Authorities in northwest China have ordered the suspension of a zinc smelting facility after it was linked to a toxic chemical leak into the Yangtze River . The alarm was raised when environmental officials in Hanzhong, a downstream city in Shaanxi province, recorded unsafe levels of thallium in a reservoir fed by a tributary of the Yangtze. Following an investigation, Chengzhou Zinc Smelter in Cheng county, Gansu province was identified as the likely culprit, according to a statement released by the county government on Friday. Officials from the environment ministry were dispatched to Gansu on Tuesday to investigate the matter further, while local authorities are checking to ensure no other industrial facilities are responsible for the leak. In the meantime, the company has been ordered to suspend production and halt all discharges of waste water. Thallium is found in several ores but it is mainly obtained as a by-product of copper, zinc and lead refining. Thallium poisoning can cause vomiting, diarrhoea, damage to the nervous system and vital organs, and even death. According to the Hanzhong environmental report, on Tuesday, the thallium concentration recorded at a monitoring station on the intersection of Qingni and Jialing rivers, both of which are tributaries of the Yangtze, was 0.22 micrograms per litre, or more than twice the permitted level in China. This is not the first case of thallium pollution in China. In 2017, environmental officials in Guangyuan, a city in the southwestern province of Sichuan, found the thallium concentration at a water source to be 4.6 times the allowed limit. A mining company was later found to be responsible. Four years earlier, high levels of thallium and cadmium were recorded in the Hejiang River, a tributary of the Pearl River in southern China. On that occasion, the pollution was blamed on sewage discharged from mining companies. Nearly 100 million Chinese people supplied drinking water with ‘unsafe’ levels of toxic chemicals A 2018 study by Juan Liu, an associate professor at Guangzhou University in south China’s Guangdong province, said cases of thallium pollution were “inevitable” as Chinese industry increased its use of thallium-bearing minerals. The problem was exacerbated by rampant illegal mining and waste water discharge in remote areas, it said. “China should improve its early warning and monitoring system,” said a professor at Hong Kong Polytechnic University, who asked not to be named. “Regulations in this area are not enough.” Some people in China questioned if local authorities were properly enforcing the existing regulations. “The pollution happened in Gansu but was detected in Hanzhong,” a person said on Weibo, China’s Twitter-like platform. “Does it mean that there is inadequate monitoring in Gansu?”