Spatial variation of cations, anions and physicochemical variables in the Solimões-Amazonas River, between Manaus and Jutaí, Amazon Basin

Major cations and anions are chemical species with great importance in different water compartments. However, no specific attention has been given to them in the Amazon basin. This study aimed to assess the spatial dynamic of Na⁺, Ca²⁺, K⁺, Mg²⁺, Cl^- and SO₄ ^2- in a specific section of the Solimões-Amazonas river taking into account the alkalinity, hydrogen potential (pH), conductivity electric (EC), total suspended solids (TSS) and total organic carbon (TOC). Samples were collected in May, July and September 2012 in seven points in the Solimões-Amazonas River and in the mouth of some of its main tributaries: rivers Jutaí, Juruá, Japurá and Purus. Cations and anions were analyzed by ion chromatography, TSS by gravimetry, pH by potentiometry, alkalinity by potentiometric titration, EC by conductimetry and TOC by catalytic oxidation combustion. The average cation concentrations followed this order: Ca²⁺ > Na⁺ > Mg²⁺ > K⁺ and anion SO₄ ^2- > Cl. From the upstream of the river towards its downstream section there was a tendency for a decrease in cations and anions accumulation, pH, EC, TSS and alkalinity. On the other hand, there was an increase in TOC. The present study showed changes in chemical characteristics of Solimões River along its course. It was observed the contribution of the tributary rivers as one of the responsible factors for these changes.

hydrochemistry; ions; Amazonas River