Different tracer, different bias: using radon to reveal flow paths beyond the Window of Detection

Abstract. Slug tracer experiments have greatly advanced our understanding of solute transport in streams. Breakthrough curves (BTCs) from these experiments are biased toward faster flow paths, highlighting the need for alternative tracers to cover longer timescales. The radioactive tracer radon (²²²Rn) is increasingly used to quantify transit times in subsurface transient storage zones, as it traces transit times of up to 21 days. However, it remains unclear whether calibrating transient storage models (TSMs) with radon yields longer subsurface timescales of transit times – and thus greater transient storage areas – than with slug tracers such as sodium chloride (NaCl). To address this, we conducted radon measurements and NaCl slug tracer experiments in Oak Creek (Oregon, USA) and jointly and individually calibrated TSM parameters with both tracers. We applied parameter identifiability analysis and evaluated the information provided by both tracers in constraining model parameters. Our results show that calibrating the TSM with radon and chloride increases information on model parameters compared to calibrating the TSM with each tracer individually. This suggests that incorporating radon in calibration improves solute transport estimates in future studies. However, when calibrating the TSM with only radon measurements, all resulting parameters of the TSM were non-identifiable. This non-identifiability arises from steady state activity of radon in streams and radon's high sensitivity to the amount and location of groundwater inflow, which is not explicitly accounted for in TSMs. As a result, radon measurements are biased toward longer-timescale flow paths, limiting its usefulness for characterizing solute transport in calibrating TSMs without chloride.