèle numérique.ResumenSe define el rol del karst en el flujo de agua subterránea a gran escala para el acuífero Chalk de Alta Normandía (oeste de la cuenca de París), Francia. En el contexto regional, las mesetas de creta ocupan la mayor parte de las cuencas y son los principales sitios de la recarga del agua subterránea. Los estudios previos se han enfocado en los egresos de los sistemas kársticos en los valles y en menor grado en las variaciones de los niveles aguas arriba en las zonas de recarga. Este estudio evalúa los procesos hidrogeológicos relevantes usando datos de series de tiempo (pozos y manantiales) registrados a lo largo de una sección hidrogeológica transversal gradiente abajo en dos cuencas seleccionadas. Estos datos hidrológicos son interpretados en el marco de las descripciones previas de la organización morfológica en la red kárstica del área de estudio. Los resultados resaltan el rol hidrológico de (1) el ingreso al karst (conductos verticales) que drenan el agua de recarga, (2) el egreso del karst (conductos subhorizontales ampliamente desarrollados en las proximidades de los valle en las cuencas de superficie) que drenan los flujos de egresos, y (3) las conexiones entre estas dos redes (ingresos y egresos), que controlan los niveles aguas arriba y permiten una transferencia rápida a los manantiales, particularmente después de eventos de fuertes precipitaciones. Se establece un modelo conceptual del funcionamiento hidrológico de este acuífero kártico cubierto, lo cual servirá para la estructurar y parametrizar el modelo numérico.摘要本文定义了法国上诺曼底(西巴黎盆地)白垩含水层中岩溶对大范围地下水流的作用。在区域范围内,白垩高原占据汇水区的大部,是地下水的主要补给区。过去的研究集中在河谷的岩溶输出系统,对上游补给区水位的变化研究的 较少。本研究利用两个选择出的汇水区沿顺梯度水文地质剖面记录下的时间序列资料(钻孔和泉)评价了相关的水文地质过程。这些水文地质资料在研究区岩溶网络形态组织先前描述的框架下得到解译。结果强调了水文作用,即(1)传送补给水的输入岩溶(垂直通道)的水文作用,(2)传送输出水流的输出岩溶(地表汇水区河谷附近广泛发育的地下通道)的水文作用,(3)这两个(输入和输出)网络的连接水文作用,这两个网络控制上游水位,使水能快速传输到泉,特别是强降雨之后。建立了这个被覆盖的岩溶含水层水文功能的概念模型,这个模型可以用在数值模型的结构和参数建设上。ResumoDefine-se a função do carso no escoamento em grande escala da água subterrânea para o aquífero do Cré (Craie) da Alta Normandia (Bacia de Paris ocidental). No contexto regional, os planaltos de cré ocupam a maior parte das bacias hidrográficas e são os principais locais de recarga da água subterrânea. Estudos anteriores concentraram-se nos sistemas cársicos de descarga nos vales e menos nas variações dos níveis de água nas zonas de recarga a montante. Este estudo avalia os processos hidrogeológicos relevantes através da utilização de dados de series temporais (furos e nascentes) registados ao longo de um corte hidrogeológico segundo um gradiente descendente em duas bacias selecionadas. Estes dados hidrogeológicos são interpretados no quadro de descrições anteriores da organização morfológica da rede cársica da área de estudo. Os resultados destacam a função hidrogeológica (1) das entradas cársicas (condutas verticais) que drenam a água de recarga, (2) das descargas cársicas (condutas sub-horizontais extensamente desenvolvidas nas vizinhanças dos vales nas bacias hidrográficas superficiais) que drenam os fluxos de descarga e (3) das conexões entre estas duas redes (de entrada e de descarga) que controlam os níveis de água a montante e permitem a transferência rápida para as nascentes, em particular após fortes eventos pluviais. Estabeleceu-se um modelo conceptual do funcionamento hidrológico desde aquífero coberto por um carso que deverá servir para a estruturação e parametrização de um modelo numérico.