Vincent Bailly-Comte

Seasonal variations of CO 2 and 222 Rn in a mediterranean sinkhole - spring (Causse d’Aumelas, SE France)

INTRODUCTION Carbon dioxide dynamics and transfer from soil to the deepest parts of an aquifer underlie the main karstification processes. Cavity ventilation plays an important role in speleothem formation and chemical equilibrium of water in the unsaturated zone of the aquifer. Meteorological parameters (barometric pressure and temperature), cave geometry and fracture networks control exchanges between subterranean and outside atmosphere (De Freitas et al., 1982; De Freitas & Littlejohn, 1987; Smithson, 1991; Christoforou, 1996; Buecher, 1999; Fernandez-Cortes, 2005). Gases currently studied in caves are CO2 and 222Rn. Their concentrations and variations can be explained by different parameters. Carbon dioxide concentrations in cavities vary from 0.03 % to more than 6 % and may have different sources : Batiot-Guilhe C., Seidel J.L., Jourde H., Hébrard O., Bailly-Comte V. 2007. Seasonal variations of CO2 and 222Rn in a Mediterranean sinkhole spring (Causse d’Aumelas, SE France). International Journal of Speleology, 36 (1), 51-56. Bologna (Italy). ISSN 0392-6672.

Interactions of diffuse and focused allogenic recharge in an eogenetic karst aquifer (Florida, USA)

The karstic upper Floridan aquifer in north-central Florida (USA) is recharged by both diffuse and allogenic recharge. To understand how recharged water moves within the aquifer, water levels and specific conductivities were monitored and slug tests were conducted in wells installed in the aquifer surrounding the Santa Fe River Sink and Rise. Results indicate that diffuse recharge does not mix rapidly within the aquifer but instead flows horizontally. Stratification may be aided by the high matrix porosity of the eogenetic karst aquifer. Purging wells for sample collection perturbed conductivity for several days, reflecting mixing of the stratified water and rendering collection of representative samples difficult. Interpretive numerical simulations suggest that diffuse recharge impacts the intrusion of allogenic water from the conduit by increasing hydraulic head in the surrounding aquifer and thereby reducing influx to the aquifer from the conduit. In turn, the increase of head within the conduits affects flow paths of diffuse recharge by moving newly recharged water vertically as the water table rises and falls. This movement may result in a broad vertical zone of dissolution at the water table above the conduit system, with thinner and more focused water-table dissolution at greater distance from the conduit.RésuméL’aquifère karstique supérieur du Centre-Nord de Floride (Etats Unis d’Amérique) est alimenté à la fois de façon diffuse et de façon ponctuelle allogène. Afin de comprendre la circulation de l’eau infiltrée dans l’aquifère, les niveaux piézométriques ainsi que les conductivités électriques ont été enregistrés, et des tests d’infiltration réalisés dans des forages situés aux alentours des pertes et émergences de la rivière Santa Fe. Les résultats montrent que l’eau infiltrée issue de la recharge diffuse ne se mélange pas rapidement dans l’aquifère mais s’écoule plutôt de manière horizontale. La stratification des circulations de l’eau infiltrée peut être favorisée par la très forte porosité de matrice de l’aquifère et par son caractère éogène. La purge des forages pour le prélèvement d’échantillons a perturbé la conductivité électrique durant plusieurs jours, reflétant le mélange d’eau des d’horizons stratifiés et rendant difficile la collecte d’échantillons représentatifs. Des simulations numériques suggèrent que la recharge diffuse influence l’intrusion d’eau allogène des conduits, en augmentant la charge hydraulique dans l’aquifère environnant et en diminuant ainsi le flux dans l’aquifère provenant des conduits. Inversement, l’augmentation de la charge hydraulique dans les conduits influence les écoulements de recharge diffuse en déplaçant verticalement l’eau récemment infiltrée lors de la variation du niveau piézométrique de l’aquifère. Ce déplacement peut aboutir à la présence d’une large zone verticale de la dissolution dans la zone de fluctuation du niveau piézométrique, au-dessus du réseau de conduits, la dissolution affectant une zone plus mince de l’aquifère et de manière plus localisée à une plus grande distance des conduits.ResumenEl acuífero kárstico superior de Floridan en el norte centro de Florida (EEUU) está recargado por recarga difusa y alogénica. Para entender como se mueve el agua recargada dentro del acuífero, se monitorearon los niveles de agua y conductividades específicas fueron monitoreadas y se llevaron a cabo ensayos “slug” en pozos instalados en el acuífero que rodea la elevación y depresión del Río Santa Fe. Los resultados indican que la recarga difusa no se mezcla rápidamente dentro del acuífero sino que fluye horizontalmente. La estratificación verse favorecida por la alta porosidad de la matriz del acuífero kárstico eogenético. El purgado de los pozos para la recolección de muestras perturba la conductividad por varios días, reflejando la mezcla del agua estratificada haciendo dificultosa la recolección de muestras representativas. La interpretación de las simulaciones numéricas sugieren que la recarga difusa impacta la intrusión de aguas alogénicas del conducto incrementando la carga hidráulica del acuífero circundante y reduciendo por lo tanto el influjo entrante al acuífero a partir del conducto. A su vez, el incremento de la carga hidráulica dentro de los conductos afecta las trayectorias del flujo de la recarga difusa moviendo el agua nueva recargada verticalmente a medida que el nivel freático asciende y desciende. Este movimiento puede dar como resultado una amplia zona vertical de disolución en la capa freática por encima del sistema de conducto, con una disolución de la capa freática más delgada y más concentrado a una mayor distancia desde el conducto.摘要位于美国佛罗里达州中北部的佛罗里达喀斯特上部含水层同时接受弥散型补给和外源补给。为了了解补给水在含水层中是如何运动的,我们对潜水水位和特定的导水系数进行了监测,并对环绕圣达菲河流进流出处含水层中布置的井进行了水文地质压水试验。试验结果显示,在含水层中弥散型补给不会与地下水迅速混合,而是水平地流动。早期喀斯特含水层介质的高孔隙度特征可能会有助于层理的形成。为取样而洗井对含水层的导水性造成了若干天的干扰,反映出层状水的混合,这使得代表性水样的采集变得困难。解释性数值模拟显示,弥散型补给通过增大周围含水层的水头来影响导水管道中外源水的入侵,进而减少从导水管道进入含水层的地下水通量。反过来,随着潜水面的升降,导水管道中水头的增大会通过新补给水的垂直移动来影响弥散型补给水的流动路径。这种运动可能会导致在导水管道系统以上的潜水面处形成一个宽阔的垂向溶解区域,在远离导水管道的地方,溶解带变薄且集中。ResumoO aquífero cársico superior da Flórida (upper Floridan aquifer), no centro-norte da Flórida (EUA), é recarregado tanto por recarga difusa como por recarga alogénica. Para se entender como a água de recarga se move dentro do aquífero, foram monitorizados os níveis de água e as condutividades específicas e realizados testes de slug em furos instalados no aquífero em torno de sumidouros e exsurgências do Rio Santa Fé. Os resultados indicam que a recarga difusa não se mistura imediatamente no aquífero, em vez disso flui horizontalmente. A estratificação pode ser auxiliada pela alta porosidade da matriz do aquífero cársico eogenético. Furos de purga para a recolha de amostras perturbaram a condutividade durante vários dias, refletindo a mistura da água estratificada e dificultando a recolha e processamento de amostras representativas. A interpretação de simulações numéricas sugere que a recarga difusa afeta a intrusão de água alogénica a partir da conduta cársica, ao aumentar a carga hidráulica nas vizinhanças do aquífero, reduzindo, desta forma, o influxo para o aquífero a partir da conduta cársica. Por sua vez, o aumento do potencial hidráulico dentro das condutas cársicas afeta as linhas de fluxo da recarga difusa, através do movimento vertical da água recém recarregada, à medida que o nível freático sobe e desce. Este movimento pode resultar na dissolução, na zona de oscilação da superfície freática, de uma ampla zona vertical acima do sistema de condutas cársicas, com uma dissolução mais estreita e focada na zona de oscilação da superfície freática a uma maior distância dessas condutas cársicas.

