Seawater intrusion angle controls colloidal chromium migration across coastal groundwater interfaces

The coastal mixing zone between seawater and freshwater is a critical interface for the exchange and transformation of contaminants. Despite its significance, the influence of seawater intrusion angle on contaminant transport has been largely overlooked. In this study, we combine laboratory column experiments with reactive transport modeling to investigate how varying seawater intrusion angles affect chromium (Cr) migration, particularly in colloid-facilitated forms. Our findings demonstrate that a 45° intrusion angle enhances the downward migration of colloidal Cr(III) into seawater, with concentrations reaching 3.88 mg/L-approximately 40 % higher than at vertical (90°) or near-horizontal (5°) intrusion angles. This angle also corresponds to the lowest concentrations of Cr and colloids in submarine groundwater discharge (SGD), suggesting reduced nearshore export. Long-term simulations reveal that steeper intrusion angles not only facilitate the migration of dissolved Cr into shallow marine zones but also open a "breakthrough window" for colloidal Cr to bypass natural attenuation zones and reach deeper ocean layers. These findings introduce seawater intrusion angle as a critical but previously underappreciated control on contaminant dynamics in coastal aquifers. This study provides valuable insights for improving coastal water quality management, pollution risk assessments, and future strategies for mitigating contaminant transport in coastal aquifers.