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

Abstract

Lake Ngami is an endorheic lake in the distal portion of the Okavango Delta in semiarid Botswana. The lake was dry until 2009 when it began filling with water. The physical, chemical, and stable isotopic composition of the lake water was documented in order to evaluate the processes that control water properties, and establish baseline values for future temporal and spatial comparisons. Field measurements were made and water samples were collected 25 cm below the surface along a ~18 km axial transect from the inflow river to the distal end of the lake. The major ionic concentrations (e.g., Cl-, Na+, Ca2+) and the stable isotope ratios of oxygen (�18O) and hydrogen (�D) showed three distinct regions of increasing concentrations and isotopic enrichment, respectively along the transect. The �18O vs. �D data plot along the Okavango Delta evaporation line and suggest modification of lake chemistry by evaporation. Because the lake's inflow showed little chemical and isotopic variation over the past three years, it is possible that the increased ionic concentrations in lake water are due to evapoconcentration and that the segmentation of major solutes, �18O, and �D into three regions along the transect, is a result of differential evaporation of lake recharge from 2010, 2011 and 2012, and are thus controlled by the residence time of recharge in the lake. Unlike the major ions, the dissolved inorganic carbon (DIC) concentrations and the stable carbon isotopic ratios (�13CDIC) increase along the transect to about midway in the lake, and then reach steady state. Carbon cycling in the lake is controlled by evaporation and equilibration between carbon in DIC and atmospheric CO2(g). The results show the importance of evaporation and residence time in controlling the solute chemistry and the dominance of the atmospheric CO2(g) in controlling the carbon isotopic signature during the filling stage of an endohreic lake in an arid environment.