Arsenic contamination in northern Thailand has reached critical levels, with toxic pollutants now spreading into the Mekong River, according to Thailand’s Pollution Control Department (PCD).
The scope of contamination is alarming. Between 26-30 May, the PCD tested 20 sites across three major waterways, and every single location showed arsenic levels exceeding both national surface water quality standards and the World Health Organization’s safety threshold of 0.01 milligrams per litre.
All 15 monitoring points along the Kok River, three in Chiang Mai Province and twelve in Chiang Rai Province, registered arsenic concentrations between 0.013 and 0.023 milligrams per litre, with the highest levels detected at Pong Na Kham village.
The Sai River showed even more severe contamination, with all three sampling locations recording concentrations ranging from 0.027 to 0.038 milligrams per litre, peaking at the 2nd Friendship Bridge in Mae Sai District. Even the Mekong River itself showed elevated arsenic levels of 0.018 and 0.014 milligrams per litre at two points in Chiang Saen District.

Water samples collected near the Thai-Myanmar border revealed both abnormally high turbidity and elevated heavy metal concentrations, particularly arsenic, providing clear evidence of pollution from upstream mining operations in Myanmar’s Shan State.
The environmental and health impacts are already visible.
The Living River Association reported on 10 June that communities along the Kok and Mekong rivers have observed increasing numbers of fish with visible infections and nodules, clear indicators of heavy metal bioaccumulation in the aquatic food chain.

Recognizing the transboundary nature of this environmental disaster, the PCD has recommended coordinated water testing from the Lao side of the river to fully understand the contamination’s regional scope. The department will launch another round of comprehensive testing, though seasonal factors complicate assessment as the initial testing occurred during the early rainy season when water conditions typically fluctuate.
The contamination’s severity first became apparent in late April, when researchers from Mae Fah Luang University and the Northern Natural Disaster Research and Prediction Institute discovered arsenic levels reaching 0.19 milligrams per litre, nearly 19 times the WHO safety threshold, at nine sites following flash floods in Chiang Rai. The worst contamination was found at the Golden Triangle, where the Ruak River meets the Mekong at the convergence of Thailand, Myanmar, and Laos.
Meanwhile, environmental activists and residents are calling on the Thai government to take immediate action, including investigating and potentially shutting down unregulated or illegal mining activities upstream, particularly those believed to be operating in Myanmar’s Shan State.

Boonsuk Suwandee, a fisherman from Sop Kham Village in Chiang Saen District, told the Living River Association that toxic substances in the rivers are severely affecting the livelihoods of local fishermen.
The fisherman stressed that fish vendors have completely stopped buying their catch and are now unreachable, with some even turning off their phones. He urged the government and relevant agencies to step in and provide immediate support to those affected, noting that, since the contamination incident, their income from fishing has nearly vanished.