Geochemistry Of High Iodine Groundwater System Of Datong Basin, Northern China

Iodine is an essential micronutrient in animals and humans, necessary for the production of thyroid hormones and for the proper functioning of the thyroid gland. Insufficient iodine intake can lead to several iodine deficiency disorders, such as goiter, cretinism and even sterility, while excessive iodine intake can inhibit several thyroid processes and cause thyroid autoimmunity (Hashimoto’s thyroidities) and even some thyroid cancers. Since marine source is the major source for the global biogeochemical cycle of iodine, major previous work has been focused on iodine geochemistry in coastal areas such as Switzerland and Denmark where the residents have been suffering from excessive iodine intake.In this study, a first effort to understand the hydrogeochemistry of high geogenic iodine groundwater was made at Datong Basin which was evolved from a fossil lake. An integrated investigation of hydrogeochemical parameters and several isotopes (H, O, Cl, Sr and Ca) was conducted to describe the iodine geochemical behavior and the major factors controlling iodine mobilization in the groundwater system of Datong Basin.The maximum iodine concentration in groundwater reported in this study is up to 1890 μg/L, which is much higher than 150 μg/L, the MCL concentration of drinking water in China. Natural high iodine groundwater with concentrations higher than 500 μg/L mainly occurs medium-deep aquifers with depths higher than 65 m. In addition, high iodine concentration was also observed in shallow groundwater with well depth less than 25 m. For shallow groundwater, strong evapotranspiration is an important process causing iodine concentration elevation in groundwater. High iodine groundwater mainly occurs in the flat alluvial plain regions near the Sanggan River, which is characterized by high pH value, high concentrations of HCO3-,and low concentrations of NO3-, SO42- and U. Weak oxidizing environment in groundwater system of Datong Basin favors iodine enrichment in groundwater.The HPLC-ICP-MS analysis results show that iodide is the dominant species of high iodine groundwater. When Eh values are lower than -50 mV, iodide is the sole iodine form in groundwater. The speciation of inorganic iodine in groundwater is mainly controlled by redox conditions of groundwater system, and extremely reducing condition restricts iodine release from sediments into groundwater. In addition, the difference between total iodine and inorganic iodine implies the importance of organic iodine in groundwater. Iodine concentration in groundwater has a positive relationship with DOM concentration. Meanwhile, the results of EEM-PARAFAC model demonstrate the dominance of terrestrial DOM sources in the sediments and the importance of microbial activities in groundwater systems.Iodine contents range from 0.18 to 1.46 mg/kg in the sediments of the borehole DXZ with depth approximately 122 m. The results of sequential extraction experiment conducted on DXZ sediment show that organic matter and metal (oxy)hydroxides are significant carriers of iodine in the groundwater system of Datong Basin. In comparison with sediments, organic matter contents in surface soils have a range of 1.51～2.48%. The results of iodine species isothermal adsorption experiment show that iodate has a stronger affinity on the natural surface soils and sediment than iodide. Organic matter content in sediment is the major factor controlling iodate adsorption behavior. The results of conditional adsorption experiment show that organic matter in surface soils mainly exists in the mineral surface, which is easily removed by the chemical oxidant, while organic carbon in sediments is mainly associated and protected via the combination with metal (oxy)hydroxides, which is harder to be removed by chemical oxidants. After the treatment of the organic matter, in comparison with original samples, the lower adsorption coefficient of surface soils suggests the dominant effects of organic matter on iodine mobilization for surface soils. The observed higher adsorption coefficients on sediment samples imply that exposed metal minerals have stronger affinity for iodine.All groundwater wells in Datong Basin can be divided into two types:multiple screen wells (MSW) and single screen wells (SSW), which exhibit significant isotope stratification of H and O isotope. Periodic agricultural irrigation activities are very common in Datong Basin. During periodic irrigation, the upstream reservoir water as the major sources of irrigation could leach saline soil and result in hydrochemical and isotopic changes of shallow groundwater. The results of Cl/Br molar ratio model show that less than 5% of halite in surface saline soil was estimated to be flushed into shallow groundwater. Cl isotope variation is the result of isotope fractionation occurring water-rock interaction along groundwater flowpaths and affected by halite in saline soil with relatively lower Cl isotope composition. The contribution of saline soil flushing to iodine concentrations was calculated to be approximately 3 μg/L, which is very limited. However, vertical mixing processes due to irrigation could create oxidizing condition in shallow aquifers, which favors iodine mobilization in shallow groundwater systems of Datong Basin as mentioned above.The results of Sr isotope composition in groundwater show that the directions of groundwater flow in Datong Basin are mainly from southwest margin and south area to the central area. High iodine groundwater mainly occurs in the down-gradient area of regional groundwater flow. The groundwater flow rates in recharge, flow-through and discharge areas as calculated using Sr isotope composition are 0.0015,0.001 and 0.0004 m/yr, respectively. The biodegradation of organic matter in groundwater system is the major factor affecting the Ca isotope fractionation in the system due to carbonate precipitation. Although the relatively lower groundwater flow velocities have significant contributions on the generation of high iodine groundwater in the central area of Datong Basin, iodine mobilization and iodine species transformation are more dependent on organic matter and microbial activities in aquifers.