Geogenic fluorine-contaminated groundwater increases fluorosis risk in communities of northern cold regions: Genesis mechanism and exposure pathways.

Geogenic fluorine-contaminated groundwater (F >1 mg/L) prevails in cold Mollisol regions of the world. Seasonal variation of F concentration in groundwater likely renders multiple pathways of toxic-level F exposure, posing unrecognized health risk to many economically challenged communities. Herein, different types of samples within the groundwater-soil-crop-human hair network were collected from the Mollisol regions of northeastern China and assessed by joint approach of medical geochemical assay, hydrogeochemical modeling, and health risk indexation. The results unravel that infiltration of dissolved organic matter from Mollisols induced by vertical infiltration led to seasonal variation of F concentration and speciation in groundwater. This is attributable primarily to biogenic dissolution-precipitation equilibria of Ca-containing minerals and altered hydrochemical types of groundwater over season, causing dynamic F- partitioning between water and minerals in aquifers. Further risk assessment suggests greater adverse effects of groundwater use on human health in summer than in autumn. Especially, two major pathways of F exposure, i.e., groundwater drinking-human and groundwater irrigation-soil-crop-human, accounted for 57.3 % of F accumulation in hair of local residents. This research highlights the underestimated health impacts of seasonal variability of F in exploited groundwater and the urgency of groundwater quality management to prevent endemic fluorosis in communities of cold regions.