The waters of Lake Chad in Central Africa are dilute and of the Ca-Mg-Na-HCO3 type with low Cl and SO4. In the Kanem region, NE of Lake Chad, such waters are subjected to evaporative concentration and brines and evaporites form in the many depressions between sand dunes. Three brine types evolve, each representing a specific subenvironment. The subenvironments can be classified as 1. (a) small, saline playas (0.5 × 2 km) with shallow groundwater tables, 2. (b) small perennial lakes (interdunal lakes) and 3. (c) lakes and playas located on islands within Lake Chad. The behavior of the major solutes can be interpreted by using chloride as a monitor of the extent of evaporative concentration. The first two subenvironments produce the alkaline brines one would expect, with trona (NaHCO3· Na2CO3·2H2O) the dominant evaporite mineral. The two environments differ with respect to the behaviour of potassium. While K is largely removed from the groundwaters of the playas, except during late-stage concentration, the interdunal lakes conserve potassium throughout evaporative concentration. The presence or absence of wetting-drying cycles are thought to be responsible for this dichotomy. The brines of the island lakes are Na-Cl-SO4-rich, with thenardite (Na2SO4) and halite (NaCl) the common evaporite minerals. These brines indicate preservation of sulfate and a very pronounced relative loss of carbonate species. Their evolution is accounted for by excess sulfate supplied through oxidation of lake sediments deposited by Megachad some 5000-9000 yr ago, a process which has been documented for near-surface waters of 'polders', arms of Lake Chad which have recently been dried up by building dams. © 1979.
