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