Direct simulation of groundwater transit-time distributions using the reservoir theory

Abstract

Groundwater transit times are of interest for the management of water resources, assessment of pollution from non-point sources, and quantitative dating of groundwaters by the use of environmental isotopes. The age of water is the time water has spent in an aquifer since it has entered the system, whereas the transit time is the age of water as it exits the system. Water at the outlet of an aquifer is a mixture of water elements with different transit times, as a consequence of the different flow-line lengths. In this paper, transit-time distributions are calculated by coupling two existing methods, the reservoir theory and a recent age-simulation method. Based on the derivation of the cumulative age distribution over the whole domain, the approach accounts for the whole hydrogeological framework. The method is tested using an analytical example and its applicability illustrated for a regional layered aquifer. Results show the asymmetry and multimodality of the transit-time distribution even in advection-only conditions, due to the aquifer geometry and to the velocity-field heterogeneity.RØsumØ Les temps de transit des eaux souterraines sont intØressants à connaître pour gØrer l'Øvaluation des ressources en eau dans le cas de pollution à partir de sources non ponctuelles, et aussi pour dater quantitativement les eaux souterraines au moyen des isotopes du milieu. L'âge de l'eau est le temps qu'elle a passØ dans un aquifre depuis qu'elle est entrØe dans le systme, alors que le temps de transit est l'âge de l'eau au moment o elle quitte le systme. L'eau à la sortie d'un aquifre est un mØlange d'eaux possØdant diffØrents temps de transit, du fait des longueurs diffØ-rentes des lignes de courant suivies. Dans ce papier, les distributions des temps de transit sont calculØes en couplant deux mØthodes, la thØorie du rØservoir et une mØthode rØcente de simulation des âges. BasØe sur la dØrivation de la distribution cumulØes des âges sur tout le domaine, l'approche prend en compte le cadre hydrogØologique dans son ensemble. La mØthode est testØe sur un exemple analytique et son applicabilitØ est illustrØe pour un aquifre stratifiØ rØgional. Les rØsultats montrent l'asymØtrie et la plurimodalitØ de la distribution des temps de transit mme dans des conditions uniquement d'advection, à cause de la gØomØtrie de l'aquifre et de l'hØtØrogØnØitØ du champ des vitesses.Resumen El estudio de los tiempos de trµnsito del agua subterrµnea es muy til para (1) la gestión de los recursos de agua frente a la contaminación por focos no puntuales, y (2) la datación de aguas mediante isó-topos ambientales. La edad de un agua subterrµnea es el tiempo que Østa ha permanecido en el acuífero contada desde el momento de su entrada, mientras que el tiempo de trµnsito corresponde a la edad del agua en el momento en que abandona el sistema. En el punto de descarga en realidad se encuentra una mezcla de aguas con distintos tiempos de trµnsito, debido a la yuxtaposición de líneas de flujo con diferentes recorridos. En este artículo se calcula...

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