Hydrological connectivity in the aquifer–river continuum: Impact of river stages on the geochemistry of groundwater floodplains

The study is based on a complex and unique data set of 222Rn activity, water stable isotopes, and physicochemical parameters (i.e., temperature, electrical conductivity, pH) produced to better understand the hydrogeochemical connectivity between groundwater and river water in a meander floodplain of the Matane River, QC, Canada. Using an array of 21 piezometers implanted into an 18 ha meander floodplain, the geochemical properties of shallow groundwater were investigated over nine sampling campaigns between 2011 and 2017. The river is in gaining conditions most of the year, although losing conditions are temporally observed along high river discharge. The objective was to explore the seasonal geochemical variations in response to river discharge with a specific focus on the spatial extent of the hyporheic zone. A thorough examination of temperature, δ18O, electrical conductivity, and 222Rn in groundwater indicated that the geochemical properties of shallow groundwater varied in space and time. Multivariate statistical analyses (i.e., clustering and canonical redundancy analysis) showed that river discharge is a key factor controlling shallow groundwater geochemistry, whatever the season. The hyporheic zone, where groundwater mixes with river water, extended far from the riverbank. These results support the concept of the hyporheic corridor and integrated river management by providing a geochemical viewpoint on river-aquifer connectivity.