Lithological and geochemical description of modern alluvial deposits in the middle reaches of the Bolshaya Kokshaga River

The paper presents the results of studying the granulometric and elemental composition of modern alluvial deposits in the middle reaches of the Bolshaya Kokshaga River. It was demonstrated that their role in the for mation of soils is significant only in the meandering sections of the riverbed in the immediate vicinity of the water edge (7-10 m), where the mass of material deposited on the riverbed ranges from 32 to 200 t/ha over the years. It decreases to 7,2 t/ha at a distance of 25 m, and rarely exceeds 100 kg/ha at a distance of 500 m. Under the side bar type of riverbed processes, the mass of alluvium in the riverine part of the floodplain is no more than 400 kg/ha. The chemical composition of alluvium reflects the influence of two mineralogical and geochemical provinces (Central Russian and Pre-Urals). It was found that alluvium of the facies of riverine ridges contains a greater amount of Si, Na, Zr, Sr, and Cr; the riverine floodplain alluvium contains more Al, Fe, K, Mg, Ti, and As, while the inner floodplain alluvium contains more Ca, P, Mn, S, Ni, Cu, Zn, Ba, and Rb. Six geochemical associations have been identified in alluvial deposits, i. e. Fe-Ca-Mn-S-P-Ba-Zn, Al-K Mg-Ti-Cr, Si-Na-Zr, Fe-Mn-Ni-As, P-Cu, and Sr-Rb. The intake of Si, Na, Sr, and Zr is associated with the sand fraction; Al, K, Mg, Ti, and Ni come from the silty fraction, and Fe, Ca, P, Mn, S, Ba, and Zn are derived from the dusty fraction. Relative to the soils of the eluvial landscape of the Mari Polesie alluvial deposits are enriched (EF > 1) with Fe, Ca, P, Mn, S and Zn (biogenic input factor), Al, K, Mg, Ti and Cr (lithogenic factor), As well as Ni and Cu (chemogenic factor). They accumulate (KK > 1,5) Mn, P, As, Cu, Ni, Zn, S, and Zr, and dissipate (KP > 1,5) Al, K, Mg, Ca, Na, Rb, Ti, and Sr; and the values of Fe, Ba, and Cr are close to the Clark ones. The concentration of elements in alluvium are higher than the hygienic standards (MPC and ODC) for S, Mn, As, Cu, Ni and Zn. The content of chemical elements in alluvium could be considered as close to the natural (background) level.