The effect of acid soil leaching on trace element abundance in a medium-sized stream, W. Finland

Abstract

The concentration variations of 16 trace elements were determined along the main stem of a medium-sized stream (catchment area=107 km 2), which drains areas covered with acid sulphate soils developed on sulphide-bearing marine sediments. During high flows in autumn, there was a strong downstream increase in the concentrations of Al, Cd, Co, Cu, Mn, Ni, Se, U and Zn and a moderate increase in those of Cr and Tl, related to extensive leaching of the acid sulphate soils, which increase in abundance from the headwater towards the basin outlet. During high flow in early summer, the downstream increase in the concentrations of these elements was not as strong as in autumn, due to decreased amounts of available mobile element fractions in the acid sulphate soils. Under baseflow conditions, the runoff from areas with acid sulphate soils is low in comparison to that in areas covered with other soils/sediments, resulting in relatively small loads of trace elements throughout the stream. The concentration variations of As, Pb, Sb, Ti and V were unrelated to catchment cover and did not vary along the stream in a regular manner. These 5 elements are, therefore, in contrast to the others, not leached more abundantly from the acid sulphate soils than from other soils/sediments. Based on the identified hydrogeochemical features and controls, it is suggested that water-quality improvement measures should include methods that primarily aim at reducing the leaching of hazardous chemical elements in the source areas (acid sulphate soils).