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Investigating Processes that Control the Chemistry During Refilling of Lake Ngami in Semiarid Northwest Botswana

Scott D. Meier1, Eliot A. Atekwana1, ans Loago Molwalefhe2
1Oklahoma State University
2University of Botswana

Lake Ngami is located in the distal portion of the Okavango Delta in semiarid Botswana and has remained dry for most of the past century until 2009. We surveyed Lake Ngami to determine processes that control water properties and to establish baseline values for future comparisons. Ionic concentrations were segmented into three distinct regions of increasing concentration along a longitudinal transect. A plot of total dissolved solids versus the stable oxygen isotopic ratio for Lake Ngami has a good correlation, suggesting that increasing solute concentrations in the lake are due to evaporation which also enriches stable oxygen isotopes. Inflow to the lake showed little chemical variation over the past three years, so we suggest that increased concentrations in lake water are due to evapoconcentration and the segmentation is a result of differential residence time of lake recharge. Unlike major ion concentrations, dissolved inorganic carbon concentrations and the stable carbon isotopic ratio increase along the transect to the lake s midpoint, and then reach steady state. We find that increasing dissolved inorganic carbon concentrations are mainly due to evapoconcentration, though a longitudinal increase in the stable isotopic ratio suggests that carbon cycling is influenced by equilibration between atmospheric and dissolved carbon species. In addition to baseline data obtained for future comparative studies, our results demonstrate the importance of evaporation and residence time in controlling solute and carbon behavior, along with the importance of atmospheric equilibration in controlling the carbon isotopic signature during the filling of an endohreic lake in an arid environment.

AAPG Search and Discovery Article #90182©2013 AAPG/SEG Student Expo, Houston, Texas, September 16-17, 2013