Stormflow hydrochemistry of a small Welsh upland catchment

Abstract

The hydrochemistry of a small upland catchment during a summer storm, following six weeks of low flow, is described. The storm exhibited several unusual features which may be attributed to the dry antecedent conditions. Streamwater concentrations of all ions analysed increased during the event but the timing of the increases varied according to the initiation of flows controlling the supply of particular ions. The increase in the concentration of calcium in the stream coincided with onset of overland flow, and appears to result from flushing of accumulated weathering products from a narrow strip of mire which provides a major pathway for storm water in the catchment. Natural soil pipes appear to be the main source of the increase in streamwater concentrations of both organically and inorganically complexed aluminium. Concentrations of inorganic aluminium in pipe water were considerably greater than those observed during previous events, possibly another result of the long period without flow. During the stream recession, inorganic aluminium remained high, reflecting a contribution from aluminium-rich throughflow. It is concluded that the chemistry of the stream at any given time is controlled by the relative importance of contributing sources of water which is, in turn, controlled by hydrological factors such as antecedent conditions and rainfall intensity and duration.