Sources and controls of calcium and magnesium in storm runoff: the role of groundwater and ion exchange reactions along water flowpaths P. J. Chapman1,2,*, B. Reynolds1, and H. S. Wheater2 1Institute of Terrestrial Ecology, Bangor Research Unit, Deiniol, Bangor, Gwynedd, LL57 2UP, UK. 2Department of Civil Engineering, Imperial College, London, SW7 2BU, UK. *Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen, AB15 8QH, UK (address for correspondence)
Abstract. 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.
Citation: Chapman, P. J., Reynolds, B., and Wheater, H. S.: Sources and controls of calcium and magnesium in storm runoff: the role of groundwater and ion exchange reactions along water flowpaths, Hydrol. Earth Syst. Sci., 1, 671-685, doi:10.5194/hess-1-671-1997, 1997.