CATCHMENT BASED ANALYSIS OF MATTER FLOWS IN THE SELENGA–BAIKAL SYSTEM.

A regional basinwide assessment of the formation of the water and suspended matter runoff was carried out for theSelengaRiver–Baikal Lakesystem. It is based on the results of integrated hydrological and geochemical research of more than 100 locations withinRussiaandMongoliaperformed by the authors in 2011–2013 using the unified methodology during various hydrological periods. It is shown that the quantitative parameters of the suspended sediment transport are largely determined by the seasonal hydrology. Contribution of floods in the suspended sediment transport ranges from 52% of annual runoff for the large rivers up to 99% for the small ones. Chemical composition of 400 samples of the water and suspended sediment was analyzed by ICPMS / ICP-AES method. The obtained results demonstrate that Selenga River basin is enriched relative to the average content in the World Rivers by dissolved Sr, Li, U, Br, B, Mo, As, migrating predominantly in the ionic form, as well as by Fe, Al, Zn and Pb, migrating in the form of complexes with organic matter. Suspended sediments of the Selenga River and its tributaries are enriched with As, Cd, Mo, Pb, Zn, Mn, Co, which is due to both the geochemical specialization of the region (As) and the technogenic influence (Cd, Mo, Pb).

Analysis of particulate/dissolved modes of chemical elements in river water showed the prevalence of particulate modes of most heavy metals and metalloids (HMM) in the upper part of the basin and dissolved ones in the middle and lower parts. The share of suspended forms of migration increases dramatically during the floods, especially in the upper reaches of the rivers; in the lower part of the basin the impact of floods on the ratio of forms decreases, which is especially noticeable in theSelengadelta. During the floods geochemical fluxes of dissolved HMM increased towards theSelengadelta with increasing water runoff. Extremely high flows of suspended forms caused by heavy rainfall were determined in the upper part of the basin that rapidly decreased downstream due to sedimentation. In the lower part of the basin, the flows increased again due to the confluence of major tributaries. The largest anthropogenic changes in fluxes are typical for small rivers (Modonkul, Hangalyngol, Tuul, Uda et al.), impacted by the towns and mining facilities. Extremely high pollution by Cd and other metals was revealed for theModonkulRiverthat is significantly affecting the geochemical flows of HMM in theDzhidaRiverup to its confluence with theSelengaRiver.

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