Thai authorities found dangerous arsenic contamination in burma-linked sediment from the Mekong River mainstream in March tests, the first time the pollutant has been detected there. Sediment from three monitoring stations along the mainstream showed concentrations from 73 to 296 milligrams per kilogram, well above levels Thailand’s Pollution Control Department calls dangerous.
The department says less than 10 mg/kg is broadly safe for aquatic life, while levels above 33 mg/kg are dangerous. The March results also showed arsenic in sediment from the Kok, Sai and Ruak rivers, with readings ranging from below 33 mg/kg to 57 mg/kg.
Thailand's March tests
The March sampling covered the Mekong River and three tributaries in Chiang Mai and Chiang Rai, in northern Thailand. The result that matters most is the mainstream finding: three separate monitoring stations on the Mekong itself all showed arsenic, with the highest reading hitting 296 mg/kg.
That is the first mainstream detection Thailand has recorded. Previous heavy metal pollution had been reported in key tributaries, but this test extends the concern to the river channel that carries water and sediment across borders.
Mekong Basin mining links
Thailand’s Pollution Control Department says the contamination appears to be spreading through the river system. Heavy metal pollution in the Mekong Basin has been widely linked to unregulated mining farther upstream in Myanmar, and a Stimson Center satellite imagery analysis identified 833 unregulated mines across the basin.
Of those mines, 86 are thought to be rare earth mines, and more than half of the 86 opened between 2024 and 2026. In the river system now under scrutiny, the contamination already runs through the Mekong mainstream and into the Kok, Sai and Ruak rivers.
For communities along the Lower Mekong River Basin, the stakes are immediate: more than 50 million people rely on the river for water, fish and livelihoods, and the basin supports some 20,000 species of plants, 1,200 bird species, 800 reptile and amphibian species, and 430 mammal species. The river also supports the critically endangered Mekong giant catfish and the Mekong subspecies of Irrawaddy dolphin, with fewer than 100 individuals thought to remain.
Chiang Mai and Chiang Rai
The contamination was found in northern provinces that sit closest to the sampled stretches of the river system, putting Chiang Mai and Chiang Rai at the center of the response. In practical terms, the March tests point to a river corridor where sediment, tributaries and downstream users now face the same pollutant signal rather than separate local incidents.
The next step is for the Pollution Control Department and other authorities to track whether the arsenic readings persist across the same stations and tributaries, because the department says the contamination appears to be spreading through the river system. For people who depend on the Mekong in Thailand, Laos, Myanmar, Cambodia and Vietnam, the newest data shifts the issue from isolated tributary pollution to a mainstream river problem.
