H131-04
A Framework for Untangling Transient Groundwater Mixing and Travel Times

Monday, 14 December 2020: 04:12
Virtual
Andrea L. Popp1, Álvaro Pardo-Álvarez2, Oliver Schilling2, Andreas Scheidegger3, Stéphanie Musy4, Morgan Peel2, Philip Brunner5, Roland Purtschert4, Daniel Hunkeler6 and Rolf Kipfer7, (1)University of Oslo, Oslo, Norway, (2)Centre d'Hydrogéologie et de Géothermie, University of Neuchâtel, Neuchâtel, Switzerland, (3)EAWAG Swiss Federal Institute of Aquatic Science and Technology, Duebendorf, Switzerland, (4)University of Bern, Bern, Switzerland, (5)University of Neuchâtel, Neuchâtel, Switzerland, (6)University of Neuchâtel, Centre for Hydrogeology and Geothermics, Neuchâtel, Switzerland, (7)Swiss Federal Institute of Aqu, Dubendorf, Switzerland
Recorded Presentation
Abstract:
Understanding the mixing between surface water and groundwater as well as groundwater travel times in vulnerable aquifers is crucial to sustaining a safe water supply. Age dating tracers used to infer apparent travel times typically refer to the entire groundwater sample. A groundwater sample, however, consists of a mixture of waters with a distribution of travel times. Thus, the interpretation of age dating tracers can be biased and apparent water ages typically only reflect the dating range of the used tracer. Additionally, hydrological mixing models are subjected to various sources of uncertainties, which often originate from insufficient tracer data. In this study, we introduce a new framework that untangles groundwater mixing ratios and travel times using a novel combination of quasi-real-time, in-situ noble gas analyses. We applied this approach during a groundwater pumping test carried out in a pre-alpine Swiss valley. First, we calculated transient mixing ratios between recently infiltrated river water and regional groundwater present in a wellfield, using helium-4 concentrations combined with a Bayesian end-member mixing model. Having identified the groundwater fraction of recently infiltrated river water (Frw) consequently allowed us to infer the travel times from the river to the wellfield, estimated based on radon-222 activities of Frw. Furthermore, we compared and validated our tracer-based estimates of Frw using a calibrated surface water-groundwater model. With this work, we demonstrate (i) that partitioning of major water sources enables the interpretation of an age dating tracer of the water fraction of interest and (ii) that end-members in fast-changing systems can be transient. Moreover, our findings indicate that (i) during most of the experiment, Frw is relatively high (~70%), (ii) travel times of Frw are in the order of two weeks and (iii) increased groundwater pumping only has a marginal effect on groundwater mixing ratios and travel times.
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