CARBON CYCLING AND GROUNDWATER EVOLUTION IN THE OKAVANGO DELTA, BOTSWANA

The Okavango Delta is one of the few examples of large river systems where carbon is cycled entirely in the terrestrial system. The Okavango Delta is an endorheic basin in an arid environment, and thus carbon and solutes supplied to the groundwater originate from the soil zone through rain recharge or from river water recharge. Mass transfer estimates of carbon from the Okavango River to the atmosphere, land surface and groundwater are needed in order to quantify the carbon balance, and determine the importance of the endorheic system in the regional and global carbon balance.

We measured concentrations of Dissolved Inorganic Carbon (DIC), major ions, the stable isotopes of carbon in DIC and hydrogen (δD) and oxygen (δ¹⁸O) in groundwater and used published chemical and isotopic data for groundwater and surface water in order to determine the origin of DIC in groundwater in the Okavango Delta.

Our results show that (1) the predominant ions in both the surface (river) and groundwater of Okavango Delta water are Ca²⁺>Mg²⁺>Na⁺>K⁺ and HCO₃^->Cl^->SO₄^2- (2) alkalinity shows a progressive increase with increase in total cation concentration (3) DIC concentrations increase progressively from surface, shallow and deep groundwater and (4) the stable isotopes of carbon in DIC, hydrogen and oxygen for surface water were heavier than for deep groundwater. The waters are characterized as calcium-sodium-bicarbonate (Ca-Na-HCO₃) water and the increase in DIC concentration with depth show that there is a continuous enrichment of carbon with depth. Surface and groundwater are isotopically enriched by evaporation. The carbonate chemistry suggests that the cations especially Na plays a major role in the sequestration of carbon as carbonate. Based on the stable isotopes of DIC, δD and δ¹⁸O ratios, the groundwater in the Okavango delta evolves from surface (river) water and the carbon is from surface water. We therefore conclude that carbon in the groundwater in the Okavango Delta is derived from evapo-concentration of surface (river) water and that the groundwater is traceable from the chemical stable isotope (C, H, and O) evolution of river recharge. It appears that recharge from direct precipitation has only a minor effect on solute mass transfer and the chemical evolution of groundwater.