The ecogeochemical condition of surface waters and bed sediments in the delta of the Mekong River (Vietnam)
The general characteristic of a chemical composition and quality of surface waters and bed sediments in the delta of the Mekong River in the territory of the Socialist Republic of Vietnam on the basis of the given geochemical research in 2001-2014 is received. Ranges of change and average values of рН, specific electric conductivity, bichromate oxidability and content of main ions (Ca <sup>2</sup>+, Mg <sup>2</sup>+, Na+, K+, HCO <sub>3</sub>", SO <sub>4</sub> <sup>2-</sup>, Cl"), biogenic substances (NH <sub>4</sub>+, NO <sub>3</sub>", NO <sub>2</sub>", PO <sub>4</sub> <sup>3-</sup>, Si), iron and microelements (Al, Mn, Zn, Cu, Pb, Cd, As, Hg) in the waters and sediments of the delta arm and branches are established. Comparison of the data on the chemical composition of bed sediments in the delta of the Mekong River, on upper sites and in other large rivers of Southern and Southeast Asia is carried out. The conclusion about the comparability of heavy metals and arsenic content is made. It is established that the content of some elements in the delta of the Mekong appreciably exceeds the corresponding parameters for other natural zones in Asia (Zn - by 9-11 times higher than in tundra and forest-tundra, about twice higher in comparison with taiga; Cu - by 6-20 times higher than in tundra and forest-tundra, and on the average by 1.5 times higher than in taiga). The increase in the sum of the dissolved substances in surface waters and water extracts from bed sediments is marked depending on the distance to the sea (in the process of mixture of sea and river waters) and the time of interaction of water with minerals and organic-mineral substances (within the hydrological year from summer-autumnal floods to winter-spring low water and in the transition from surface waters to porous solutions of bed sediments). A condition change of the system "water - gas - organic substance - rock" is also simultaneously observed. This change is connected with the reduction of unsaturation of surface waters, achievement of balance and even some supersaturation with carbonate minerals and humic acids compounds in water extracts of bed sediments on a site up to 10 km from the sea edge of the delta. The formation and subsequent removing of poorly soluble substances from water environment as well as the sorption processes connected with them can serve as a reason of accumulation of some microelements in bed sediments. The general condition of surface waters and bed sediments in the delta of the Mekong River can be characterized as unsatisfactory by the content of nitrogen compounds (NO3 <sup>-</sup>, NH4 <sup>+</sup>), iron and some microelements (Hg, Cu, Zn, Mn, Fe, As, Al). It limits the use of waters in the economic-drinking purposes and determines a necessity of special preparation of surface waters and bed sediments for the needs of agriculture and municipal services. Similar preparation should be directed on the activation of poorly soluble substance formation that are less toxic than initial substances.