Interest of flow and transport models to shape residence and transit time distributions from observed groundwater ages

Abstract : Groundwater age tracers provide essential characteristics of residence time distributions in aquifers. In few cases, enough tracers are available to identify a priori selected distribution shapes (lumped parameter models). In most cases, however, the information contained in the tracer concentrations is not enough to constrain any relevant distribution shape. Other issues also arise including the lack of definition of the distribution shape and strong temporal variations of the younger part of the distribution that cannot be followed by the measured tracers. We investigate the combination of flow and transport models with groundwater age tracers to assess residence and transit time distributions. We base our study on shallow aquifers in a crystalline region (Brittany, France). The oceanic climate and limited topographic gradients promote infiltration and transfers within the shallow aquifer of the upper weathered zone and within the soil horizon. A parsimonious hillslope flow and transport model is developed to model water and element transfers. In the aquifer, Boussinesq flows result in a broad distribution of residence times. High residence times come from the large aquifer storage. Low residence times only show up in the wet season when the repartition of saturation induce seepage and quick transfers in the soil to the stream network. A gravity-driven flow in the soil layer is introduced to process the quick transfer to the river. With the combined use of anthropogenic tracers (CFCs) and the previous hillslope models, we show first that dissolved silicates can be used as a tracer of the mean transit times. Five studied sites show remarkably consistent weathering rates, qualifying dissolved silicates as an effective regional tracer. Second, hillslope models have been successfully calibrated to river discharges and silicate concentrations. They show relatively stationary residence time distributions in the aquifer but highly dynamical transit time distributions in the river. The relations of residence time to transit time distributions give process-based foundations to storage age selection functions. The “preference” of the catchment to release younger or older groundwater to the streams derives from the dynamical interactions between the stratification of the residence times in the aquifer, the repartition of saturation in the catchment and the intermittent seepage of the aquifer to the soil layer.