[A-HW19] Tracer Hydrology: Advances in Measurement and Modelling

[AHW19-P05] Stable water isotopes, tritium and noble gases as an ideal tracer combination for managed aquifer recharge systems

*Jared van Rooyen^1,2, Torsten Vennemann³, Roland Purtschert⁴, Oliver Schilling^1,2 (1.Eawag, Swiss Federal Institute of Aquatic Science and Technology - Dübendorf, Switzerland. , 2.Hydrogeology, Department of Environmental Sciences, University of Basel - Basel, Switzerland., 3.Institute of Earth Surface Dynamics, University of Lausanne, Lausanne, Switzerland., 4.Climate and Environmental Physics and Oeschger Center for Climate Change Research, University of Bern, Bern, Switzerland)

Keywords:Isotope Tracers, Noble Gas, Tritium, MAR

Isotopes of the water molecule, both stable and radiogenic, have proven effective and accessible tracers of transport, flux, and mixing of waters in the hydrological cycle. Physical processes in the groundwater system are typically difficult to constrain and have been elucidated through our understanding of these water isotopes. This is especially the case in younger groundwater systems that contain tritium produced through thermonuclear bomb testing in the 1960s, therefore indicating modern recharge. The efficacy of using bomb tritium as a modern tracer has, however, dissipated, as the abundance of tritium in precipitation has now returned to natural levels. Nevertheless, many systems still contain anthropogenically produced tritium (e.g. from nuclear power generation), providing unique opportunities to use tritium as a tracer of modern waters in such systems.

In Switzerland, multiple nuclear power plants release tritiated water to the river Rhine, creating significant and semi-regular spikes in tritium abundance downstream. Managed aquifer recharge (MAR) schemes artificially infiltrate river water into shallow unconsolidated aquifers for storage and natural purification of domestic water supply. The Hardwald MAR scheme near the city of Basel uses such Rhine waters to infiltrate over 100,000 m3 of water per day to provide clean drinking water to over 80k people. With (seasonal) water scarcity predicted to increase strongly in Central Europe, improving the efficiency of MAR schemes will contribute strongly to achieving several of the UN Sustainable Development Goals and EU agendas.

In this study, we monitored stable (daily) and radiogenic (weekly) isotopes of water in both the infiltrated and the abstracted waters of the MAR scheme in order to investigate the efficacy of these tracers in informing groundwater models of fluctuations in the hydrological behavior of the system. Accompanying this, we continuously monitored gases dissolved in groundwater (He, Kr, Ar, N₂, O₂, CH₄ and CO₂). Tritium abundances in the Rhine infiltrate varied from <10 TU to >35 TU, and spikes fluctuated over 20 TU. While these signals could be tracked in the abstraction wells, the results also showed significant and rather unexpected mixing of infiltrated waters with a wide range of residence times within the MAR scheme. Results of the one-year monitoring period allowed revising the conceptual model of the entrainment of deep regional groundwaters in the system, and the results can be used as a guide to employ tritium measurements in similar systems. Moreover, these data will serve for the calibration and validation of a 3D hydrogeological groundwater model that will be used for real-time management of the MAR site.