Okavango Delta Islands: Interaction between density-driven flow and geochemical reactions under evapo-concentration

This paper studies the interactions of density driven flow and geochemical reactions under evapo-concentration. A numerical model that couples the relevant flow, transport and chemical processes was used to analyze density-driven flow on islands in the Okavango Delta, Botswana. Evapo-concentration on the islands leads to steadily increasing solute concentrations until the onset of density-driven flow against the evaporation-induced upward flow. The modelling results suggest that lag times to the onset of density-driven flow are strongly influenced by geochemical reactions. Mineral precipitation and carbon dioxide de-gassing increase the lag time. Carbon dioxide de-gassing is enhanced if humic substances are present at elevated concentrations. Simulation results were shown to be most sensitive to the longitudinal dispersivity. Modelling results are compared to field observations from three islands in the Okavango Delta. A semi-quantitative correspondence between modelled and observed concentration patterns is established for the major chemical constituents.