The water column chemistry of lake St Eloy (St Eloyles Mines, Allier, France) was monitored over a period of a year. This is a small stratified lake created by subsidence of an underground coal mine. The results show a permanent stratification and the development of two water bodies with contrasting chemistry. The superficial oxic layer (i.e. the upper 5 m) is of high water quality and the water, not affected by seasonal changes, is suitable as a source of drinking water. In contrast, the deeper layer is permanently anoxic and the waters have a conductivity which is six times greater than in the epilimnion. Due to density difference and the small size of the lake, no turnover occurs and all elements that diffuse from bottom sediments are strongly concentrated in the hypolimnion. Arsenic concentrations are very low in the epilimnion, but they increase quickly with depth below the oxycline, to reach up to 7 μm below 8 m depth. Different solid phases could control the fate of arsenic in this lake such as gypsum for As(V) in the hypolimnion, mackinawite for As(V) and As(III) in the hypolimnion, and close to the oxycline, Fe(III) oxyhydroxides are possible hosts for both As(III) and As(V).

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