Mapping fluoride footprints: Fractionation, bioaccumulation, sources, and sophisticated risk assessment in El-Bardawil Lake, Sinai Peninsula, Egypt.

Understanding fluoride (F) behavior in sediments and associated risks in hypersaline environments such as El-Bardawil Lake remains limited. This study investigated fluoride concentrations, distribution, sources, fractionation, mobility, and ecological and human risks to provide a comprehensive environmental assessment of this economically important lake. Sampling was conducted at twelve locations during 2023, targeting sediments, surface water, and two key bioindicator fish species, Sparus aurata and Mugil cephalus. Sequential extraction was applied to characterize sediment fluoride fractions: water-soluble (WsF), exchangeable (ExchF), Fe-Mn oxide-bound (Fe-MnF), organic-bound (OrgF), and residual (ResF). Environmental risk was assessed using contamination factors (CF), geo-accumulation indices (Igeo), individual contamination factors (ICF), risk assessment codes (RAC), bioavailability (BI), mobility indices (MI), availability indices (AI), and pollution coefficients (logK_d). Results indicated residual fluoride dominated sediment fractions (ResF: 64.51 %), followed by Fe-MnF (12.74 %), ExchF (10.59 %), OrgF (7.50 %), and WsF (4.66 %) forms. Sediment contamination was minimal (CF < 1; Igeo < 1; ICF < 0) with moderate ecological risk (RAC 11-15 %). Bioavailability and mobility (BI, MI: 0.1-0.4) and significant availability (AI > 0.4), facilitating transfer to the water column (logK_d < 3). Fluoride bioaccumulation primarily in liver tissues of both fish species. Although hazard quotient (HQ > 1) for Mugil cephalus suggests health risks for children via consumption, exposure via sediments and water (THQ < 1) remains negligible. Multivariate analyses (PCA, correlation matrix) revealed fluoride behavior is governed by sediment characteristics, organic matter content, and physicochemical parameters (pH, salinity). Fe-Mn oxides fraction as intermediate risks under reducing or high TOC conditions, while residual and organic fractions represent more stable or bioavailable pools. These findings provide critical insights for policymakers aiming to safeguard ecosystem health and ensure sustainable management of hypersaline lakes under increasing anthropogenic and climatic pressures.