Abstract: The incidence of iodine deficiency disorder in China has been greatly reduced since the promotion of iodized salt in the late 1990s, however, the issue of excess iodine causing disorder is also attracting more and more concerns. A total of 4136 groups of water samples were collected from both shallow and deep underground dringking water sources in the North China Plain for analysis of iodine content to explore spatial distribution of iodine in groundwater and its hydrogeological effect. And based on the epidemiology, a reasonable iodine content in drinking water was put forward. On such a basis, the North China Plain was divided into six regions, namely, extremely iodine-deficient region (≤4μg•L^-1), iodine-deficient region (＞4-8μg•L^-1), iodine-proper region (＞8-50μg•L^-1), drinkable high iodine region (＞50-100μg•L^-1), high iodine region (＞100-200μg•L^-1) and extremely high iodine region (>200μg•L^-1). A proper iodine amendment rate in salt was worked out for each region. And based on iodine contents in groundwater, the plain could be divided into three regions, namely, iodine required region, iodine unrequired region and iodine-amendment unfit region. Such divisions are of some scientific significance for guiding optimization of salt iodization and solution of the problem of negative impact of the salt iodization. Results indicate that iodine-deficient groundwater is mainly distributed in the pediment alluvial-diluvial plain of the Yanshan Mountains-Taihang Mountains and the Puyang section of the Yellow River alluvial-diluvial plain; extremely iodine-deficient groundwater in the alluvial-diluvial plains adjacent to the mountains; and high iodine groundwater in the Yellow River alluvial-diluvial plain and the eastern coastal plain. The calculation shows that addition of iodine is required to the table salt for the extremely iodine-deficient regions and iodine-deficient regions, where the currently used 20-50μg•g^-1 iodized salt is adequate to meet the iodine demand of human body; there is no need to add any iodine to the table salt for the iodine-proper region and drinkable high iodine region, where the iodine content in the table salt should not exceed 9.2μg • g^-1; and it is not fit to add any iodine in the table salt in the high iodine regions and extremely high iodine region, where iodine-free salt is recommended.

Key words: North China Plain, high iodine groundwater, iodized salt, iodine deficiency disorder (IDD), iodine-deficient groundwater