Source and transport enrichment mechanism of iodine in shallow saline groundwater in Tianjin coastal area

Iodine is one of the essential trace elements in the human body, and excessive or insufficient intake will affect human health. To ensure the safety of drinking water resources, the spatial distribution of iodine content and migration enrichment factors in shallow underground salty water in Tianjin coastal area were studied. The results show that the iodine (as I⁻) content of shallow groundwater ranges from not detected (ND) to 1320 μg/L, and high iodine groundwater (100–300 μg/L) and super-high iodine groundwater (> 300 μg/L) account for 37.5% of the groundwater samples, distributed in the east, west, central and south of the study area; iodine-deficient groundwater (< 25 μg/L) accounts for 10% of the groundwater samples, and iodine-suitable groundwater (25–100 μg/L) accounts for 15% of the groundwater samples. From north to south and from northwest to southeast, the shallow groundwater in the study area changed from freshwater and brackish water to saline and salt water; the I⁻ concentrations in freshwater, brackish water, and saltwater were significantly different, and the I⁻ concentrations tended to increase with the increase of TDS concentration; the main hydrochemical types in this direction changed as HCO₃-Ca·Na → Cl·SO₄-Na·Mg → HCO₃·Cl-Na·Ca → Cl·HCO₃-Na → Cl-Na type shift; high iodine and super-high iodine groundwater chemistry types are mainly Cl-Na type, Cl·HCO₃-Na type and HCO₃-Ca·Na type. The main source of iodine in groundwater is inferred to be marine sediments. Sedimentary iodine is released during dissolution of marine sediments. I⁻ concentrations increase along with the groundwater flow. In the eastern area, the poor permeability of the aquifer due to the main lithology of mucky clay slows down the groundwater flow, and strong evaporation and concentration on account of the shallow groundwater depth contributes significantly to iodine enrichment.