Defining the hyporheic zone in a large tidally influenced river

An investigation was conducted to characterize the spatial and temporal distribution of the hyporheic zone of a large tidally influenced river. The field site is located on the Fraser River in British Columbia, Canada, approximately 30 km from its outlet to the ocean. The physical attributes of the riverbed were mapped using geophysical techniques coupled with sediment sampling. The spatial and temporal distribution of groundwater composition beneath the riverbed was determined through detailed profiling. Contaminated (fresh) groundwater discharges through a narrow band of the riverbed at a distance approximately 88-105 m from the shoreline coinciding with the termination of a massive silty unit. Saline groundwater, as part of a regional flow system, dominates the riverbed sediments from 105 m beyond the shoreline towards the centre of channel. Three water types occur within the upper 2 m of the riverbed sediments; a result of both mixing of river water, contaminated (fresh) groundwater, and saline groundwater and modification by cation exchange reactions. The interaction of these waters produced distinct zones of Ca-HCO3-, Na-Cl, and Ca-Cl type waters. The distribution of groundwater solutes indicates 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 m below the river bed. (C) 2011 Elsevier B.V. All rights reserved.