Aluminium mobilization in soil and stream waters at three Norwegian catchments with different acid deposition and site characteristics

Abstract

Streamwater, soil water, and soil chemistry data are collated for three small (18–41 ha) Norwegian headwater catchments and intercomparisons are made to allow assessment of the effects of acid deposition on aluminium mobilization. Two of the sites (Birkenes I and II), situated in the most heavily impacted area of southern Norway, show pronounced differences in streamwater chemistry, especially at highflow. The pH and inorganic monomeric aluminium (Al i) at Birkenes I reach 4.2 and 20 μM respectively, compared with pH 5-4.6 and 3 μM Al i at Birkenes II. The third catchment (Ingabekken) is in a pristine area in mid-Norway where streamwater pH changes from 7.2 at baseflow to ∼ 5 at highflow; in this case, Al i is generally < 1 μM. The differences in streamwater chemistry are mirrored in the soil solution composition at the three sites. Major differences also occur in the compositions of exchangeable ions, even though cation exchange capacities are similar. In the pristine area, exchangeable aluminium is low, with the exchange complex being dominated by H +; this contrasts with the impacted sites where exchangeable aluminium is much more predominant, especially at Birkenes I. It is concluded that one of the main changes observed during acidification is the conversion of the soil exchange complex from a hydrogen-dominated form to one where aluminium plays an increasingly important role. Such a transformation is not included in most acidification models and should therefore be a focus for further model development.