Spatial distribution and formation mechanisms of high‑iodine groundwater throughout China

High‑iodine groundwater poses a severe threat to the health of millions of people worldwide, especially in China. Understanding iodine mobilization in aquifers is crucial for sustainable exploitation of groundwater resources. In this Review, we summarize the spatial distribution characteristics of high‑iodine groundwater across China, elucidate the sources and hosts of iodine, and discuss the hydrogeological and biogeochemical processes responsible for iodine enrichment in aquifers. High‑iodine groundwater is widely distributed in inland semi-arid/arid basins/plains and coastal areas, occurring in both shallow and deep layers with iodide as the dominant iodine species. Terrestrial/marine-derived sedimentary organic matter and organic matter-bound iron minerals are the primary sources of iodine. The mobilization, transformation and enrichment of iodine in aquifers are controlled by both hydrogeological processes (i.e., evaporation concentration, compaction release and burial dissolution) and biogeochemical processes, including iodate reduction by iodate-reducing bacteria, Fe(III)-reducing bacteria, sulfate-reducing bacteria and anaerobic methane-oxidizing microorganisms, and organic iodine degradation/dehalogenation. Future studies should focus on the investigation of radioiodine-contaminated groundwater, identification and quantification of organic iodine species, characterization of anaerobic oxidation of methane coupled with iodate reduction, iodine oxidation and methylation, and cost-effective remediation of high‑iodine groundwater.