Aqueous aluminium chemistry response to episodic increases in discharge

Abstract

Increased aluminium mobilization and transport from edaphic reservoirs to surface waters have been documented as major effects of atmospheric deposition of mineral acids^1,2. High concentrations of hydrogen ions and aqueous aluminium in stream water tend to occur at times of heavy runoff^3–6. Some forms of inorganic monomeric aluminium are highly toxic to freshwater biota. It is therefore important to know more about changes in aqueous aluminium speciation and associated chemical parameters during high–discharge episodes. We present here recent data from the Birkenes catchment in southernmost Norway, demonstrating that short-term fluctuations in inorganic aluminium in stream water cannot be explained by assuming equilibrium with a single mineral such as gibbsite (Al(OH)₃). In some conditions [Al³⁺] may decrease or remain constant with decreasing pH, thus violating the gibbsite equilibrium assumption. Aluminium in stream water is contrasted with data from soil lysimeters which showed higher pH and concentrations of inorganic aluminium. Lysimeter concentrations were less variable with hydrological conditions. To explain the observed patterns of aluminium fraction concentrations and pH, we postulate changes in flow paths with discharge and/or kinetically controlled dissolution of aluminium.