Long-Term Variations of Water Quality of the Selenga and Kiran Rivers at the Russian-Mongolian Boundary

Principal-Component Method is used to identify the major factors governing the long-term variations of water chemistry in the transboundary rivers of Selenga and Kiran. Their preliminary interpretation was made. Water chemistry formation in both large (Selenga) and small (Kiran) rivers is shown to be mostly governed by natural-climatic factors. Thus, an increase in water abundance has a dual effect on water chemistry: on the one hand, water is enriched with substances increasingly entering it because of the more intense erosion and dissolution of solid rocks, and, on the other hand, the concentrations of chemicals of anthropogenic origin decrease because of their greater dilution. A conclusion was made regarding a relationship between a positive trend in water temperature and an increase in the concentration of chlorine ion in water. A preliminary estimate is given to the effect of municipal and industrial wastes, as well as surface and subsurface waters of various genesis on long-term variations of river water chemistry. Trends in changes in water chemistry were studied by using approximations of long-term series of the variables that determine the interpretation of principal factors with the help of a slow trend by "caterpillar" method.