Solute transport in stratified coastal aquifers with a subsurface dam

Subsurface dams are a common engineering measure for mitigating seawater intrusion in coastal areas. However, the environmental impact of subsurface dams remains poorly understood, especially for stratified unconfined coastal aquifers. This study, using laboratory experiments and numerical simulations, investigated the effects of subsurface dams on land-derived solute transport in a three-layered aquifer [a low-permeability layer (LPL) in between two high-permeability layers]. The results show that, compared to a stratified aquifer, the construction of a subsurface dam would prolong the residence time of solute plume by 200%. Also, the addition of a subsurface dam to a three-layered aquifer may constrain the spreading area of solute plume, but enhance its vertical variance. Temporally, the solute discharge duration is prolonged by a subsurface dam by 237%. Nevertheless, the dam barely modifies the solute discharge zone along the aquifer–ocean interface, because its impact is filtered by the LPL, above which solute discharge occurs. Sensitivity analysis further demonstrates high-level sensitivity of retention time and maximum daily integrated solute efflux to dam height and dam location. This study, for the first time, systematically revealed the synergistic effects of subsurface dams and stratified structure on land-derived solute transport in coastal aquifers. The research findings contribute to deeper insights into the migration of terrestrially sourced solutes in complex coastal groundwater systems. Also, these findings may help to evaluate the environmental impact and assist the design of subsurface dams in layered geological settings.