Effects of Tides and Subsurface Dams on the Land-sourced Contaminant Transport

The presence of tides and subsurface dams adds complexity to the migration and mixing processes of land-sourced contaminant in coastal aquifers. While prior studies have explored the individual effect of tides and subsurface dams, their combined impact on the transport characteristics of land-sourced contaminant remains unclear. This study conducted laboratory experiments and numerical simulations to thoroughly investigate the migration and discharge behaviors of land-sourced contaminant in an unconfined coastal aquifer. The spatiotemporal variation, transport pathways, spreading, residence time and mass fluxes were analyzed in detail. Results demonstrate that a large low-velocity zone forms near the bottom corner upstream of the subsurface dam, and the mixing of the contaminant with residual saltwater in this zone substantially delays its discharge to the ocean. Compared to the nontidal condition, the addition of tides enhances seawater circulation within the saltwater wedge downstream of the subsurface dam while slowing particle transport in the freshwater zone. Moreover, increased tidal amplitude induces a time lag in the peak efflux of contaminant. The residence time of the contaminant is jointly affected by the subsurface dam, saltwater wedge and tidal forces. Sensitivity analysis indicates that a greater aquifer permeability and lower contaminant dispersiviy reduce the maximum spreading area while significantly promoting the maximum daily contaminant efflux. However, the residence time exhibits non‐monotonic relationships with respect to dam locations and aquifer permeabilities. The findings highlight the complexity of nearshore subsurface systems subjected to both natural and human factors, and have valuable insights for developing effective strategies to safeguard coastal environments.