Aluminium speciation in surface waters from a Welsh upland area

The complete aluminium speciation of surface waters from an acidified catchment, during dry weather and during storm episodes, is obtained. For the first time, estimates of adsorbed aluminium (Al(ad)) are reported together with other aluminium species. Al(ad) is calculated from the difference between monomeric aluminium content before and after separation of suspended solids by filtration (0.015 microm pore diameter). This fraction was very significant (Al(ad) >; 40 microg litre(-1)) for samples with high suspended solids content (ss >; 20 mg litre(-1)) collected during storm episodes. Plots of pAl(3+) versus pH for samples collected during dry weather are approximated by a straight line, close to the solubility line of synthetic gibbsite, and have a slope of 2.5, which suggests that these samples were saturated with respect to gibbsite. The cloud of pAl(3+) versus pH points for samples from storm episodes has a near zero slope, and shows a significant degree of undersaturation with respect to synthetic gibbsite. Various mechanisms of aluminium mobilisation are discussed.