Another look at the chemical relationships in the dissolved phase of complex river systems

Large rivers are a major pathway for the erosion products of continents to reach the oceans. The riverine transport of dissolved and particulate materials is generally related to a large number of interactions involving climate, hydrological, physico-chemical and biological aspects. Consequently, the investigation of large rivers allows the erosion processes at a global scale to be addressed, with information about biogeochemical cycles of the elements, weathering rates, physical erosion rates and CO2 consumption by the acid degradation of continental rocks. Today, good databases exist for the major dissolved ions in the world’s largest rivers. Since concentration of ions in river waters has to be considered in a compositional context, it is necessary to study the implications of considering the simplex, with its proper geometry, as the natural sample space. Using the additive (alr) or the isometric (ilr) log-ratio transformations, a composition can be represented as a real vector; but only in the second case can these coordinates be mapped onto orthogonal axes. Using data related to the dissolved load of some of the most important rivers in the world, the relationships among the major ions frequently used in molar ratio mixing diagrams have been investigated with alternative tools. Following the balances approach, an investigation of the properties of aqueous solutions of electrolytes that are often treated in terms of equilibrium constants is undertaken. The role played by the source—rain water, weathering of silic, carbonatic and evaporitic rocks, pollution from which elements and chemical species can potentially be derived, has been checked through an investigation of a probabilistic model able to describe the relationships among the different components that contribute to the chemical composition of a river water sample.