MAJOR AND TRACE ELEMENTS GEOCHEMISTRY OF THE SELENGA RIVER, TRANSBAIKALIA

Selenge River is a transboundary river, which rises in the Khanghai Mountains in Mongolia and flows into Lake Baikal in the Republic of Buryatia (Russia). We studied the Selenga River basin (446 000 km²) located in western Siberia (Transbaikalia, Russia), and northern and central Mongolia between the Tuva-Mongol and Dzabkhan microcontinents in the west, the Siberia craton in the north and the Amurian superterrane in the south. Water resources are very limited in this basin because annual precipitation is only 250-400 mm. Moreover, 90% of annual precipitation occurs in summer, which causes frequent flooding in summer and severe shortage of water during the rest of the year. The transition from a planned economy to a market economy has resulted in inefficiently operated wastewater treatment systems which pollute the river, and reckless deforestation have increased non-point pollution sources on the lower Selenge River in Russia. In recent years, of industrial water consumption more than 50% of withdrawn water was used by the mining industry in Mongolia. The mining enterprises are located in watersheds of small tributaries, the second order of the Selenge River. In this work we present the current state of the Selenge River chemistry and estimate heavy metal pollution. From 2004, the requirement of federal law of Russia «On the protection of Lake Baikal» the new variant of regional norms MAC (Maximum Allowable Concentration) were elaborated. The concentration of the main polluting substances were compared with norms MAC.

The dissolved concentrations of major, trace elements, REE (rare earth elements), and dissolved organic carbon (DOC) have been measured in Selenge River draining watersheds with various surface areas. Measured pH for all sampling sites were within the standard range for river water, which is between 6 and 9, meantime the highest measured pH 9.2 may be due to anthropogenic influences in the area. The total dissolved solids load (TDS) is 108-297 mg l^-1. Along the river Ca/Na ratio varies 2.1-9.8 that can be explained by the heterogeneity of the crystalline rock and soil.

Mongolian and Russian parts of Selenge River having different drainage areas and hydrological parameters were sampled over a 6-year period. Collected data show that the river present the same monthly seasonal variations, with higher concentrations in winter and lower concentrations during other seasons. In waters, DOC and insoluble element concentrations (e.g., Al, Fe, and Th) were strongly correlated, which show the key role of organic colloidal matter in the transport of some insoluble elements.