Toxic Metals in Runoff from Boreal Acid Sulphate Soils.
Mats Astrom, Kalmar Univ, Dept of Biology and Environmental Science, Kalmar, Sweden
Finnish acid sulphate soils, developed in an area covering approximately 3000 km2, deliver large quantities of acidity to drains during snow melt in spring and heavy rains in summer and autumn. This is a well known phenomenon, generally thought to be the main reason for the annually occurring fish death in many affected streams and estuaries. While there is no question that the acidity release is of environmental concern and deteriorate the surface-water quality, several recent studies have indicated that potentially toxic metals, accompanying the acidity, may be an even larger environmental threat. A recent nationwide hydrogeochemical investigation, based on more than 1000 headwater streams distributed throughout the country, showed that while there are several sources of low pH waters, including acid sulphate soils, metals such as Ni, Be, Co, Zn, Tl and Cd are strongly enriched only in streams draining areas where acid sulphate soils are widely spread. Other studies have shown that ditches running through acid sulphate soils have moderately to strongly elevated concentrations of a number of metals including Al (up to 300 mg/l), Mn (up to 16 mg/l), Ni (up to 1 mg/l) and Co (up to 0.6 mg/l), and that acid sulphate soil runoff carries much larger quantities of these and several other metals than is discharged in effluents from the entire Finnish industry. These new findings have not yet been fully discovered by the environmental authorities, and consequently the field techniques developed and utilised to combat the problems have, in addition to being overall inefficient, focused only on pH. Therefore, the acid sulphate soil leakage persists and is currently causing extensive loadings of toxic metals both on surface waters and downstream sediments. The forthcoming Finnish acid sulphate soil research should thus focus on three main things. First, biogeochemical and hydrological processes in order to better understand the mechanisms of metal release and dispersion. Second, development and implementation of field techniques which should significantly reduce the acidity and metal leaching. Third, efforts to make the acid sulphate soil problem known among the authorities and known in national and European environmental legislation.