INTERACTION OF DENSITY FLOW AND GEOCHEMICAL PROCESSES ON ISLANDS IN THE OKAVANGO DELTA, BOTSWANA

Abstract

The Okavango Delta in Northern Botswana is a large wetland system (about 20’000 km2) located at the downstream end of the Okavango River basin. The endorheic Okavango River originates on the crystalline Benguela Plateau in Southern Angola, flows southwards into the Kalahari sedimentary basin, where it terminates in the Okavango Delta. The system is open with respect to water (evapotranspiration) but closed with respect to solutes. Consequently, about 300’000 tons of salts accumulate in the Okavango Delta per year. Significant salt accumulation is observed on small to medium-sized islands (100- 1000 m diameter) that are scattered throughout the wetland system. Due to relatively shallow groundwater levels below the islands, groundwater is continuously withdrawn from the surrounding swamps by the combined action of transpiration and evaporation. Whereas transpiration dominates in the belt of vigorous riverine vegetation along the islands’ fringes, evaporation is the dominant process in the centre of the islands. Due to elevated groundwater salinities (up to 30 g/l), only specialized grass species can survive in the islands’ centre. Salt accumulation on three islands in the Okavango Delta was studied in detail. Electrical resistivity tomography was used to derive the 3D salinity distribution on one island and its surroundings. Multiple boreholes and piezometers were drilled to various depths and water samples were analyzed for major ions and tracers. The field data suggests that density fingering against the evaporation-induced upward flow occurs on the islands. At the same time, various minerals precipitate (carbonates, silica and others) and form cemented layers and efflorescent salt crusts. A coupled multi-species flow and reactive transport model was used to analyze the chemical evolution of the groundwater below the islands in the Okavango Delta. The model takes into account both geochemical reactions and density flow. It is based on the PHREEQC and SEAWAT codes. Of particular interest is the interaction between density flow and geochemical reactions. To study these interactions, four numerical island models of increasing complexity were developed and results were compared. In a first step, advective-dispersive transport of one conserved species (salinity) was simulated. Subsequently, variable fluid density was taken into account. The third step considers advective-dispersive transport of multiple reactive species, disregarding variable density effects. The final model formulation includes multi- species reactive transport and variable density flow. In the final model, mineral precipitation delays the onset of fingering instabilities, due to the removal of salinity. The vertical transport of solutes into deeper subsurface strata is therefore less efficient as compared with the single species, conservative density flow simulation. The results enhance our understanding of reactive variable density flow in porous media. They are relevant for all evaporative systems, where significant evapo- concentration occurs. An important application is soil and groundwater salination, which is presently one of the major global environmental problems.