Aquitard-breaching conduits control salinity dynamics and variable-density contaminant transport in coastal aquifer-aquitard systems

Contaminant transport from semi-confined aquifers to coastal waters is an emerging environmental concern, yet its prediction remains challenging due to the complexity of coastal aquifer systems. In coastal environments, aquitards are breached by conduits formed through natural processes or anthropogenic disturbances, enhancing hydraulic connectivity between otherwise isolated aquifers. Despite their widespread occurrence, the impacts of conduits on salinity distributions and land-derived contaminant migration from semi-confined aquifers to coastal waters remain poorly understood. This study systematically investigates the influences of an aquitard-breaching conduit on salinity dynamics and land-derived contaminant transport in coastal aquifer-aquitard systems. The conduit is represented as a high-permeability pathway through the low-permeability aquitard. Results show that a conduit enhances seawater intrusion into semi-confined aquifers relative to no-conduit conditions, irrespective of conduit locations. Notably, a conduit situated near an underlying saltwater wedge induces saltwater up-coning from that wedge, establishing a hydraulic connection between the overlying and underlying wedges. This up-coning redirects both freshwater-density and saltwater-density contaminant plumes from predominantly horizontal migration to vertically oriented pathways toward the ocean. In the absence of up-coning, plume behavior diverges by density contrast: freshwater-density contaminants preferentially discharge through the conduit, whereas saltwater-density plumes partition between upward discharge via the conduit and seaward transport within the semi-confined aquifer. Compared to no-conduit scenarios, conduits consistently facilitate the export of freshwater-density contaminants, whereas a conduit without saltwater up-coning can prolong the residence time and discharge duration of saltwater-density contaminants and reduce their peak efflux. These findings demonstrate a dual role for aquitard-breaching conduits in modulating seawater intrusion and contaminant fate, providing new insights for managing coastal groundwater systems.