Estimating streamwater concentrations of aluminium released from streambeds during ‘acid episodes’

Rates of release of monomeric aluminium ‐ Al_m ‐ were determined for streambed materials placed in a laboratory channel and exposed to acid water, and values of the release rate coefficient, R (μmol Al released per m² of bed per second) were calculated. Estimates of R were also made from the reported results of experiments in which streams were artificially acidified. The values of R ranged from 0.1 to 3.2 μmol m⁻² s⁻¹. They decreased with increase in pH, and were greater for beds containing substantial amounts of the liverwort Nardia compressa than for pebble and/or gravel (mineral) beds. Calculations, using, the estimates of R, were performed to assess the contribution of bed‐derived Al to streamwater concentrations, under conditions where acid water enters stream channels having exchanged negligible H⁺ for Al³⁺ or base cations in the soil. For typical high‐discharge conditions, considerable concentrations of Al are possible. For a liverwort‐rich bed exposed to water of initial pH 3.5 there may be as much as 60 μM Al in the streamwater. At the other extreme, a mineral bed subjected to pH 4.5 water would yield less than 1 μM Al_m. Streambed stores of releasable Al appear sufficient to supply Al to the water for significant lengths of time.