INFLUENCE OF ABIOTIC AND BIOTIC PROCESSES ON THE MOBILIZATION OF ARSENIC IN GROUNDWATER OF THE OKAVANGO DELTA, BOTSWANA

Arsenic (As) is a naturally occurring trace element that has been found in high concentrations in many deltaic aquifers. In arid environments with high evaporation, abiotic processes such as evapoconcentration and desorption of arsenic under alkaline pH are commonly thought to be responsible for high As concentrations. In groundwater underlying deltaic and wetland environment such as in the Bengal Basin, arsenic is mobilized via a reductive dissolution mechanism. The key driver for reductive dissolution is dissolved organic matter (DOM), which serves as an energy source for bacteria that reductively dissolve iron and/or arsenic and lead to arsenic desorption in reducing groundwater. The Okavango Delta is an arid-zone wetland that is punctuated by tens of thousands of islands, and recent studies have shown that groundwater beneath these islands can have dissolved As concentrations as high as 3 ppm. Evapoconcentration and increasing pH along groundwater flowpaths are thought to contribute to elevated arsenic concentrations beneath the central part of islands in the Okavango Delta; however, the dissolution of iron-containing sediments has been proposed as the initial step in releasing arsenic from sediment to the groundwater. The purpose of our research is to evaluate how abiotic as well as biotic mechanisms influence As mobility in this setting. Our analyses of groundwater samples collected from an island transect in October 2011 suggest that although elevated arsenic was associated with high conductivity and pH, the aquifer was generally reducing throughout and reductive dissolution of Fe minerals in initial segments of the flowpath was likely. We also observed a transformation in DOM quality that may be due to sorption of fulvic acids along the groundwater flowpath or biotransformation of DOM by sulfate reducing bacteria (SRB), which were also quantified. We are currently testing whether SRB utilize fresh DOM (supplied to groundwater by vegetation-derived surface water) to produce calcium carbonate and increase pH, thereby contributing to greater desorption of As under alkaline conditions. In this arid deltaic groundwater environment, where abiotic processes are expected to dominate, microbial processes and DOM transformations may have an additional role in arsenic mobility.