Co-occurrence of arsenic and iodine in the middle-deep groundwater of the Datong Basin: From the perspective of optical properties and isotopic characteristics.

Redox processes can induce arsenic (As) and iodine (I) transformation and thus change As and I co-occurrence, yet there is no evidence that Fe-C-S coupled redox processes have such an impact on the co-occurrence of As and I. To fill this gap, middle-deep groundwater from the Datong Basin were samples for the purpose of exploring how dissolved organic matter (DOM) reactivity affects As and I enrichment and how iron reduction and sulfate reduction processes influence As and I co-occurrence. We identified three DOM components: reduced and oxidized quinone compounds (C1 and C3) and a labile DOM from terrestrial inputs (C2). Two pathways of DOM processing take place in the aquifer, including the degradation of labile DOM to HCO3- and the transformation of oxidized quinone compounds to reduced quinone compounds. Electrons transfer drives the reduction of the terminal electron acceptors. The supply of electrons promotes the reduction of iron and sulfate by microbes, enhancing As and I co-enrichment in groundwater. Thus, the reduction processes of iron and sulfate triggered by the dual roles of DOM affect dissolved As and I co-enrichment. As and I biogeochemical cycling interacts with C, Fe, and S cycling. These results provide isotopic and fluorescence evidence that explains the co-occurrence of arsenic and iodine in middle-deep aquifers.