Transport Mechanisms of Conservative Contaminants From Tidally Influenced Semi-Confined Aquifers to the Sea

Groundwater contamination in confined aquifers is an increasingly critical issue, yet the dynamic processes governing contaminant transport between semi-confined aquifers and the ocean remain insufficiently understood. This study explores these processes within semi-confined aquifers using laboratory experiments and numerical simulations. The results reveal, for the first time, that contaminants from semi-confined aquifers mix within the interiors of overlying saltwater wedges in unconfined aquifers. As aquitard permeability decreases, contaminant composition mixed within overlying wedges changes from saltwater-density to freshwater-density counterparts. Two primary retention mechanisms are identified for deep-layer contaminants: One involves entrapment within aquitards, facilitating the mixing of contaminants into overlying wedges; the other arises from gravity-induced mixing between saltwater-density plumes and saltwater wedges. Interestingly, the residence times of freshwater-density and saltwater-density contaminants increase initially and then decline with reduced aquitard permeability. These findings enhance our understanding of the complex processes involved in deep-layer contaminant transport and retention in coastal aquifers.