Combined effect of inland groundwater input and tides on flow and salinization in the coastal reservoir and adjacent aquifer

•Water/salt exchange between coastal reservoir and aquifer were quantified.•More than 75% of inland groundwater would discharge to reservoir then to sea.•Salinization was decreased nonlinearly as inland groundwater input increased. Coastal reservoirs can alleviate the shortage of freshwater in the nearshore zone. Inland groundwater and tides can cause hydraulic gradients between seawater and reservoir water and affect the salt concentration of coastal reservoirs; however, the mechanism is poorly understood. Laboratory experiments and numerical simulations were conducted to investigate the combined effect of inland groundwater inputs and tides on the surface/subsurface flow and salinization dynamics of the coastal reservoir and adjacent aquifer. The results indicated that inland groundwater followed two pathways before discharging into the sea, regardless of the tidal conditions. The predominant pathway led inland groundwater to be discharged to the coastal reservoir and then to sea. The other pathway encompassed the entire aquifer underlying the reservoir bed and terminated at the aquifer–sea interface. As the inland groundwater input increased, the groundwater entering the reservoir also increased, whereas the ratio of the fresh groundwater transported through the reservoir to the total groundwater input decreased. Tidal pumping allowed more seawater to intrude into the coastal reservoir and adjacent aquifer compared with nontidal conditions. This caused a larger proportion of groundwater to be discharged to the coastal reservoir, with the density-driven flow and water/salt exchange across the aquifer-reservoir interface also enhanced. As the inland groundwater input increased, salinization of the coastal reservoir decreased and the inhibition of the inland groundwater input on salinization also decreased. This study contributes to the improved management of such coastal infrastructures.