Comparing the behaviour of aluminium species within the hydrological cycle in Westland, New Zealand, and the Fichtelgebirge, Germany

Abstract

In order to compare the chemodynamics of aluminium in acid-sensitive river catchments with and without atmospheric deposition of strong mineral acids, we measured ionic compounds in soil, stream and ground waters in the Fichtelgebirge, Bavaria, and pristine Westland, N.Z. In the Fichtelgebirge high concentrations of H 3O +, NO 3 −, SO 4 2− and NH 4 + in precipitation indicate an impact of ‘acid rain’ from densely-populated and industrialized central Europe, whereas in pristine Westland concentrations were generally very low except for seaspray derived ions. The low pH values of the Fichtelgebirge (pH≈ 4.2) are due to an excess of strong mineral acids, whereas the acidity of rain in Westland is controlled by dissolved CO 2 (pH≈ 5.6). Soil, stream and ground waters in the Fichtelgebirge were more acidic and contained significantly more sulfuric acid derived SO 4 2− than waters in Westland. In the Fichtelgebirge we measured, at peak runoff, total monomeric Al in soil water of c(Al-tot)∼ 80 μmol1 −1 with no vertical decline, and in stream water c(Al-tot)∼ 50 μmol1 −1. In Westland, however, we measured in soil water c(Al-tot)∼ 40 μmol1 −1 declining to c(Al-tot)∼ 4 μmol1 −1 half a meter deep. Thus, in stream water of Westland, the peak concentration was only c(Al-tot)∼ 20 μmol1 −1. The difference in Al-species distribution is even greater: In Westland we observed predominantly organic complexed Al in soil and stream water, whereas in the Fichtelgebirge inorganic Al plays the dominant role with free Al accounting for more than 60%. As sulfate is highly mobile in the hydrological cycle, and the acidity of rain and snow is neutralized mainly by exchange of basic cations and Al within the soils, we observe considerable transport of Al along the hydrological pathway. In Westland, however, where the exchange of basic cations neutralizes most of the input of acids, low concentrations of Al are associated with alkalinity. Our observations of temporal variations in saturation indices in stream waters seem to confirm the conclusion of Neal et al. (1990) that mixing of soil and ground water controls the solubility of Al.