Seasonal and spatial variations in major and trace elements in a regulated boreal river (Esse River) affected by acid sulphate soils

Abstract

AbstractIn coastal areas of Finland, extensive artificial drainage of Holocene sulphide‐bearing marine and lacustrine sediments has resulted in development of acid sulphate (AS) soils (pH 2.5–4.5) over an estimated area of approximately 3000 km2. During heavy rains and snow melting, these soils are flushed resulting in discharge of acidic and metal‐rich waters that strongly affect small streams. However, the total and precise effects in the important and large rivers are not well understood. In this study, the impact of AS soil occurrence and hydrological changes on water quality was determined in an important regulated boreal river (Esse River) having a catchment area of 2054 km2 partially covered with AS soil (39 km2). Water samples, collected at five sites along the river during four carefully selected events, were analysed for pH, total organic carbon, conductivity and the following elements/anions: Al, Ba, Br, Ca, Cd, Cl−, Co, Cr, Cu, Fe, K, La, Mg, Mn, Na, Ni, NO, Rb, Sc, Si, SO, Sr, Th, Y and Zn. There is a clear spatial correlation between AS soil occurrence and elevated element concentrations in the river water, especially when the conditions change from dry/warm (summer) to wet/cool (autumn). During the rains in autumn these soils are extensively flushed and concentrations of Co, La, Zn, Y, Mn and Al are increased between three and nine times towards the outlet. The buffering capacity of the river was, however, high enough to prevent a detrimental drop in pH. Another intriguing feature is substantially elevated concentrations of several potentially toxic metals (Cr, Cd, Cu) in the middle reaches in winter when the river is ice‐covered. Since no external source for this was found, we suggest an internal source operating by an as yet unknown mechanism. During baseflow in summer, the concentrations of several solutes reach minimum concentrations. Copyright © 2005 John Wiley & Sons, Ltd.