Geomorphological Controls on Groundwater Transit Times: A Synthetic Analysis at the Hillslope Scale

We investigated how geomorphological structures shape Transit Time Distributions (TTDs) in shallow aquifers. Extensive three-dimensional simulations were performed to determine the TTDs for synthetic convergent, straight, and divergent hillslopes with a constant slope. The uniform recharge applied on top of the aquifer is transferred to the receiving stream through steady-state groundwater flows, return flows and saturation excess overland flows. Without seepage, TTDs evolve from uniform- to power law-like- distributions depending on the average distance of the groundwater volume to the river (barycenter). Remarkably, the coefficient of variation (ratio of the standard deviation to the mean) of the TTDs scales linearly with the barycenter in agreement with a theoretical prediction based on three analytical approximations derived for specific cases. With seepage, the TTD has three separate modes corresponding to rapid saturation excess overland flows, to the intermediate flow paths ending in seepage area and to the slower flow paths going all the way to a discharge in the river. The coefficient of variation additionally depends on the extent of the seepage area. For a natural hillslope in the crystalline basement of Normandy (France), the same synthetic analysis demonstrates that the coefficient of variation is not only determined by the extent of the seepage zone but also by its structure in relation to the local and global geomorphological organization. The results suggest the possibility to assess the variability of transit times by combining geomorphological analysis, surface soil saturation observations and environmental tracers.