NGWA Event and Education Recordings Archive

Fluvial and marine hydrological processes converge in estuaries creating a system that has both dynamic surface water flow and salinity. This unique environment sets-up subsurface niches with biogeochemical functions driven by porewater chemistry and temperature. We investigated porewater salinity and temperature dynamics along a meander reach within a small river estuary using subsurface electrical resistivity (ER) measurements, vertical temperature profiles and groundwater heads monitored within a tidal cycle. Cross channel time-lapse ER measurements reveals a deep and steeper erosional bank zone with higher subsurface ER values that does not vary with time and a shallow inner meander zone with subsurface electrical resistivities that becomes less resistive with increasing tides. Stronger upward flux potential from groundwater head measurements and steep vertical temperature gradients with colder lower subsurface temperatures indicate a stable zone of fresh groundwater discharge along the erosional bank. Subsurface temperatures closer to surface water values but with high groundwater flux potential suggests an inner meander zone with higher porewater salinity dominated by hyporheic exchange. An estuary-wide ER survey shows sinuous reaches with higher subsurface resistivities relative to straighter channel segments indicating groundwater discharge hotspots. Our study suggests that sinuous estuarine segments juxtaposes zones dominated by both groundwater discharge and hyporheic exchange whereas straighter channel reaches may be more dominated by hyporheic exchange.