Exploring an aquifer system by integrating hydraulic, hydrogeologic and environmental tracer data in a three-dimensional hydrodynamic transport model

This article presents a numerical model of a part of an aquifer that is recharged by infiltration from the Swiss pre-Alpine river Töss in the Linsental (north-eastern Switzerland). The nearby city of Winterthur makes use of this aquifer as a resource of drinking water. The presented model is part of a larger interdisciplinary research program undertaken with the goal to evaluate the possible impacts of a planned revitalization of the severely canalized river Töss. Above all it should show the extent of decrease of the groundwater residence time if the river bed is allowed to move towards the drinking water wells. The flow model was constrained and calibrated by transport modelling of tritiogenic 3He. This tracer reflects both the aging of the water (by accumulation of 3He resulting from tritium-decay) as well as the two different components of the mixture (river water free of tritiogenic 3He due to degassing, and groundwater enriched in 3He due to accumulation). By simulating a Dirac-pulse-shaped input of a conservative tracer at different sources (river cells or upstream flux boundary cells) it is possible to determine the age distributions as well as the mixing ratios of the two types of water at the two pumping stations within the model area. The same calculations for a hypothetical river course passing directly beside the pumping stations indicate a decrease of the mean residence time of the pumped water together with an increase of the amount of the younger river water component.