Dynamic partitioning of deep, stratified groundwater vs shallow subsurface flow leads to large and old transit time distributions with marked seasonal dynamics.

Abstract:

Active transfers through shallow soils and aquifers are determinant in the mitigation of growing anthropogenic pressures on water resources and to enhance the resilience of freshwater ecosystems. We develop a parsimonious hillslope-based model to capture the contribution of aquifers to stream discharges and its effect on transient transit time distributions modeling. Interactions between Boussinesq groundwater flows and subsurface flows in the soil horizon are controlled by the relative aquifer saturation. Aquifer saturation promotes the apparition of subsurface saturated areas at the soil-bedrock interface, return flows and direct precipitations on saturated soils. Consequences on advective-dispersive transport are modeled in a Lagrangian framework with an original 2D particle tracking algorithm, specifically developped for the hillslope storage Boussinesq equations.

Applied on a realistic catchment case in a temperate climate with moderate topographical gradients (Guillec, France), the calibrated flow model was able to reproduce CFC-based groundwater ages as well as discharge dynamics at the outlet of the catchment. This provides key information on the mean transit times dynamics recorded in this catchment and on the partition between groundwater and subsurface stormflows in the streamwater. The relative activation of transient shallow flows and more stationary deeper flows results in large dispersion and marked seasonal variations of the transit times to the river. This parsimonious strategy is particularly adapted to effectively model the large range of old transit times (5-100 years) due to the groundwater inflows to the stream. This is of uttermost importance for legacy contaminations in catchments where groundwater inflows predominate.