Use of dissolved gases as tracers to study the impacts of floods and river works on Surface water – Groundwater interactions.

Alluvial aquifers have a significant potential for pumping large quantities of groundwater, essential for meeting drinking water needs. Abstracted water typically consists of a mix of regional groundwater and freshly infiltrated river water. A good understanding of surface water – groundwater interactions in these types of systems is required for managing both qualitative and quantitative aspects of riverbank filtration or river renaturations. Tracers are important tools in these contexts, as they provide crucial information on travel times and mixing ratios.

We present data from a comprehensive multi-tracer approach obtained in a field experiment conducted in a pre-alpine valley in central Switzerland. Over several months, river works were undertaken in an infiltrating river in the proximity of an important field of groundwater wells used for drinking water production. We investigated the impact of these river modifications on surface water - groundwater dynamics by monitoring the natural concentrations of (noble) gases with multiple potable mass spectrometers (miniRuedi, Gasometrix) and radon detectors (Rad7, Durridge). Additionally, we injected different noble gas species as artificial tracers both in the river and in groundwater and gained valuable insights into the evolving dynamics of the system.

The combination of these different tracers provided insights that could not have been obtained by a single tracer. Our results demonstrate that during and immediately after restoration works the infiltration of river water increases temporarily and provide insights about the time it takes for a riverbed to recover after restoration works and for river-groundwater interactions to reach a new dynamic equilibrium.