Release of cationic aluminium from acidic soils into drainage water and relationships with land use

Abstract

SUMMARYThe concentration of cationic monomeric aluminium (A13+) was determined in streams draining areas in different land use. Relationships between the concentrations of A13+ and companion ions were examined both for streams and for eluates from soil leached in the laboratory with simulated rainwater that ranged in pH and salt concentration.The concentrations of A13+ were consistently greater in streams draining Sitka spruce woodland than in streams in adjacent catchments draining rough grazing. In no case was the A13+ concentration governed by the solubility product of gibbsite. The concentrations of A13+ were very closely correlated with excess anions (total inorganic anions minus basic cations) both for stream water and for eluates from soil leached with simulated rainwater at a constant pH equal to that of the soil (3.8).Exchangeable A1 was the source of A13+ in leachates from soil in the laboratory and the displacement of exchangeable Al was the dominant process accounting for the levels of A13+ in acidic streams. Hydrogen ions were much more important than basic cations in displacing exchangeable Al from the acidic soil used in the laboratory experiments and probably from soils in the field. The greater excess of inorganic anions in streams from Sitka spruce woodland probably resulted from a greater anion excess in the input water (acid rain) together with a greater NO, production in the soil. All three major anions, CI, SO4 and NO3 contributed to the greater anion excess.