The Pollution Control Department has confirmed unsafe levels of arsenic and other heavy metals in the Kok, Sai and Mekong rivers in northern Thailand, with the contamination traced to upstream mining operations across the border in Shan State of Myanmar.
The PCD released its findings from three rounds of water and sediment sampling conducted between March 19 and May 16, revealing widespread pollution in northern waterways.
The highest levels of contamination were detected in the Kok River, particularly from Tha Ton in Chiang Mai’s Mae Ai district, to Chalerm Phrakiat Bridge in Muang district of Chiang Rai. In some locations, arsenic concentrations reached 0.44 milligrammes per litre (mg/l) — more than 40 times above the safety threshold of 0.01 mg/l.
Lead levels also exceeded safe standards, peaking at 0.076 mg/l near the Thai–Myanmar border in Chiang Rai. While other metals such as cadmium, mercury, and manganese were found within safe limits, the persistence of arsenic is raising concerns among health and environmental authorities.
“Water quality along the Myanmar border shows elevated turbidity and arsenic, a clear indicator of mining impacts,” a PCD source said. Officials said historical monitoring from 2014 to 2015 had shown no such contamination, suggesting a recent and worsening trend.
Sediment testing in March and April confirmed that levels of arsenic, lead, and other heavy metals had exceeded those considered safe to benthic (bottom-dwelling) organisms. High-risk areas include Tha Ton, Ban Mae Salak, Ban Ja Der, and Chalerm Phrakiat Bridge 1, where health experts warn that regular consumption of locally caught fish could pose long-term health risks.
The PCD expressed concern that chronic exposure to these metals, especially arsenic, could lead to cumulative health effects, even if symptoms are not immediately apparent.
Further downstream, including at the Yonok Nakorn Bridge and Ban Saeo in Chiang Saen district in Chiang Rai, arsenic levels were found to be within acceptable limits. Authorities attribute this to sedimentation at Chiang Rai weir, which has slowed water flow and allowed contaminated particles to settle.
In the Sai River, which flows into the Mekong in Chiang Rai, arsenic levels exceeded safety standards at all sampling sites, ranging from 0.044 to 0.049 mg/l, higher than those observed in the Kok River. Lead levels also initially exceeded limits before returning to normal in the third round of testing.
While the Mekong River did not show excessive levels of heavy metals overall, arsenic concentrations remained a concern. In Chiang Saen district, readings peaked at 0.036 mg/l before slightly falling to 0.025–0.026 mg/l in the latest samples.
Tributaries such as the Fang, Korn, Lao, and Suai rivers showed sporadic spikes in nickel, arsenic and chromium, with the Korn River recording the highest range of contaminants. The PCD believes that arsenic detected in the Mekong may be linked to inflows from the Sai and Ruak rivers, which originate in Myanmar.
Natural Resources and Environment Minister Chalermchai Sri-on acknowledged the country’s limited ability to address pollution that originates beyond its borders. Mining operations in Shan State, controlled by the United Wa State Army, are seen as the primary source of contamination.
“This issue cannot be solved overnight, as its root cause lies in foreign territory,” Mr Chalermchai said. “We must proceed carefully, using diplomatic channels step by step.”
In the short term, the ministry has proposed check dams to trap contaminated sediment before it flows further downstream. The Department of Water Resources is designing the structure, which could cost more than 7 billion baht, he said.
Water resources expert Sitang Pilailar of Kasetsart University says the proposed check dams must be carefully engineered to contain fine sediment and withstand high pressure.
“If the dam fails, it could worsen the situation,” she warned. “In the meantime, urgent efforts are needed to locate clean water sources for tap water production and invest in reverse osmosis systems capable of filtering out heavy metals.”
Ms Sitang also urged the government to take a stronger stance on the issue. “Thailand must not act as though it is powerless to speak out against harm to its own territory. We need a robust national response to protect public health.”
Chiang Rai residents plan to rally on Thursday to demand an end to mining activities in Myanmar, and they will urge the embassies to intervene.