Estimating amount and spatial distribution of groundwater recharge in the Iullemmeden basin (Niger) based on 3 H, 3 He and CFC-11 measurements

A new approach is presented to estimate the amounts and spatial distribution of groundwater recharge based on the groundwater dating tracers 3H, 3He and CFC‐11. The method was applied in a groundwater project in southwest Niger (Africa). The model developed to simulate observed tracer concentrations is composed of a model to account for the water transported through the unsaturated zone followed by a dispersion box model to determine the age distribution of the sampled groundwater volume. For the unsaturated zone transport, a one‐dimensional approach was used considering tracer transport in the water and the gas phase. The calculated tracer concentrations at the groundwater table are then used as input functions for the subsequent dispersion box model. Using a dispersion box model it is possible to outline regions where recharge preferably occurs. To account for the high variance of soil properties in space, a Monte Carlo simulation was performed. The results show estimated recharge rates of 50–300 mm year−1 (±50%). The large error bars are mainly due to high uncertainties of the soil parameters. Therefore, a better estimation of the average soil moisture would lead to more accurate results. The resulting spatial distribution of dispersion parameters indicates that recharge mainly occurs in the ancient riverbeds where surface runoff from the laterite plateaux is gathered and leads to spatially limited high recharge events. Where the soil is not permeable enough, these events lead to the observed temporary or permanent lakes. This new conceptual view of major recharge mechanisms contradicts the former picture of recharge occurring in this region, but it coincides better with other independent measurements, such as the stable isotope ratios of water