Deriving celerity from monitoring data in carbonate aquifers

Abstract

Celerity (the speed of a pressure wave) in fracture networks in bedrock can result in large and rapid water level responses in both aquifers and streams, but has been little documented in detail. To examine the role played by preferential flow in fracture networks, groundwater and stream data were analysed in a headwater catchment in a chalk aquifer in the UK. Hourly data show that aquifer water levels and streamflow both respond promptly to rain and have a high correlation. The stream exhibits double peaks in flow during stormflow events. The initial peak hardly lags rainfall, is composed of dilute water, and is attributed to direct flow (i.e. overland flow and/or interflow). The second peak occurs several days after rainfall, has high solute concentrations and a high correlation with aquifer water levels, indicating that the second peak is composed of groundwater. Groundwater inputs provide 89% of quickflow in the stream. Calibration of a numerical model gave a fissure porosity (calculated as an equivalent specific yield) of 0.0015, with the low porosity being associated with high celerities and a fissure residence time of 0.14 years. Conversely, the low matrix permeability results in a residence time of 26 years. A review of a further 13 carbonate aquifers in six countries shows that similar dual-porosity behaviour is common. High celerities in fissure networks provide a potent process for the rapid transfer of groundwater to stormwater hydrographs in streams.