Quantifying hyporheic exchange in a tidal river using temperature time series

An investigation into groundwater-surface water interaction (GSWI) beneath a large tidally influenced river was conducted to determine the effect of tides on the development of a hyporheic zone (HZ) and to quantify mixing of river water and groundwater. Temperature measurements, coupled with independent hydraulic head measurements, were used to detect groundwater flow within the riverbed. GWSI under tidal forcing produced a 1 m deep HZ. Time-averaged riverbed temperature profiles displayed a distinct compressed convex pattern: clear evidence of net groundwater discharge. However, the instantaneous time series data indicate that riverbed temperatures were affected by tidal forcing to a depth of 1 m. Heat transport modeling revealed that instantaneous velocities within the shallow sediments of the riverbed are rather high, creating a zone of vigorous exchange during either a flooding or ebbing tide. Furthermore, the magnitude of the tidal pressure gradient was found to be significantly greater than the pressure gradient expected across 0.8 m high dunes, evidence that bed-form-driven exchange under these conditions, and this scale of observation, did not contribute to the development of the HZ. Conditions for exchange induced by shear and current bed form are favorable during ebbing tidal conditions only; flow paths are therefore limited in depth. Exchange flow paths in an estuary setting are complex; they are limited in duration and space and dominated by tidal pumping.