Spatial Variability of Radon Production Rates in an Alluvial Aquifer Affects Travel Time Estimates of Groundwater Originating From a Losing Stream

Assessing water travel times in hyporheic and adjacent alluvial sediments is important to quantify exchange rates, biogeochemical turnover, and pollution dynamics across groundwater-surface water interfaces and in floodplain aquifers. Heat and ²²²Rn are useful natural tracers for this purpose. While heat transport is commonly simulated via the convection-conduction equation, the transport of ²²²Rn is often simulated assuming purely advective transport and a homogenous distribution of ²²²Rn production rates across the model domain. In the present study, we explicitly model the production, transport, and radioactive decay of ²²²Rn after surface water infiltration into an alluvial aquifer with the numerical models MODFLOW-NWT and MT3D-USGS. Using field observations, numerical modeling, and laboratory experiments, we show that ²²²Rn production rates vary substantially in the alluvial aquifer of a lowland river. Distributions of mean river-to-groundwater travel time in the alluvial aquifer, estimated via a Bayesian approach, shifted considerably depending on whether ²²²Rn and time series of groundwater heads and temperature were jointly inverted and whether ²²²Rn production rates were allowed to vary spatially. With distance to the river, differences between the median river-to-groundwater travel time and apparent ²²²Rn ages increased by factors ranging from 1.4 at 4 m to 11.9 at 59 m, a finding that highlights the need to simulate ²²²Rn transport explicitly. The joint inversion of ²²²Rn, groundwater heads, and temperature reduced uncertainty associated with mean travel times, suggesting that the uncertainty introduced by spatially varying ²²²Rn production rates can at least partly be compensated by using a combination of natural tracers.