Abundance, sources and speciation of trace elements in humus-rich streams affected by acid sulphate soils

The behaviour of trace elements (Al, As, Cd, Co, Cr, Cu, Fe, Mn, Ni, V, Zn) was studied in five humus-rich streams (dissolved organic carbon = 14-40 mg/L) impacted by acid sulphate soils developed in marine sulphide-bearing fine-grained sediments. During heavy rainfalls in autumn, on which the study focusses, the metals Al, Cd, Co, Cu, Mn, Ni and Zn are extensively leached from these acidic soils (pH = 2.5-4.5), while As, Cr, Fe and V are not leached more strongly from this soil type than from areas of till and peat. A speciation experiment, based on anion and cation exchange of the stream waters in the field, showed that (1) the metals Al, Cd, Co, Mn, Ni and Zn are transported in the streams mainly as inorganic cations, (2) Cu exists mainly in cationic form but is also to a significant extent associated with dissolved humic substances, (3) Fe occurs mainly in the anionic fraction explained by organic coating on colloidal Fe oxyhydoxides and (4) the hydrochemistry of As, Cr and V is complex as these elements may exist in several unquantified anionic fractions and to a minor extent in cationic species/forms. Whereas the proportion of acid sulphate soils in the catchments had a large impact on concentrations levels of several elements in the stream waters, these soils did not have a large affect on the speciation of elements in water.