INVESTIGATING PROCESSES THAT CONTROL THE CHEMISTRY DURING REFILLING OF LAKE NGAMI IN SEMIARID NORTHWEST BOTSWANA

Lake Ngami is an endorheic system located in the distal portion of the Okavango Delta in semiarid Botswana. The lake has remained dry for most of the past century up until 2009 when it began filling with water. We surveyed Lake Ngami and documented the physical, chemical, and stable isotopic compositions of the lake water in order to evaluate the processes that control water properties and to establish baseline values for future temporal and spatial comparisons. We made physical measurements and collected water samples for chemical and stable isotope analysis at 25 cm below the surface along a ~19 km axial transect from the inflow river to the distal end of the lake. The major ionic concentrations (e.g., Cl^-, Na⁺ Ca²⁺) were segmented into three distinct regions of increasing concentration along the transect. Stable isotopic ratios of oxygen (δ¹⁸O) also increased along the transect and showed the same three regions of segmentation. The δ¹⁸O vs. the stable hydrogen isotope (δD) plot along the Okavango Delta evaporation line and suggest modification of lake water by evaporation. Because the lake’s inflow showed little chemical and isotopic variation for the past three years, we suggest that the increased concentrations in lake water are due to evapoconcentration and that the segmentation is a result of differential evaporation of lake recharge from 2010, 2011 and 2012; this is controlled by the residence time of temporal recharge into the lake. Unlike the major ions, the dissolved inorganic carbon (DIC) concentrations and the stable carbon isotope (δ¹³C_DIC) increase along the transect to about the lake’s midpoint, and then reach steady state. The increasing DIC concentrations are mainly due to evapoconcentration. Increase in the δ¹³C_DIC is the result of carbon equilibration between the DIC and atmospheric CO_2(g). The δ¹³C_DIC results suggest that from the middle of the lake to the distal portion, which represents recharge from 2010 and 2011, carbon cycling is at steady state. In addition to baseline data obtained for future comparative studies, our results show the importance of evaporation and residence time in controlling solute and carbon behavior, as well as the dominance of atmospheric CO_2(g) in controlling the carbon isotopic signature during the filling stage of an endohreic lake in an arid environment.