Integrated source apportionment, co-enrichment mechanisms analysis, and risk assessment of arsenic and fluoride in groundwater of large-scale irrigation districts in semi-arid regions.

To address the widespread concern of arsenic (As) and fluoride (F) contamination in irrigated areas, this study analyzed 51 groundwater samples from the Jiaokou Irrigation District, China. An integrated approach of isotope tracing (δD and δ18O) and self-organizing map (SOM) was used to reveal the spatial distribution and enrichment mechanisms of As and F. Health risks were further evaluated via Monte Carlo simulation. The results indicated that groundwater in the study area was weakly alkaline brackish water. The groundwater was classified into three hydrochemical types: Cl·SO4-Na, HCO3-Na, and HCO3-Ca by SOM cluster analysis. Weakly alkaline conditions inhibited As adsorption onto minerals (hematite, goethite) and fluorite precipitation, while promoting OH⁻ substitution for F⁻ in F-bearing minerals (biotite, amphibole), thereby facilitating the co-enrichment of As and F. Dissolution of feldspathic silicates increased water-rock interaction time and further enhanced alkalinity. Meanwhile, strong evaporation-concentration processes and agricultural activities contributed to As-F co-enrichment in groundwater. Overall, F posed a greater non-carcinogenic risk than As, with children being more susceptible. For carcinogenic risk, adults were the more sensitive group to arsenic. Specifically, F was the major contributor to non-carcinogenic risks for both adults (62.2%) and children (59.6%). As, however, showed higher sensitivity for carcinogenic risk in adults (92.6%). Management priorities should include safeguarding drinking water for children in high-risk areas and controlling groundwater fluoride within agricultural water strategies. These findings provide a scientific foundation for mitigating health risks and ensuring sustainable groundwater use in semi-arid irrigation districts.