Introduction Karde dam's lake is a carbonate-precipitating and diamictic lake, which supplies irrigation and potable water for more than 4 million inhabitant of Mashhad city (NE Iran). In the following survey, Vertical (five depths 1, 5, 10, 15, 20) and temporal distribution of major cations and anions (especially nitrogen) in lakes water are investigated during 15 months (Jan 2010 to June 2011). Results The Calcite, Dolomite and Aragonite saturation index of lakes water and evolution of Na, Mg, Ca concentration in lake, local ground water and rock samples in ternary diagram indicate that Na, Ca and Mg concentrations are mainly governed by incongruent dissolution of local carbonate (Mozdooran formation) and silicate (Neogen red formation). Identification of dissolved minerals with ion activity (a) and Log(aCa2+/a(H+)2) versus Log(aMg2+/a(H+)2) graph for Carbonate minerals and Log(aMg2+/a(H+)2) and Log(aCa2+/a(H+)2) versus Log (aNa+/a(H+)2) for silicate minerals show that carbonate minerals "Calcite and Dolomite" and silicate minerals "Kaolinite, Laumontite and Clinochlore" are the dominate dissolved minerals within dam lake water. Vertical distribution of N in lake water shows significant increasing in NH4+ and NO2- concentrations with depth when the NO3- concentration decreases. Principal Component Analysis (PCA) of hydrochemical data revealed 3 components which explain 67% of the variance observed. In the first component, strong relationship between EC, TDS, Mg2+, HCO3- and hardness reflect carbonate weathering. The variables in the second component are NH3, NH4+, dissolved oxygen and depth. All the variables are directly related except dissolved oxygen which has inverse relationship with this component. This factor represents high organic production in Karde dams' lake water. The third component shows relationships in the concentrations of Na+, Ca2+, SO42- indicating the weathering of evaporative minerals especially gypsum (CaSO4) and Glabuer's salt (Na2SO4) in the upper hand basin. In the classical plot of δ 18O vs δ 2H, the isotopic signature of lake water samples (-6.82‰, -48.3‰) plot further to (LMWL) comparing to the local ground water (-8.21‰, -57.5‰) and upstream river (-8.7‰,-61.8‰) which appear to be controlled by evaporation loss of the lake water. References [1] Cioni et al. (2002) Volcanology and geothermal research 120, 179-195. [2] Marini et al. (2000) Geochemica et Cosmochimica Acta, 64, 2617-2635. [3] Hong-jun et al. (2006) Enviromental science, 19, 689-695.

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