Occurrence of non-monomeric species of aluminium in undersaturated soil and surface waters: Consequences for the determination of mineral saturation indices

The occurrence of a polymeric and/or colloidal Al pool was demonstrated on an experimental basis in various natural acid soil solutions and stream waters, even in the pH range 3.3-4. A number of these waters were found to be undersaturated with respect to various relevant mineral phases (amorphous AI(OH)(3) and AlPO4, gibbsite, boehmite, variscite, imogolite, kaolinite, halloysite, jurbanite, basaluminite and alunite). Non-monomeric Al species should not occur in undersaturated aqueous systems in equilibrium; therefore the polymeric/colloidal Al pool observed in these undersaturated waters cannot be in equilibrium with monomeric Al. As a consequence, the traditional determination of saturation indices (SI) by model equilibrium calculation using total concentrations of every element is considered to be unreliable, and any valid procedure should involve the determination of monomeric species (or a part of them) by analytical speciation.