Residents in Chiang Mai and Chiang Rai provinces have been cautioned against using water from the Kok River after Thai environmental authorities reported arsenic levels exceeding safety limits. The contamination, discovered during routine sampling in March, is believed to be linked to upstream mining operations in Myanmar’s Shan State.
On March 19, the Chiang Mai Environmental and Pollution Control Office collected water samples from three locations in Mae Ai district. Tests showed arsenic concentrations at 0.026 milligrams per litre—over twice the standard limit of 0.01 milligrams per litre for natural water sources. The river’s turbidity was also found to be nearly ten times higher than acceptable levels, raising further concern.
Alongside arsenic, traces of other heavy metals, including cadmium and lead, were detected. Officials warned that long-term exposure to arsenic could lead to severe health effects, such as gastrointestinal distress, skin issues, and a heightened risk of cancer.
The results of additional water samples from the river in Chiang Rai’s Muang district are expected in the coming days. Meanwhile, local health agencies have advised residents to avoid contact with the river water, particularly children and pregnant women. If use is unavoidable, officials recommend boiling or filtering the water before consumption.
A rights organization based in Myanmar noted that gold mining activities near the Kok River began around 2020, following the arrival of Chinese companies. Currently, four Chinese-operated open-pit mines are active near Mong Hsai, employing over 300 workers. The group alleges that wastewater from these operations is discharged directly into the river without treatment, contributing to the pollution downstream in Thailand.
Public health and environmental officials in Thailand, including Dr Waranyu Chamnongprasatporn from the Chiang Mai Provincial Public Health Office, continue to monitor the situation and have pledged support for affected communities.
