Dynamic Mechanisms of Flow and Transport in Coastal Aquifer-Aquitard Systems

Alternating deposition of marine and terrestrial sediments commonly produces multi-layered aquifer-aquitard systems in coastal zones. Under the influence of vertical hydraulic gradients, hydraulic connections may develop between adjacent aquifers, leading to the formation of interlayer leakage. However, the extent to which the vertical leakage influences groundwater flow and freshwater-saltwater mixing processes in coastal aquifers remains poorly understood. Moreover, submarine groundwater discharge (SGD) and salinity dynamics vary across daily, monthly, and annual timescales in response to tidal, spring-neap and seasonal forcings. To date, no study has systematically compared the cross-timescale dynamics of salinity distribution and submarine groundwater discharge (SGD) in unconfined and semi-confined aquifers under leakage conditions. To address these knowledge gaps, this study combines laboratory experiments and numerical simulations to investigate the dynamic mechanisms of flow and transport in coastal aquifer-aquitard systems. The results demonstrate that upward leakage induces unstable freshwater fingering within the saltwater wedge of the unconfined aquifer, promoting the extension of the mixing zone from the wedge margin into its interior. Compared with steady-state conditions, tidal fluctuations reduce upward leakage from the semi-confined aquifer to the unconfined aquifer, thereby increasing horizontal freshwater discharge from the semi-confined aquifer to the sea and further alleviating seawater intrusion within it. When seasonal inland recharge is considered, both saltwater wedges and SGD in the unconfined and semi-confined aquifers exhibit pronounced periodic variations; however, leakage-affected saltwater wedges and internal saltwater circulation display irregular interannual variability. Relative to non-tidal conditions, tidal forcing reduces the amplitude of saltwater-wedge fluctuations in the unconfined aquifer driven by seasonal inland input, while amplifying the corresponding variability in the semi-confined aquifer. Furthermore, the combined effects of spring-neap tides and inland input variability result in dual monthly and quarterly fluctuations in SGD from both aquifers, whereas saltwater-wedge dynamics in the semi-confined aquifer respond to this coupling primarily at the quarterly timescale.