Influence of soil hydrological pathways on stream aluminium chemistry at Llyn Brianne, mid-wales

Storm runoff in afforested catchments at Llyn Brianne is acidic and Al-bearing. At baseflows, stream water is well-buffered with low Al levels. This paper presents the results of a study into how hydrological pathways account for these variations in stream-water chemistry. The investigation was carried out in the LI1 catchment; a 0.4-ha subcatchment covered by stagnohumic gley soils was monitored between October 1988 and September 1989. An instrumented hill-slope was established to identify the hydrological pathways that control the hydrochemistry of storm runoff draining from the subcatchment. Perched watertables developed in the surface horizons of the soil during storm episodes and produced lateral flow above the impeding subsoil. This near-surface flow path was responsible for generating acid, Al-rich storm runoff. Some water drained vertically through the soil profile into the underlying slope drift; seepage from groundwater in the drift sustained baseflows. Buffering reactions in the groundwater zone reduced the acidity and Al levels of baseflows. These hydrochemical characteristics are likely to be representative of other areas of stagnohumic gley soils, which cover 19% of the LI1 catchment: these soils may therefore provide a substantial source of acid, Al-bearing storm runoff in LI1 and similar afforested catchments.