Volcanic ash as a factor for radioecological conditions in Kamchatka part 1. The abundance and hypergene transformation of Th and U in recent volcanic ash

Concentrations of Th and U in fresh ash ejected by active volcanoes in Kamchatka are generally considerably below their Clarke values in their respective types of rock. The bulk concentrations of Th and U in andesitic and basaltic ashes do not show any obvious differences. The concentrations of their mobile forms decrease with increasing basicity of the ash. We revealed overall positive linear trends in the series of bulk concentrations for Th and U and a negative trend of Th/U in ashes that have been arranged over eruption year. Considerable quantities of mobile forms of radioactive elements fall during ashfalls in dissolved form or are sorbed by falling snow directly from the dispersive medium of volcanic aerosols. The original fractions of all of the mobile forms are 0.68–1.93% for Th and 1.16–3.21% for U. The bulk of these come to the Earth’s surface during eruptions when no snow cover was available. The quantities of mobile forms of Th and U were considerably decreased when ejected by winter-time eruptions during the time they resided in the snow.