A civil society group and several academics in Chiang Rai and Chiang Mai have issued an urgent letter to Prime Minister Paetongtarn Shinawatra, demanding government intervention to address severe pollution in the Kok and Sai rivers, which flow into Thailand from Myanmar's Shan State.
The letter highlights "alarming" levels of sediment and heavy metal contamination in the rivers.
The pollution, they warned, poses a serious threat to the health and livelihoods of more than 1.2 million residents in Chiang Rai and Chiang Mai provinces, who depend on the rivers for consumption and farming.
"The Kok and Sai rivers have become increasingly polluted due to unregulated mining upstream in Myanmar," the letter says, citing 14 high-risk mining zones identified by the Pollution Control Department in 2023.
Five of those sites are located near rivers that cross into Thailand. Satellite images show large-scale land excavation in upstream areas.
Despite the growing threat, civil society groups say there has been no systematic effort to solve the transboundary pollution.
The World Health Organisation (WHO) has warned that long-term exposure to cyanide, detected in the rivers at levels as low as 0.05 milligrams per kilogramme of body weight, can have serious health consequences.
In September 2024, communities along the Kok and Sai river basins endured devastating flash floods and landslides -- a disaster described as unprecedented in the region's history.
Many families lost homes, belongings, and jobs, yet comprehensive recovery efforts remain absent. With the rainy season only a month away, residents are expressing frustration over the government's inaction, the letter says.
Suebsakun Kidnukorn, a lecturer at Mae Fah Luang University, said it was a classic case of cross-border pollution.
"The contamination originates in Myanmar, but it is Thai communities who are suffering the consequences," he said. "Now is the time for environmental diplomacy. The prime minister must raise this issue with Myanmar's leadership and use regional platforms for negotiation."
He said that with the Malaysian PM set to meet Myanmar military leader Min Aung Hlaing in Thailand yesterday, Thailand should push the issue onto the regional agenda.
Pianporn Deetes, secretary-general of the Hill Area and Community Development Foundation and Southeast Asia Campaign Director for International Rivers, echoed the call for swift action.
"These rivers are transnational. Gold mining in upstream Shan State is wreaking havoc on communities and ecosystems downstream in Thailand. The government must act now to stop widespread land clearing and mining."
The Environmental and Pollution Control Office early this month warned residents to avoid direct contact with the water in both rivers due to the findings of lead and arsenic levels, which are far above safety thresholds.
Despite assurances by the Provincial Waterworks Authority that treated tap water remains safe, public confidence is waning, said Ms Pianporn.
"The government tells us the water is fine, but we feel unsafe," she said. "We're being told not to touch the river water, yet we're still drinking water derived from it. There's no clear plan for solving the root of the problem, and meanwhile, the ecosystems that depend on these rivers continue to collapse."