For the first time, a team of researchers has mapped the distribution of groundwater iodine – which plays a critical role in human health – across China. This is the first time that groundwater iodine levels have been mapped on a continental scale.
Ingestion of too much or too little iodine is known to cause thyroid disorders that affect about one-quarter of the world’s population.
The team from Peking University, Fujian Agriculture and Forestry University, the University of Plymouth, the University of Edinburgh, and Southern University of Science and Technology Shenzhen carried out the study, which has been published in Nature Communications.
Iodine is a micronutrient which plays a major role in human health. It has a critical influence on the production of thyroid hormones which are central to human metabolism, physical growth, mental capability, and the body’s ability to regulate its internal temperature.
Excessive or insufficient iodine intake has been shown to cause health disorders such as hyperthyroidism, hypothyroidism, and nodular goitre.
In many parts of the world, iodine is ingested from drinking water from underground sources –known as groundwater. However, groundwater iodine is not evenly distributed, and tends to be at higher concentrations in low-lying coastal plains and lower concentrations in mountainous uplands.
To date, little information has been recorded on the distribution of groundwater iodine at a continental scale.
The team analysed groundwater samples taken from 686 wells in 31 provinces of China during 2016 and 2017. The sampling sites were selected to be representative of most of China, including plains, plateaus, river basins, and urban areas.
In each case, the researchers measured water quality parameters such as pH, temperature, and dissolved oxygen, and tested the groundwater for alkalinity, ion content and total iodine content.
The results confirm that health risks associated with too much iodine in China occur more often in densely populated lowland areas which are over-exploited by humans, and from too little iodine in highlands.
The team also found that the transformation of iodine from one form (i.e., species) to another in groundwater is affected by the biogeochemical cycle, and influences thyroid health risks in areas of higher and lower groundwater iodine.
The study discovered that the lowest and highest levels of iodine found in groundwater across China were beyond World Health Organisation (WHO) safe thresholds for human health*.
Mapping a varied terrain
The team found that the distribution of iodine in groundwater follows the continent’s laddered terrain as it rises in several distinct steps from the sea in the east to the Himalayas in the west.
Low iodine groundwater is found in higher altitude areas of magmatic and metamorphic rocks, while high iodine groundwater occurs in lower lying river deltas and plateaus.
Excessive pumping of water from aquifers and subsequent land subsidence have acted to increase iodine contamination of groundwater. In deep aquifers, iodate in sediment may be converted to iodide. Human activities lead to the highest levels of iodine, with organo-iodine particularly prevalent in the groundwater of eastern deltas and plains.
The research suggests that uneven distribution of groundwater iodine throughout China will translate into different levels of health risk.
The study recommends that more effort should be placed by government agencies, policy makers and industry on regulating iodine levels in drinking water from groundwater sources, to reduce the risk of thyroid disorders.
Professor Alistair Borthwick, Emeritus Professor at the University of Edinburgh’s School of Engineering said, “Iodine is both a blessing and a curse. It is well known that too much or too little ingestion of iodine is damaging to health by affecting thyroid hormones.
"Our study shows that iodine in groundwater can vary greatly across a continent, being affected by its geology, terrain, and human land-use activities. Our work should assist authorities in China and perhaps globally in adopting local mitigation measures for groundwater iodine.”
Professor Jinren Ni of Peking University, who led the study, commented “For the first time, the methods of groundwater iodine environmental monitoring and thyroid disease epidemiological survey were combined to conduct health risk assessment at a continental scale, which enabled us to reveal the response relationship between dominant iodine forms in high and low iodine regions and major thyroid diseases.”
Professor Alistair Borthwick, pictured in the garden of Peking University
Find out more
- Peking University: Ruoqi Ma, Mingquan Yan, Peng Han, Ting Wang, Bin Li, Tong Zheng, Yandi Hu, and Jinren Ni (study lead)
- Fujian Agriculture and Forestry University: Shungui Zhou
- University of Edinburgh and University of Plymouth: Alistair Borthwick
- Southern University of Science and Technology Shenzhen: Chunmiao Zheng
*WHO. Urinary iodine concentrations for determining iodine status deficiency in populations. Vitamin and Mineral Nutrition Information System. Geneva: World Health Organization; 2013 http://www.who.int/nutrition/vmnis/indicators/urinaryiodine, accessed 6/12/2022