Variable 222Rn emanation rates in an alluvial aquifer: Limits on using 222Rn as a tracer of surface water-Groundwater interactions

Radon-222 (222Rn) is routinely used as a tracer of surface water (SW) - groundwater (GW) interactions. However, methods that rely on this tracer to quantify SW-GW exchange fluxes, GW residence time, SW-GW mixing ratios, or GW flow velocities usually depend on the assumption of spatially constant 222Rn production within aquifers and/or hyporheic sediments. In the present study, we measure the natural variability of 222Rn emanation rates in a highly instrumented alluvial aquifer and underline some important issues in assuming homogenous 222Rn production in alluvial systems. A robust experimental setup was used to determine 222Rn emanation rates from sediments. High-resolution measurements reveal that 222Rn emanation rates are highest within the first few meters below the surface. These results have significant implications for the interpretation of 222Rn activities in GW, notably in the vicinity of infiltration zones, as changes in hydraulic conditions can lead to variations in 222Rn signatures that can only be resolved with proper knowledge of the spatial distribution of 222Rn production rates. In such contexts, the assumption of constant 222Rn production leads to an inadequate understanding of SW infiltration patterns, and biased 222Rn-based estimates of SW-GW exchange.