Distributions and geochemical behaviors of oxyanion-forming trace elements and uranium in the Hövsgöl-Baikal-Yenisei water system of Mongolia and Russia

We investigated the distributions and geochemical behaviors of two oxyanion-forming elements: vanadium (V) and molybdenum (Mo), and of uranium (U) in the Hövsgöl-Baikal-Yenisei water system. The vertical profiles of these elements in Lakes Hövsgöl and Baikal were almost constant, except V showed slightly lower concentrations in the epilimnion, probably because of biological activity. Based on residence time calculations, it is inferred that V and U are removed from Lake Baikal via some biogeochemical processes, whereas Mo is almost nonreactive within the lake. Such differences might be attributable to their adsorptive behaviors onto solid phases, including sediments and suspended particles. The dissolved concentrations of these elements showed similar variation in the Egiin Gol and Selenga Rivers, and the highest concentration within these rivers was observed after the confluence of the Orkhon River, a tributary of the Selenga River. The concentrations in the Angara River were almost constant along its length. On the other hand, in the Yenisei River, the inflow of a tributary (the Nyzhnyaya Tunguska River) strongly affected the distributions of the trace elements. The concentrations of V and Mo at several sampling stations, particularly on the Selenga and Orkhon rivers, exceeded the maximum permitted concentration of Russian fishing industry regulations (MPCfish). The high concentrations in the Orkhon River might be attributable to pollution, whereas those in the Selenga River are probably natural because its watershed has been affected little by mining and other anthropogenic activities.