EVAPORATIVE CONCENTRATION, DISSOLVED ORGANIC MATTER DEGRADATION AND SULFATE REDUCTION AS CONTROLS ON ARSENIC MOBILITY IN GROUNDWATER OF THE OKAVANGO DELTA, BOTSWANA

The detrimental health effects of arsenic (As) contamination have motivated the study of As mobility around the globe. In arid environments with high evaporation, abiotic processes such as evapoconcentration and As desorption under alkaline pH are thought to be responsible for high As concentrations. In reducing groundwater, on the other hand, microbial iron (Fe) reductive dissolution is known to release As into solution. In such environments, microbial sulfate (SO₄^2-) reduction can precipitate sulfide minerals that incorporate As. The purpose of this study is to evaluate how microorganisms and DOM impact As distribution along the groundwater flow path beneath the islands of the Okavango Delta. The Okavango Delta is an arid-zone wetland punctuated by ten of thousands of islands, and the reducing groundwater beneath these islands have dissolved As as high as 3ppm. Abiotic processes are thought to contribute to the elevated As level; however dissolution of Fe-containing sediments has been proposed as the initial step in releasing As from sediment to the groundwater. To test the consistency of the hypothesized mechanism in other islands of the delta, four islands were sampled in January 2013. Our analyses suggest microbial Fe- and SO₄^2--reductions are significant in releasing and sequestering As along the flow path of each island. DNA sequences collected were that grouped within lineages that contain organisms capable of dissimilatory Fe and SO₄^2- reduction, including Geobacteracea and Desulfovibrionaceae. Further analyses with most probable number (MPN) technique for SO₄^2- reducing bacteria (SRB), indicated that in locations with elevated As levels, SRB were found in very low numbers. In addition, the formation of thioarsenic compounds may maintain As in solution at higher pH. Microbial processes that oxidize DOM and reduce SO₄^2- earlier along the flow path produce carbonate alkalinity and increase pH, and may thereby contribute to As mobility further along the flow path. In this arid and reducing groundwater, we find that coupled abiotic-biotic processes and DOM transformations have a fundamental role in As mobility. Our findings also have important implications for assessing microbial SO₄^2- -reduction as a promising inexpensive remedial method for As contamination and the limitations of this process.