Hydrodynamics primarily drive fluoride migration in coastal aquifers under seawater intrusion

The escalating severity of seawater intrusion (SWI) and managed aquifer recharge (MAR) disrupt coastal aquifer biogeochemistry, challenging contaminant transport models. This study investigated the transport dynamics of fluoride, a pervasive geogenic contaminant in coastal aquifers, under sequential SWI and MAR. The results showed that both SWI and MAR processes substantially enhanced fluoride mobilization and altered its spatial distribution patterns. Specifically, SWI caused fluoride accumulation near the seawater wedge toe and upward migration into shallow aquifers. Subsequent MAR further amplified mobilization rates and promoted vertical transport. Interfacial shear stresses induced by hydraulic fluctuations of SWI-MAR mobilized colloidal fluoride, increasing its contribution to 41 ± 3 % of total fluoride. Contrary to prevailing assumptions, our findings identified hydraulic field variations, rather than geochemical processes such as ionic interactions or shifts in water chemistry, as the primary driver of fluoride release and migration. Transport model validated the hydrodynamic-dominated fluoride enrichment and predicted fluoride mobilization potential. Long-term SWI exacerbated contamination severity, but strategic recharge well siting could mitigate this pollution. Overall, this study proposes a novel mechanism for the transport of fluoride contamination in aquifer during SWI and MAR, providing critical insights for the sustainable management and protection of coastal aquifers.