A field investigation characterizing the hyporheic zone of a tidally-influenced river

This research investigated groundwater surface water interaction (GWSi) of the Fraser River, in the Lower Mainland of British Columbia. At the Braid Street site, GPR, seismic reflection surveys, bulk resistivity profiling, and sediment sampling were used to map the sediments of the riverbed and the area of contaminated groundwater discharge. Groundwater profiling revealed that three water types occur within the upper 2 m of the riverbed sediments a result of mixing of river water, contaminated (fresh) groundwater, and saline groundwater. The distribution of groundwater solutes indicate that during a single tidal cycle, river water penetrates the riverbed to a depth of approximately 15 cm but the long term effects of tidal pumping of river water into the riverbed is observed to a depth of approximately 1 meter below river bed (m.b.r.b.) Temperature measurements coupled with independent hydraulic head measurements within the riverbed confirmed that GWSi under tidal forcing produced a 1 m-deep hyporheic zone (HZ). Time-averaged riverbed temperature profiles displayed a distinct compressed convex-upward pattern, clear evidence of net groundwater discharge. However, the instantaneous time series data indicate that riverbed temperatures, to a depth of 1 m were affected by tidal-forcing. Heat transport modeling revealed that instantaneous velocities within the shallow sediments of the riverbed are rather high during either a flooding or ebbing tide. Further, the magnitude of the tidally-induced pressure gradient was found to be significantly greater than the pressure gradient attributable to flow across large river bottom bedforms, indicating that bedform-driven exchange is limited to within a few centimeters beneath the riverbed. At the Kidd2 site, where saline encroachment occurs up river, hyporheic and hypoaigic (recharge by saline water) processes occur. During low winter river flows saline encroachment up river occurs and hypoaigic exchange dominates. During low tide the saline water is pushed out of the river and hyporheic exchange dominates. Hyporheic exchange dominates exchange during freshet leading to deep freshening of the aquifer on the order of 2+ m.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat