Untangling transient groundwater mixing and travel times with noble gas time series and numerical modeling

The quality and quantity of alluvial groundwater in mountainous areas are particularly susceptible to the effects of climate change, as well as increasing pollution from agriculture and urbanization. Understanding mixing between surface water and groundwater as well as groundwater travel times in such systems is thus crucial to sustain a safe and sufficient water supply. We used a novel combination of real-time, in-situ noble gas analysis to quantify groundwater mixing of recently infiltrated river water (F_rw) and regional groundwater, as well as travel times of F_rw during a two-month groundwater pumping test carried out at a drinking water wellfield in a prealpine valley in Switzerland. Transient groundwater mixing ratios were calculated using helium-4 concentrations combined with a Bayesian end-member mixing model. Having identified the groundwater fraction of F_rw consequently allowed us to infer the travel times from the stream to the wellfield, estimated based on radon-222 activities of F_rw. Additionally, we compared and validated our tracer-based estimates of F_rw using a calibrated surface water-groundwater model. Our findings show that (i) mean travel times of F_rw are in the order of two weeks, (ii) during most of the experiment, F_rw is substantially high (~70\%), and (iii) increased groundwater pumping only has a marginal effect on groundwater mixing ratios and travel times. The high fraction of F_rw in the abstracted groundwater and its short travel times emphasize the vulnerability of mountainous regions to present and predicted environmental changes.