Water Geochemistry of the Crater Lake on Maly Semyachik Volcano

We used data acquired during field surveys conducted over the years to study the macro component and micro component composition of water in Lake Zelenoe in the Troitsky Crater of active Maly Semyachik Volcano. We showed that the lake contains ultra-acid (pH < 1) water of sulfate–chloride composition with mineral concentrations between 8 and 42 g/L, depending on the state of the volcano. The anion composition of this lake is formed by inflow and subsequent dissolution of acid volcanic gases in an aquifer horizon situated immediately below the lake. The cation composition of the water is controlled by practically congruent dissolution of host rocks. A long period of repose was followed in 2008 by a new phase in the hydrothermal activity of the volcano lasting until today. Upon the background of a constantly increasing volume we observed increasing concentrations of main macro components (SO₄, Cl, Al, Fe), and of mineral content in general.