and deployment; ii) no need for sample purging; iii) a calibration in a synthetic river solution that reproduces well the speciation changes of the natural samples and allows the oxygen interferences to be minimised; iv) the addition of a background electrolyte up to 0.01 mol L-1 in both the calibration and freshwater samples; v) chemical stripping chronopotentiometry as the quantification stage of AGNES. This procedure minimizes the effects of working at low ionic strength and in the presence of dissolved oxygen. In the laboratory, the methodology was checked with different natural samples taken from rivers Garonne, Gave de Cauterets and Gave de Pau in the Pyrenees. Results appeared in good agreement with theoretical estimations computed from Visual Minteq. On-site measurements were performed for the first time with AGNES in the Gave de Cauterets in Soulom (France) and the results were corroborated with purged measurements performed in the laboratory.