Mobility of Al, P and alkali and alkaline earth metals in acid sulphate soils in Finland

The concentrations and forms of Al, Ba, Ca, K, Mg, Na, P and Sr were determined in acid sulphate soils and in the parent materials of the soils (sulphide-bearing fine-grained sediments) at five sites in western Finland. Samples were taken from each vertical 10-cm section, except from the topsoil, in pits ranging in depth between 2 and 3 m. In addition, a bulk sample was collected from each of three zones in each pit: (1) the middle section of the acid sulphate soil; (2) the transition zone (characterised by a steep pH gradient); and (3) the reduced zone. The former samples were dissolved in aqua regia (3:1:2 HCl:HNO3:H2O), while the latter were dissolved in aqua regia and in concentrated acids (HClO4–HNO3–HCl–HF at 200°C) and were subjected to extractions with ammonium acetate; element concentrations in each leachate were determined with ICP-AES. In the soils and sediments studied, Al, Ba, Ca, K, Na and Sr are only to a limited extent extracted in acid (aqua regia), which shows that they are associated mainly with `resistantʹ minerals such as feldspars, dioctahedral mica and amphibole. In contrast, Mg and P are largely extracted in acid. It is therefore argued, that Mg is mainly held in the structures of trioctahedral mica/clay minerals and that P is in the form of phosphate. It is also shown that (1) Ca, Na and Sr have been lost in considerable amounts from the acid sulphate soils, (2) Mg, K and Al have been lost from one or several soil profiles, but only in small proportions and (3) Ba and P have not been lost from the acid sulphate soils in detectable quantities. The losses of Al, Ca, K, Mg, Na and Sr are assumed to be a result of leaching of trapped metal salts and of fractions released in cation exchange reactions and from altered silicates. The limited losses of Ba and P are ascribed to precipitation of Ba sulphate and retention of phosphate on positively charged compounds, respectively. Existing hydrogeochemical data show that the concentrations of Al, Ca, K, Mg, Na and Sr are elevated in the water of streams draining areas of acid sulphate soils and that the concentrations of Ba and P are not elevated in the water of such streams. There is thus agreement between the geochemical data presented in this study and existing water data. It can therefore be concluded, that artificial drainage of sulphide-bearing fine-grained sediments will have a strong impact on the loads of Al, Ca, K, Mg, Na and Sr in stream water, but no effect on the stream-water loads of Ba and P.