Iodine in major Danish aquifers

Iodine in groundwater can have direct importance for human dietary iodine intake in areas where drinking water is of groundwater origin, as in Denmark. Knowledge on the sources and processes for the varying iodine concentrations in groundwater is of utmost importance for understanding the variation in iodine intake of the population via drinking water. The aim of this study was to characterize groundwater with elevated iodine concentrations and to investigate the sources and processes controlling natural iodine speciation and concentration at four study sites in Denmark with postglacial sandy, Quaternary sandy, and Cretaceous limestone aquifers. Analyses included iodide (I ⁻), iodate (IO ₃ ⁻), total iodine (TI), major ions, and stable H and O isotopes. Dissolved organic iodine (DOI) was calculated by subtracting I ⁻ and IO ⁻ ₃ from TI. A diagram of stable δ ¹⁸O and δ ²H isotopes in Danish groundwater was compiled in order to interpret the groundwater iodine geochemistry. Groundwater had TI concentrations from 5 to 14,500 µg/L. Iodine speciation reflected the prevailing neutral to alkaline and reduced conditions at the investigated sites with domination of I ⁻ and DOI correlated with dissolved organic carbon. We found three different explanations for elevated TI concentrations at the four Danish sites: (1) leaching from soil enriched in iodine due to atmospheric deposition and proximity to the sea, (2) influence from the marine origin of the aquifer sediment due to desorption of iodine from iodine-enriched organic matter or minerals, and (3) influence from residual saline water due to upward advective or/and diffusive transport of iodine.