Rare earth elements in surface waters of Priamurye. Features of accumulation and fractionation
In the last few decades the geochemistry and distribution of rare earth elements (REEs) in natural surface waters have become increasingly important so REEs are often used as geochemical tracers of the geochemical processes in water-bedrock interaction. The ability to use REEs as a marker is determined by their three main features: absolute content, diffraction manner and abnormal behaviour of some REEs. In the present time there are numerous studies where the REEs geochemistry and distribution in natural waters of the Earth are investigated. This helped establish that the suspended form of REEs prevails during the water migration, however their amount depends on a number of factors (value of pH, occurrence of colloids, etc.). The main goal of this study is to investigate the distribution of REEs in the surface waters of Priamurye, to test REEs applicability for geochemical typification of these waters and for allocating the sources of aqua. In addition, new data about trace elements content in the surface water of the region is presented in this study. The detailed hydrochemical data obtained for the surface streams from three rivers basin (the Amur, the Lena and the Uda) is the framework of this study. More that 15 watercourses were investigated during three years (2011-2013). Main ions were determined by ion-chromatographic methods (HPLC-10AVp, SHIMADZU), minor and trace elements were identified using IOP-AES (Plasmaquant-110) and IOP-MS (Agilent 7500c) methods in the analytical centre of the Far East Geological Institute FEB RAS (Vladivostok). The analytical precision for all REEs was better than 5 % of the relative standard deviation (RSD). The speciation of the REEs was resolved by the approaches outlined in Johannesson et al. (1996). This does not require the input of REE concentration so trace element complexation in solution is controlled by the free concentrations of complexing ligands in the natural aqua along with the corresponding stability constants for the metal-ligand complexes. Free inorganic ligand contents used in the simulation of migration forms were calculated from the main solute composition by the computer program PHREEQE (Parkhurst et al., 1980). For the result verification the computer program SELECTOR-WINDOWS (Chudnenko, Karpov, 2003) was used. The obtained results indicate that the natural surface waters of Priamurye have only a slight content of dissolved REEs (up to 1.1 ^g/l), whereas the streams with strong man-made pollution REE concentration increases considerably (up to 77.92 ^g/l). A distinct correlation between the content of aluminum and REEs testifies about the preferential water migration of REEs as clay particles sized to colloids. The surface waters of Priamurye have remarkably more light REEs in comparison with heavy REEs, the amount of LREEs makes up 8292 %. According to REE patterns in the studied waters, there is selective enrichment of middle REEs, mainly gadolinium and terbium. The distribution of REEs in the natural surface waters of Priamurye reflects the REE pattern of bedrocks; however, the absolute values of the REE content differ and depend strongly on the following factors: the residence time, the rate of the water-bedrock interaction and REE-bearing minerals located in the bedrock. The principal dissolved form of REEs in all the studied surface streams is REE <sup>3</sup>+.