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As an element relevant to human health, iodine is highly worthy of researchers' attention, especially the mechanism of iodine migration and enrichment in groundwater systems. A total of 43 groundwater, 1 seawater, 107 sediment and 111 pore water samples from two boreholes (toward to Bohai Sea: BT, HH) were collected along a groundwater ow path at the North China Plain to investigate hydro-geochemical processes controlling groundwater iodine. High iodine groundwater (> 100 µg/L) was characterized by Na-Cl type, with high TDS values (827-2,400 mg/L) and high Cl (110-705 mg/L) and Br (416-1,180 µg/L) concentrations, which may be related to marine in uence. Borehole BT and HH had pore water I concentration ranges of 1.4-132 µg/L and 3.6-830 µg/L, with high level occurred near to coastline and corresponded to ancient transgression events. The results of sequential extraction of borehole sediments indicate that the fractions of sediment inorganic iodine were mainly consist of exchangeable, carbonate and Fe-oxides associated fractions. Fe-oxides associated iodine was the main occurrence state in borehole BT far from the coastline, but high exchangeable iodine fractions (up to 92% of total extracted iodine) were observed in a high salinity borehole HH located near Bohai Bay, corresponding to the occurrence of high iodine pore water and groundwater. The analysis of iodine species indicates that iodide with strong migration ability dominated high iodine groundwater, pore water and exchangeable sediment iodine, re ecting the occurrence of adsorption/desorption processes of iodine in groundwater system. High iodine groundwater and pore water exhibited iodine enrichment relative to Cl and Br, suggests that iodine adsorbed on sediment desorbed under suitable pH and high solution ionic strength and subsequently released to pore water and aquifers. Inverse geochemical modeling stressed that ion exchange play an important role in iodine enrichment of groundwater system.

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