Iodine enrichment and the underlying mechanism in deep groundwater in the Cangzhou Region, North China.

The lack of information on the origin and behavior of iodine in deep groundwater restricts the development and use of groundwater resources. To address this issue, the Cangzhou region in the eastern North China Plain (NCP) was selected for a case study. In total, 296 deep groundwater samples were collected, their iodine concentrations were determined, and the distribution characteristics of iodine concentrations were analyzed. Iodine concentrations ranged from < 0.002 to 1.22 mg/L, with a mean of 0.19 mg/L; 42% of the samples had high iodine concentrations. The levels were higher in the east than in the west, and most of the samples with high iodine concentrations were obtained from sites east of the boundary between the Cangxian uplift and the Huanghua depression. The weathering and dissolution of iodine-bearing minerals and the leaching of marine sediments can facilitate iodine enrichment. In the Cangxian uplift, iodine was mainly a result of the conversion of organic iodine, while in the Huanghua depression, iodine enrichment was a factor of the conversion of IO. Overall, the main factors for the enrichment of iodine are the sedimentary environmental and the hydrodynamic conditions. Our results provide a theoretical basis to understand the occurrence of high iodine concentrations in deep groundwater.