Iodine enrichment in the groundwater in South China and its hydrogeochemical control

In North China, iodine-rich groundwater has been extensively studied, but few in South China. This study aimed to investigate the characteristics of iodine-rich groundwater in South China and identify potential contamination sources. The results revealed that the average concentration of iodine in groundwater was 890 mu g/L, with a maximum concentration of 6350 mu g/L, exceeding the permitted levels recommended by the World Health Organization (5-300 mu g/L). Notably, the enrichment of iodide occurred in acidic conditions (pH = 6.6) and a relatively low Eh environment (Eh = 198.4 mV). Pearson correlation and cluster analyses suggested that the enrichment of iodide could be attributed to the intensified redox process involving Mn(II), iodine (I-2), or iodate (IO3-) in the soil. The strong affinity between Mn(II) and I-2/IO3- facilitated their interaction, resulting in the formation and mobilization of I- from the soil to the groundwater. Leaching experiments further confirmed that reducing substances (such as sodium sulfides, ascorbic acids, and fulvic acids) in the soil with low dissolved oxygen (DO) levels (< 1.0 mg/L) enhanced the dissolution of iodine species. Conversely, higher DO content (> 3.8 mg/L) promoted the oxidation of I- into I-2 or IO3-, leading to its stabilization. This research provides new insights into the characteristics and mechanisms of I- enrichment in groundwater in South China, and emphasizes the significance of the redox reactions involving Mn(II) and I-2/IO3-, as well as the influence of soil properties in regulating the occurrence and transportation of iodine species within groundwater systems.