Exploring groundwater's journey through fluvial deposits and its impact on recharge estimation with a landscape evolution model

Geological heterogeneities are prevalent throughout the subsurface, yet, for convenience, they are often disregarded in estimating recharge from groundwater age, which relies on analytical models that assume a homogeneous aquifer. We propose to explore the impact of heterogeneities in fluvial deposits on residence times and on recharge estimates. First, we use CHILD, a landscape evolution model, to generate plausible deposits after tens of thousands of years of river evolution. Then, we use the fractional packing model to compute the porosity and permeability of those unconsolidated deposits. Finally, we use PFLOTRAN to simulate groundwater flow and mean age over a thousand years of spatially uniform recharge at 25 mm/yr. This reproduces the boundary conditions underlying Vogel's analytical model to estimate recharge rates. Our results are based on 15 000 realizations from varying inputs: grain sizes, aggradation rate of the river, porosity, etc. For most models, the mean estimate of the recharge rate remains below an absolute error of 25 %. Although the channel belt itself funnels the flow, its heterogeneities lead to spatially heterogeneous residence times, even over short distances. Thus, fluvial heterogeneities can lead to estimates ranging from close to zero to several thousand mm/yr, and mean estimates could drastically vary depending on sampling locations. A sensitivity analysis using the delta-importance measure and Cusunoro curves exposes how fluvial processes influence the range and distribution of the error. For instance, a high bank erodibility and small coarse grain size under low aggradation rates favor the reworking of deposits, which increases heterogeneity and leads to high errors. This work highlights that the wide variability observed in recharge estimates from environmental tracers can to a large extent be explained by geological heterogeneities.