Rivers can be considered as arteries, which bring suspended and dissolved matter of organic and inorganic origin to the ocean. The characteristics and properties of the matter depend on the climatic, geographical, and geological features of water-catchment area and on human activity. Natural and anthropogenic components of load material induce intense biogeochemical processes in the region of mixing of river and seawater. According to the modern concept of the geochemical "river-sea" barrier, the river flow is not a simple mechanical delivery of matter from land to ocean. According to Lisitsyn [7], the main accumulation of sedimentary material of the Earth occurs in that geographically small area. He considered the estuaries and deltas of rivers of the world as the first global level of avalanche sedimentation. Biological processes, the intensity of which in estuaries is several times higher than in adjacent areas of the sea, are rather significant for substance flows. It is quite apparent that carbon dioxide is a key factor, as CO 2 is the basis of construction of organic matter and is extracted from the environment during photosynthesis. At the same time, it is excreted in the environment during biological oxidation of suspended and dissolved forms of organic carbon. Moreover, carbon dioxide is a gas influential the greenhouse effect, which affects the climate of the planet. The intensity of processes occurring in estuaries with the participation of carbon dioxide determine to a significant extent the global cycling of carbon [15,30]. One may support Cai and Wang's opinion [15] that the number of publications dedicated to study of the carbonate system of estuaries does not correspond to the relevance of the problem. The existing situation, in our opinion, is caused by the presence of methodological problems related to measurement and calculation of the parameters of the carbonate system of estuaries. Now techniques [17] enabling sufficiently high reproducibility of measurement of all four parameters of the carbonate system of seawater have been developed: DIC-dissolved inorganic carbon, pCO 2 -partial pressure of carbon dioxide, TA-the total alkalinity, and pH. The concentration constants of carbonic acid for the range of salinity of 30-40‰ were measured reliably enough [25]. Nevertheless, there are some unsolved methodological problems in studying the carbonate system of seawater, first of all, the problems of intercalibration of measurements of pH and alkalinity [26], the intrinsic coherence of the four gauged parameters and the constants of carbonic acid [23], and precise pCO 2 measurements in discrete assays. The given problems are essentially complicated when a study concerns riverine waters and areas of riverine and seawater mix [20]. In this case, standard methods and tech-BIOGEOCHEMISTRY Abstract -Two methods, the total alkalinity measurement by Bruevich [4] and pH measurement in a cell without liquid junction [11], were suggested for study of the carbonate system of estuaries. Based on new measurements...
