Evaluating the recharge mechanism of the Lower Kuiseb Dune area using mixing cell modeling and residence time data

A mixing cell approach was extended by a method incorporating mean cell residence times derived from ¹⁴C to further constrain and validate the modeling results. This extended approach was used to model the groundwater system of the Lower Kuiseb Dune area in Namibia. The Kuiseb river is a 560 km long ephemeral river that crosses the Namib Desert from east to west. Transmission losses from the riverbed during flood events are an important source of groundwater recharge to the underlying aquifer system. The Lower Kuiseb area is located in a hyper arid region with annual precipitation less than 25 mm/yr. Hydrochemical data from 13 wells in the area were used in the mixing cell model (MCM). End members were identified as sources for the groundwater found in the Lower Kuiseb, including inflow from the crystalline basement plateau north of the Kuiseb as well as floodwater from the Kuiseb river. A conceptual groundwater recharge and flow model was developed, and then inverse mixing cell modeling was carried out using hydrochemical tracers. This approach generally allows for several possible solutions. After completing the inverse modeling, a forward mixing cell model was developed by varying the mean residence time of each cell to fit calculated ¹⁴C data to the measured ¹⁴C data. This new approach joins previously developed methods solely based on conservative mixing or residence time optimization. Based on the results of the model the fraction of floodwater in different sections of the Lower Kuiseb groundwater systems was calculated, ranging from 61% to 98.2%. In addition to floodwater, groundwater inflow from the crystalline basement north of the Kuiseb was shown to contribute to the Lower Kuiseb aquifer, accounting for salinization during periods without flooding.