Exploring the reliability of 222Rn as a tracer of groundwater age in alluvial aquifers: Insights from the explicit simulation of variable 222Rn production

Abstract

Knowledge of groundwater residence times (GRT; the time elapsed since surface water infiltration) between losing rivers and pumping wells is crucial for management of water resources in alluvial aquifers. The radioactive noble gas radon-222 (²²²Rn) has been used for decades as a natural indicator of surface water infiltration, as it can provide quantitative information on GRT. However, models using ²²²Rn as a tracer of GRT are often based on a set of highly simplifying assumptions, including spatially homogenous ²²²Rn production and exclusively advective mass transport within the aquifer. In this paper, we use the integrated surface-subsurface hydrological model HydroGeoSphere (HGS) to simulate ²²²Rn transport, production, and decay in a bank filtration context. Spatially variable ²²²Rn production, based on experimental data, is explicitly considered. We show that variable ²²²Rn production rates, coupled with hydrodispersive mixing of groundwater, may lead to large biases in GRT estimates. Under certain transient conditions however, changes in tracer-derived GRTs correlate well with changes in mean groundwater age. Whereas ²²²Rn-derived GRTs may only be reliable under a narrow range of field conditions, ²²²Rn may serve as a powerful tracer of changes in mean GRT even in complex and heterogenous environments.