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. In our study 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 longitudinal 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. A plot of TDS vs. δ¹⁸O for Lake Ngami has a good correlation coefficient (R²) of 0.91 and suggests that the increase in solute concentrations in the lake is due to evaporation which is also the mechanism for enrichment of δ¹⁸O. Because the lake's inflow showed little chemical and isotopic variation over 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 the temporal recharge into the lake. Unlike the major ions, 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 but an increase in the longitudinal δ¹³C_DIC suggests that other processes such as photosynthesis and carbon equilibration between the DIC and atmospheric CO_2(g) are influencing the lake's carbon cycle. The leveling off of the δ¹³C_DIC from the middle of the lake to the distal portion, which represents recharge from 2010 and 2011, suggests that carbon cycling of the lake water attains a steady state over time. 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.