Effects of tides and subsurface dams on the land-sourced contaminant transport: Laboratory and numerical investigation

Abstract

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 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. Spatiotemporal variation, transport pathways, spreading, residence time and mass fluxes were analyzed considering effects of tides and subsurface dams. Results demonstrate that a large low-velocity zone forms near the bottom corner upstream of the dam, and the contaminant mixing with residual saltwater in this zone substantially delays its discharge to the ocean. Compared to the nontidal condition, tides enhance seawater circulation within the saltwater wedge and shorten the transit time by 1.5 times while slowing particle transport in the freshwater zone. Moreover, increased tidal amplitude induces a time lag of 9000 s in the peak efflux. The residence time of 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.