Thai authorities have found dangerous arsenic contamination in sediment from the Mekong River and three of its tributaries in northern Thailand, marking the first time the toxic metal has been detected on the river’s mainstream. Tests carried out in March by Thailand’s Pollution Control Department found arsenic in sediment at three monitoring stations along the Mekong ranging from 73 milligrams per kilogram to 296 milligrams per kilogram.
The department says sediment with less than 10 mg/kg is broadly safe for aquatic life, while levels above 33 mg/kg are dangerous. In the same round of testing, sediment from the Kok, Sai and Ruak rivers ranged from below that threshold to 57 mg/kg, and the department said the contamination appears to be spreading through the river system.
The results, published in mid-April, point to a pollution problem that has moved from tributaries into the main river. For more than a year, heavy metal contamination has been reported from key waterways feeding the Mekong, a shift that now puts one of Southeast Asia’s most important rivers under sharper scrutiny.
That matters because the Mekong Basin has been widely linked to unregulated mining farther upstream in Myanmar. The Stimson Center used satellite imagery to identify 833 unregulated mines across the basin, including 86 thought to be rare earth mines, and more than half of those suspected rare earth sites opened between 2024 and 2026.
The river is central to life across the lower basin, where more than 50 million people depend on it for water, fish and livelihoods. It also supports some 20,000 species of plants and a vast range of wildlife, including the critically endangered Mekong giant catfish and the endangered Irrawaddy dolphin.
What happens next is whether Thai and regional authorities can keep the contamination from moving farther downstream. The test results suggest the burden is already moving through the system, and the longer the source remains uncontrolled, the harder it will be to stop the river from carrying it with it.
