The objective of this study is to clarify the biogeochemical characteristics of the Hövsgöl–Ustilimsk water system in Mongolia and Russia. For this purpose, we measured dissolved major elements, minor elements, and nutrients in surface water throughout the system. Calcium ions and HCO3 + 2CO3 were the dominant cations and anions in Lake Hövsgöl, respectively. As the water flows down from Lake Hövsgöl to the Egiin Gol and Selenga Rivers in Mongolia, the water quality derived from carbonate rock was found to be influenced by river confluences. In the Selenga River in Russia, major elements were diluted with low-salinity waters from the tributaries. At the boundaries between the Angara River and Bratsk or Ustilimsk Reservoirs, the behaviors of nutrients were affected by the transition of the water area from a riverine zone to a lacustrine zone. Although the composition of cations and anions changes gradually as river water flows further downstream, the water type remains to be Ca–HCO3 throughout the system. Thus, it can be concluded that the fundamental water quality of this system was determined in northern Mongolia, the source area of this water system, and that the environmental factors such as the climate, geology and, geography in its basin from Hövsgöl to Ustilimsk regulated the dissolved chemical components.
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