Saltwater intrusion in estuarine aquifers through tidal river-groundwater interactions: Three-dimensional experiments and fully-coupled numerical simulations

Abstract

Saltwater intrusion in estuarine aquifers through interactions with the saline river water has been shown an important contribution to coastal groundwater salinization. In this study, we performed three-dimensional laboratory experiments to investigate the saltwater intrusion dynamics in estuarine aquifers under tidal conditions. Fully-coupled numerical models were constructed considering surface water-groundwater interactions and tidal fluctuations. The simulations were used to interpret the experimental observations and to investigate the sensitivities of tidal amplitude and aquifer hydraulic conductivity on saltwater intrusion with an extended model size. Our results show that flow and salt transport in estuarine aquifer is significantly influenced by the interactions between the river water and groundwater at different tidal stages. At high tide, saltwater rapidly enters into the river channel and penetrates into the upstream aquifer driven by density gradient. At this time, flow is converged from the aquifer to the river close to the sea boundary and dispersed from the river to the aquifer at the inland side. At low tide, inland freshwater flushes the river and leads to the retreat of the salt. Yet, since the flow velocity in the aquifer is very slow, salt in the aquifer is not completely flushed out but accumulated. The flow in the aquifer is focused into the river at the inland side and defocused close to the sea boundary. The salt flux quantification indicates a net salt flux from the river and sea to the aquifer at high tide and a remarkable discharge to the sea at low tide and falling mid-tide. Both saltwater intrusion area and mass in the aquifers evidence a much stronger saltwater intrusion degree in estuarine aquifers in comparison to the classical coastal seawater intrusion without river. Furthermore, a higher tidal amplitude or a larger hydraulic conductivity result to a more significant saltwater intrusion in estuarine aquifers. The former contradicts to the trend observed in the classical coastal seawater intrusion without river due to the additional path of groundwater salinization the river offered in estuarine systems.