HYDROCHEMICAL CHANGES ALONG STORMFLOW PATHWAYS IN A SMALL MOORLAND HEADWATER CATCHMENT IN MID-WALES, UK

A hydrogeochemical investigation of a small moorland catchment in Mid- Wales has determined the importance of chemical changes along stormflo w pathways to stream water chemistry. Flow from a network of ephemeral , natural soil pipes was identified as a major source of solute-rich w ater to the stream during storm events. Water was sampled during five events, of different rainfall magnitudes and antecedent conditions. Sa mples were collected at several points within the pipe network, along the water pathway from a major pipe outlet to the head of the stream a nd within the stream channel. There were significant changes in the ch emical composition of water along the flow pathways to the stream; ant ecedent conditions, size of event and season were important in determi ning the magnitude of these changes. Between the outlet of the main pi pe and the stream channel, concentrations of Ca, Mg and Si increased, whereas concentrations of dissolved organic carbon (DOC), Fe, H+ and A l species decreased. The response of K and NO3-N Varied with season. C oncentrations of Na, Cl and SO4 varied little along the pathway. The m ost significant change in chemical composition was the release of Ca a nd Mg and the associated consumption of Hf within the drift material a t the head of the stream. This reaction has important consequences for the degree to which the stream is buffered against inputs of acidic p ipe water and hence for the acidity of the stream water. Solutes whose concentration and speciation are controlled by pH are also affected. The results emphasize the significance of spatial variability within t he catchment soils and the importance of chemical and biological react ions along water pathways in determining stream water chemistry.