Water Chemistry and Stable Isotopes of Different Water Types in Tajikistan

The concentration of major ions and their molar ratios are used to identify sources of the solutes and highlight their hydrochemical evolutions in surface waters. Oxygen and hydrogen isotopes of stream and lake waters are ideal to characterize sources and transport pathways of incoming precipitation. In order to analyze the spatial distribution characteristics and further explore the formation reason and environmental significance, major ions and isotopes of hydrogen and oxygen were measured in lake, river and spring water samples collected in Tajikistan from September to October 2011. Ca²⁺ and HCO₃⁻ were the dominant ions of river and spring waters, and originated mainly from carbonate weathering. The northwestern waters were of the Ca-SO₄²⁻ type resulting from silicate weathering. Most lakes of Tajikistan are located in the eastern, mountainous area and are of the Mg-SO₄²⁻ and Na-Cl⁻ type, reflecting persistent, long-term evaporation under an arid climate. The hydrogen and oxygen isotope values of river and spring water in Tajikistan ranged from −129 to −65‰ and −17 to −9.3‰, respectively, and display larger variance in spatial distribution, indicating that river and spring waters in the east are supplied mainly by glaciers and waters in the west come mainly from precipitation. Regression lines between hydrogen and oxygen isotopes in waters from the eastern (δD = 9.2δ¹⁸O + 28.0) and western (δD = 5.5δ¹⁸O - 10.7) rivers and springs differ as a consequence of different moisture sources in the two areas. Significant correlations between hydrochemistry and isotope values in lake water mainly reflect the intensity of evaporation under arid climate conditions.