The objective herein is to describe the present hydrochemical state of the Large Aral Sea and quantify the ongoing changes accompanying the contemporary desiccation. Compared with the predesiccation period before 1960, the sulfate-to-chloride mass ratio decreased by about 40%, whilst the relative content of calcium decreased by a factor of nine in the western basin and a factor of 40 in the eastern basin. However, the reduction of the sulfate-to-chloride ratio in the eastern basin is smaller than that for the western basin. Because the eastern basin water, penetrating into the western trench through the connecting channel, sinks to the bottom layer and forms there a water mass partly retaining the properties of the eastern basin, the relative concentration of calcium in the western basin, generally, decreases downwards, while the sulfate-to-chloride ratio increases.
The ongoing desiccation has also resulted in significant changes in the distributions of dissolved gases in the residual water body. The once fully oxygenized sea developed anoxic conditions and intermittent hydrogen sulfide contamination in the bottom layers. However, H2S is a variable rather than a permanent feature of the present Aral Sea.