Sources and controls of aluminium in storm runoff from a headwater catchment in Mid-Wales

A combined hydrological and chemical investigation was undertaken in a small moorland catchment at Plynlimon, Mid-Wales, to determine both the dominant runoff generation mechanisms and the controls of storm runoff chemistry. Natural soil pipes, overland flow from peat soils and streamflow were gauged and catchment waters were intensively sampled during five storm events. Samples were analysed for total monomeric and non-labile monomeric aluminum. The stormflow response appears to be dominated by overland flow with a significant contribution from pipeflow, particularly during small events. Pipes appear to be the major source of Al during storms, and contribute to increased concentrations of both organic and inorganic complexes of Al in the stream during high flow. However, for a given stream discharge, a wide range of Al concentrations was observed. Highest concentrations (maximum 0.477 mg l-1) were observed in throughflow from the mineral soil, but the contribution of throughflow to stream discharge appears to be limited. Al was rarely detected in overland flow. The Al chemistry varied considerably between pipe and throughflow water. Al concentrations in these waters reflect the availability of Al in pipe and throughflow source areas and are not primarily dependent on hydrogen ion content. Concentrations in pipe water varied with antecedent conditions; highest inorganic Al was observed after a long flow-free period, whereas in very wet conditions both inorganic and organic Al became depleted. In contrast, the highest concentrations in throughflow were observed after the wettest antecedent conditions. The Al-hydrogen ion relationship in stream water also showed considerable scatter. Al concentrations in the stream reflected contributions from pipeflow and throughflow but were greater than the levels suggested by the storm runoff generation hypothesis. This indicates that further sources of Al may exist. Organic Al is strongly related to dissolved organic carbon (DOC) in individual events, but there was great variation between events.