Sources and controls of calcium and magnesium in storm runoff: the role of groundwater and ion exchange reactions along water flowpaths

A combined hydrological and chemical investigation was undertaken in a small moorland catchment at Plynlimon to determine the processes controlling storm runoff chemistry. Flow from natural soil pipes, overland flow from peat soils, throughflow from a mineral horizon and streamflow were gauged and sampled intensively during seven storms. Stormflow Ca and Mg concentrations in stream water consistently exceeded those observed in overland flow, pipeflow and throughflow. The response of Ca and Mg to increases in streamflow varied between the storms and could not be explained readily by the mixing of the dominant source waters monitored within the catchment. Intensive sampling of pipe water along a major stormflow pathway revealed a large and consistent increase in the concentration of dissolved Ca and Mg accompanied by a corresponding decrease in acidity, the magnitude of which was strongly influenced by antecedent conditions. Analyses of soil exchangeable cations along the stormflow pathway revealed soils enriched in divalent cations probably derived from a groundwater source. Laboratory leaching experiments confirmed that rapid cation exchange reactions could explain the changes in pipe water chemistry along the stormflow pathway. The relative contribution of flow from pathways where these ion exchange reactions occur strongly influences the stormflow response of Ca and Mg in the stream. The results also highlight a potentially important, indirect role for base-rich groundwater in modifying storm runoff chemistry along water flowpaths.