Use of a flood-routing model to assess lateral flows in a karstic stream: implications to the hydrogeological functioning of the Grands Causses area (Tarn River, Southern France)

The aim of this study is to assess the spatio-temporal variability of lateral flows in the streams of a large karstic basin to construct a conceptual model of karst contributions to flood generation. The lateral flows of the Tarn River, which crosses the Grands Causses karst zone in Southern France, were investigated between several gauging stations along the river. First, through analysing the lateral flows on an event time scale of 30 floods, it was possible to identify the losing and gaining reaches, highlighting a highly variable attenuation/amplification role of karsts on flood generation. Second, the diffusive wave model was used to quantify the lateral flows on an hourly time step within a flood event. The simulations show a high variability of lateral outflows and inflows within a same reach according to the hydrometeorological conditions, with in some cases an inversion of the lateral flow direction during the flood. The results highlight complex surface/groundwater exchanges during a single flood event, with high river losses despite the concurrent flow of large springs feeding the stream. This spatio-temporal variability of the karst influence on flood generation was linked to the aquifer’s structure, which has improved the understanding of the hydro(geo)logical functioning of the Grands Causses massif. Finally, the new methodology proposed here opens challenging perspectives towards a framework for the analysis of surface–groundwater exchanges in karstic rivers.

How Karst Areas Amplify or Attenuate River Flood Peaks? A Response Using a Diffusive Wave Model with Lateral Flows

This paper investigates the role of karst aquifers on flood generation and propagation using the Hayami Diffusive Wave (DW) model accounting for uniformly distributed lateral flows. The inverse model was applied on the main channel reaches of the Tarn basin at Millau (2,400 km2) in southern France to assess lateral inflows from karstic springs as well as lateral outflows from river losses. Results show that the DW model, which is simple, parsimonious, and easy-to-use, is able to quantify lateral flows avoiding difficult parameterisation. Surface/groundwater exchanges were characterised on several reaches along the stream, showing a highly variable attenuation/amplification influence of flood peak by karst units during a single flood event. We showed that the upstream part of the karst area have a dominant attenuation role by re-infiltrating part of runoff from the head-water basin in hard-rock areas, while the downstream part have a dominant amplification role due to high contributions of karst groundwater. These results improved the conceptual hydrogeological model of the Grands Causses region.