Geogenic fluoride in groundwater of hard rock terrains and defluoridation options for rural water security: a mini review

Hard rock terrains, defined here as crystalline igneous-metamorphic aquifers in which groundwater is stored mainly in weathered mantles and fracture networks, are especially prone to borehole-scale fluoride variability that complicates rural drinking-water security. This mini-review synthesizes recent evidence on geogenic fluoride occurrence, exposure pathways, and defluoridation strategies relevant to such settings. Elevated fluoride is most consistently associated with weathering of F-bearing minerals, low Ca2+ activity, alkaline HCO3-rich water, cation exchange, and long residence time or evapoconcentration. These coupled controls explain why neighboring wells can differ markedly in fluoride concentration and why dry-season dependence on deeper or older groundwater may intensify exposure. Defluoridation options are critically compared using fluoride-removal efficiency, chemistry sensitivity, operator burden, cost, scalability, residual management, and long-term service needs. NF/RO offers the highest intrinsic rejection, whereas alum-lime, activated alumina, bone char, and electrocoagulation can perform well only when dosing, regeneration, media replacement, or electrical control are reliably supported. The review argues that durable mitigation depends less on peak laboratory efficiency than on seasonal monitoring, source selection, user acceptance, service continuity, and safe handling of sludge, regenerants, and brines. Integrating hydrogeochemical screening with accountable rural service models is therefore essential for sustained fluoride risk reduction.