Distribution characteristics of iodine in karst groundwater in Xintian county, Hunan Province and the analysis on the causes of high iodine

Iodine is one of the essential trace elements for human body, which maintains the growth and normal metabolism of the organism. Iodine deficiency or excess will have different degrees of impact on human health. A large field of strontium-rich mineral water was discovered in Xintian county, Hunan Province. However, the iodine content in some strontium-rich groundwater is abnormal, threatening the drinking water safety of local residents. Therefore, it is important to find out the distribution characteristics of iodine in groundwater and the controlling factors of the formation of high iodine groundwater so as to implement the project for the safety of drinking water and to prevent endemic iodine diseases in the study area.

In a hydrogeological survey, 66 groups of spring water samples and 45 groups of well water samples were collected in Xintian county to analyze their hydrochemical characteristics, identify the spatial distribution characteristics of iodine in groundwater and analyze the main factors controlling iodine content in groundwater by means of hydrochemical graphical method, principal component analysis and GIS technology. The results showed that the iodine concentration in spring water and well water ranged from 2.7 to 92.8 μg·L⁻¹ and 4.15 to 3,861 μg·L⁻¹, respectively, with the respective median value of 5.4 μg·L⁻¹ and 168 μg·L⁻¹. It can be seen that all the groundwater with high iodine was well water, and 53.3% of well water samples had iodine concentration exceeding the permitted national standard of 100 μg·L⁻¹ (GB 19380-2016). In contrast, the iodine content in spring water was relatively low, and the overall iodine-deficient groundwater was predominant. High iodine groundwater was mainly distributed along a river valley in the northeast-southwest direction. The iodine content in groundwater showed a gradual increase from the peak-forest valley to the plain area of low-lying river valley, and the hydrochemical type also changed from water with single HCO₃-Ca type to water with complex HCO₃-Na, HCO₃-Na·Ca type, etc.

The marl stratum formed by marine sedimentation of Shetianqiao Formation is rich in iodine and organic matter, which provides good geological conditions for the enrichment of iodine in groundwater. The microbial degradation of organic matter occurs in the closed and partially reducible groundwater environment of the marl aquifer. CO₂ generated by organic matter decomposition exists in groundwater mainly in the form of ${\rm{HCO}}_3^{-}$, which will compete with I^- for adsorption. Meanwhile, the process of organic matter decomposition to form ${\rm{HCO}}_3^{-}$ is accompanied by the release and migration of iodine deposited on it, making the iodine content in the groundwater increases with the increase of ${\rm{HCO}}_3^{-}$ content. At the same time, the alkaline karst water environment can increase the electronegativity of organic matter surface to reduce the adsorption of iodine ions by organic matter. OH⁻ in groundwater also competes with I⁻ for adsorption, which accelerates the release and migration of adsorbed iodine into groundwater, and hence increases the iodine content. The distribution area of high iodine groundwater is located in a groundwater discharge area. It is also an area distributed with strata rich in iodine and organic matter. In this area, the sluggish groundwater flow due to flat topography, together with a relatively closed groundwater environment, further facilitates the enrichment of iodine. The strata rich in iodine and organic matter formed by marine sediments, closed and partial reduction chemical environment with karst water of weak alkalinity and sluggish groundwater runoff are the main factors controlling the formation of high iodine groundwater in the study area.