Groundwater flow regime, recharge and regional-scale solute transport in the semi-arid Kalahari of Botswana derived from isotope hydrology and hydrochemistry

A synthesis of isotope hydrological (2H, 3H, 18O, 13C, 14C, 3He, 4He) and hydrogeochemical tools was used to analyze flow regimes and their time scales, to determine recharge conditions, and to describe the regional-scale hydrogeochemical evolution of groundwater from the Triassic Ntane Sandstone Aquifer between Serowe and Orapa in the Kalahari of Botswana. This location is taken as an example of a complex hydrodynamic setting relating to semi-arid climatic conditions, low recharge amounts, thick unsaturated zones (up to 100 m) and long groundwater residence time scales. Capabilities and limits of the individual methods are discussed, and relations to solute transport are demonstrated for nitrate. Chemical classification allows the distinction of six different groundwater types. This characterization is reflected in the isotopic composition and in part can be attributed to different age ranges (up to greater than 20 ka). We show that the regional-scale groundwater flow regime in the aquifer is only slow, and that groundwater is recharged in Serowe and Orapa. However in both investigated regions, mixing between over- and underlying aquifers is evident which limits a clear derivation of one regional flow regime. The challenges imposed on the identification of the regional groundwater flow regime by resulting mixing processes and for groundwater dating are discussed and evaluated.