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The main interest of this study is to assess whether uranium mineral deposits located in the San Marcos outcrops (NW of Chihuahua City,Mexico) could be considered as a source of U-isotopes present in suspended matter of surface water of its surrounding environment.The aim of this work was to assess the fractionation and distribution of isotopic uranium contained on both suspended matter and water.Samples were taken in three points along the San Marcos dam: water input,midpoint,and near to dam wall.To evaluate the uranium behavior in surface water,every water sample was separated in suspended particulate matter fraction and in dissolved fraction,above and below of 0.1 μm,respectively.Likewise,suspended particulate matter was divided and analyzed in three particle size fractions: coarse fraction (>20 μm),medium fraction (20 – 11 μm),and fine fraction (11 – 0.1 μm).Activity concentration of U-isotopes in samples was determined by liquid scintillation spectrometry.The results show that most uranium was in the suspended particulate matter.The distribution coefficient kd,calculated by the activity concentration ratio between suspended particulate matter and dissolved fractions,ranged between 1.6 ·101 to 1·103 L/g.In dissolved fraction,high concentrations of 234U were found,where the activity ratio (AR,238U/234U) ranged from 2.1 to 3.5.In suspended particulate matter,U was fractionated depending of particle size.In average,uranium contents tend to be concentrated in coarse fraction,with AR near to unit.Medium and fine fractions showed 238U concentration higher than 234U.Uranium distribution varies according to sampling point.The highest uranium concentration was at water input point.The highest 238U concentrations were found: in the coarse fraction at the input point,in the medium fraction at the midpoint of the dam,and in the fine fraction by the nearest point of the dam wall.The results have shown a possible lixiviation of uranium from geological substrate into the surface water and an important fractionation of isotopic uranium,which has in consequence that uranium is distributed non homogeneously along San Marcos dam.