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

Abstract 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.