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

Groundwater transit times are of interest for the management of water resou rces, assessment of pollution from non-point sources, and quantitative dati ng of groundwaters by the use of environmental isotopes. The age of water i s the time water has spent in an aquifer since it has entered the system, w hereas the transit time is the age of water as it exits the system. Water a t the outlet of an aquifer is a mixture of water elements with different tr ansit times. as a consequence of the different flow-line lengths. In this p aper, transit-time distributions are calculated by coupling two existing me thods, the reservoir theory and a recent age-simulation method. Based on th e derivation of the cumulative age distribution over the whole domain, the approach accounts for the whole hydrogeological framework. The method is te sted using an analytical example and its applicability illustrated for a re gional layered aquifer. Results show the asymmetry and multimodality of the transit-time distribution even in advection-only conditions, due to the aq uifer geometry and to the velocity-field heterogeneity.