Hydrochemical changes along stormflow pathways in a small moorland headwater catchment in Mid-Wales, UK

Abstract

A hydrogeochemical investigation of a small moorland catchment in Mid-Wales has determined the importance of chemical changes along stormflow pathways to stream water chemistry. Flow from a network of ephemeral, natural soil pipes was identified as a major source of solute-rich water to the stream during storm events. Water was sampled during five events, of different rainfall magnitudes and antecedent conditions. Samples were collected at several points within the pipe network, along the water pathway from a major pipe outlet to the head of the stream and within the stream channel. There were significant changes in the chemical composition of water along the flow pathways to the stream; antecedent conditions, size of event and season were important in determining the magnitude of these changes. Between the outlet of the main pipe and the stream channel, concentrations of Ca, Mg and Si increased, whereas concentrations of dissolved organic carbon (DOC), Fe, H + and Al species decreased. The response of K and NO 3-N varied with season. Concentrations of Na, Cl and SO 4 varied little along the pathway. The most significant change in chemical composition was the release of Ca and Mg and the associated consumption of H + within the drift material at the head of the stream. This reaction has important consequences for the degree to which the stream is buffered against inputs of acidic pipe 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 the catchment soils and the importance of chemical and biological reactions along water pathways in determining stream water chemistry.