Tracing Bank Storage and Hyporheic Exchange Dynamics using 222Rn:A Virtual Test

Abstract:

Dynamic hydrologic exchange flows in river beds and banks are important for many ecosystem functions throughout river corridors. Here we test whether the exchanges and the associated mixing between a flooding river and groundwater within the river’s bank can be effectively traced by ²²²Rn, a naturally occurring, inert, radiogenic and radioactive gas that can be analyzed and monitored in situ. The assessment was done by simulating groundwater flow and reactive transport of ²²²Rn in the bank following a single flood wave in the river. The results illustrate that ²²²Rn variations in space and time are larger than the uncertainty of common measurement methods, making ²²²Rn a suitable tracer. The individual effects of aquifer hydraulic conductivity, dispersivity, river water ²²²Rn concentration, and bank topography were analyzed through sensitivity analysis. Larger hydraulic conductivity and dispersivity, lower ²²²Rn concentration in river water relative to groundwater, and gentler bank slopes resulted in a more prominent and traceable ²²²Rn signal. The transport and residence time of exchanged water may be estimated and interpreted using reactive transport models such as those implemented here. However, such application is sensitive to fluctuations in river water ²²²Rn, requiring it to be well characterized. The assessment provides a helpful guide for field implementation of using ²²²Rn as a tracer for groundwater-surface water interactions in dynamic settings.