Fractionation of chemical species in surface water from El Granero reservoir, Chihuahua, Mexico

Chemical species are highly dependent on the physical and chemical parameters of aquatic ecosystems. The aim of this work was to determine and characterise the fractionation of several chemical species (in colloidal and dissolved phase) in water from El Granero dam. Total dissolved solids, hardness, and chemical oxygen demand were recorded in situ. Soluble ions (Ca+, Mg+, Na+, K+, Cl-, F-, and SO42-) and alkalinity were determined by atomic absorption spectrophotometry and the Mexican standard, respectively. Ultrafiltration was used to separate the fractions (colloidal and dissolved) from the water samples. Elemental, radiological, mineralogical, and morphological characteristics were assessed using inductively coupled plasma-optical emission spectrometry, alpha spectrometry, X-ray diffraction, and scanning electron microscopy with energy dispersive X-ray analysis, respectively. The results showed that easily fractionated trace elements are more concentrated in colloids. Ca and Mg composed colloids, with low contents of Fe. The most abundant trace element in colloids were Sr, Li, Zn, Pb, As, Cu, Mo, V, Ti, and Cr. Conversely, Cu and V were in dissolved fraction. Colloids had high contents of U-238. Considering mineralogy, colloids are composed by calcite (CaCO3), halite (NaCl), and sodium carbonate/sulphate (Na6CO3(SO4)(2)). The colloidal material showed the formation of agglomerates due to their charge. The colloids were mainly formed by calcium and magnesium, which is consistent with the geological and arid conditions of the zone. From colloids composition, it was possible to obtain the fractionation of selective particle-reactive elements in surface water.