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

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, capturing durations of up to 21 d. However, it remains unclear whether calibrating transient storage models (TSMs) with radon yields longer subsurface timescales of transit times than calibrating them 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 used information theory to evaluate how the two tracers constrain model parameters. Our results show that TSM calibration with both radon and chloride increases parameter information compared to TSM calibration with either tracer alone. This suggests that incorporating radon into calibration improves estimates of solute transport 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 because radon activity in streams remains at steady-state and is highly sensitive to the location and amount of groundwater inflow, as well as contributions of flow paths from subsurface transient storage zones. As a result, radon measurements are biased toward longer-timescale flow paths, limiting their applicability to uniquely constrain solute transport parameters in TSM calibration without complementary slug tracers.