CHIANG RAI – An academic researcher says arsenic contamination in rivers across Chiang Rai and Chiang Mai is continuing above legal limits. The slow, gradual spread of heavy metals through the environment, food chain, and human bodies makes the situation difficult to control and predict.
The Pollution Control Department released the results of its 22nd water-quality monitoring campaign for the Kok River and its tributaries, the Sai River, Ruak River, and Mekong River in Chiang Mai and Chiang Rai. Officials conducted the latest round of testing from August 3 to 7, 2026, to monitor water quality and contaminants at 22 locations.
The department used the surface-water quality standard for arsenic, which sets the limit at no more than 0.010 milligrams per liter (mg/L). Tests found arsenic levels above the limit in parts of all four rivers. The Sai River also recorded lead levels above the standard at three locations.
Kok River
Arsenic exceeded the standard at six monitoring points:
- Tha Ton Bridge, Tha Ton subdistrict, Mae Ai district, Chiang Mai, 0.011 mg/L
- Mae Na Wang Friendship Bridge, Mae Ai district, Chiang Mai, 0.011 mg/L
- Mae Yao-Doi Yang Friendship Bridge, Mueang Chiang Rai district, Chiang Rai, 0.013 mg/L
- Kok River Bridge, Mueang Chiang Rai district, Chiang Rai, 0.013 mg/L
- Chiang Rai Weir, Mueang Chiang Rai district, Chiang Rai, 0.013 mg/L
- Rim Kok-Wiang Nuea Ruam Jai Bridge, Mueang Chiang Rai district, Chiang Rai, 0.011 mg/L
Sai River
Arsenic and lead exceeded the standards at three locations:
- Ban Hua Fai, Mae Sai district, Chiang Rai, arsenic at 0.284 mg/L and lead at 0.266 mg/L
- The Second Sai River Friendship Bridge, Mae Sai district, Chiang Rai, arsenic at 0.208 mg/L and lead at 0.193 mg/L
- Ban Pa Sang Ngam, Mae Sai district, Chiang Rai, arsenic at 0.184 mg/L and lead at 0.176 mg/L
The standard for lead in surface water is no more than 0.05 mg/L. Other tested heavy metals remained within the standards at every monitoring location.
Ruak River
Arsenic exceeded the standard at two locations:
- Ko Chang Water Pumping Station, Provincial Waterworks Authority, Mae Sai district, Chiang Rai, 0.017 mg/L
- Wiang subdistrict, Chiang Saen district, Chiang Rai, 0.012 mg/L
Mekong River
One location recorded arsenic above the standard:
- Muang Yai subdistrict, Wiang Kaen district, Chiang Rai, 0.014 mg/L
Residents can follow water-quality monitoring results and updates on the situation at the Pollution Control Department website: www.pcd.go.th
Dr. Suebsakul Kitnukorn, an academic researcher at Mae Fah Luang University, said he recently discussed the issue with an international doctoral student. He explained that the contamination reflects the gradual accumulation of heavy metals in the environment, food chain, and human body.
According to Dr. Suebsakul, the effects develop slowly and spread across different areas, which makes them difficult to monitor, control, and predict. Aquatic ecosystems are also being damaged as rivers gradually lose aquatic plants and animals. He added that the government’s slow and limited response has led many residents to question the authorities and lose trust in official measures.
This quiet water crisis is causing a gradual collapse of the natural river ecology. Local waterways are slowly losing their important aquatic plants and native fish populations. Sadly, the government’s official response has been just as slow and uncomfortably silent. Consequently, frustrated citizens are starting to question the authorities and completely lose their trust.
