The Dynamic Hyporheic Zone: Variability of Groundwater-Surface Water Exchange at Multiple Temporal Scales

Abstract:

The pathways of exchange of surface water and groundwater can have a significant influence on the delivery of nutrient-rich groundwater to streams. Many studies have revealed how the spatial variability of physical properties (sediment permeability, bedform structures, etc.) at the interface of groundwater and surface water can impact on flow pathways and residence times of hyporheic exchange flow. Here we explore the temporal variability of flow pathways at this interface. We focus on observations made on a study reach of the River Leith, UK but also provide evidence of dynamic exchanges at a number of other study sites. Under baseflow conditions, the study reach of the River Leith shows a predominance of upwelling of groundwater to the river, and in some sections of the reach a significant groundwater discharge zone in evident. However, from observations of piezometric heads made over a two year study period, repeated reversal of flow direction was observed during storm events. By deploying novel miniature electrode sensors in the river bed we were able to monitor the migration of surface water during these events. Penetration of river water to depths of 30cm was observed during monitored events, which support the reported reversal of hydraulic gradients. We, therefore, observed event-driven hyporheic exchange flow. The duration and frequency of such events may have significant impact on the biogeochemistry of shallow river bed sediments within this reach. Furthermore, temporal variability of exchange is not limited to such events: changes in regional groundwater flow pathways over longer time scales may have a significant impact on the location of localised upwelling; at much shorter timescales we see evidence of diurnal fluctuations in hydraulic heads due to evapotranspiration processes. We report on similar observations at companion study sites and discuss implications on the management of water quality in these groundwater fed systems.