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

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Abstract
Rates of release of monomeric aluminium .sbd. Alm .sbd. were determined for steambed materials placed in a laboratory channel and exposed to acid water, and values of the release rate coefficient, R (.mu.mol Al released per m2 of bed per second) were calculated. Estimates of R were also made from the reported values of experiments in which streams were artificially acidified. The values of R ranged from 0.1 to 3.2 .mu.mol 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 Alm 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 Alm are possible. For a liverwort-richbed exposed to water of initial pH 3.5 there may be as much as 60 .mu.M Alm in the streamwater. At the other extreme, a mineral bed subjected to pH 4.5 water would yield less than 1 .mu.M Alm. Streambed stores of releasable Al appear sufficient to supply Al to the water for significant lengths of time.