Arsenic in the Kok and Sai Rivers: New Water and Health Checks

Arsenic in the Kok and Sai Rivers remains a reason to monitor water quality and residents’ health in northern Thailand. This is an update to an earlier news story on river water checks, water supply and residents’ health, not a report of new confirmed poisonings. According to Thai PBS, in 2026 (2569 in the Thai calendar), tests in 16 districts of Chiang Rai found 31 exceedances of the inorganic arsenic threshold among 546 urine tests. This does not mean that the people tested were diagnosed with poisoning. Estimates for 2025 (2568 in the Thai calendar) differ. Meanwhile, the broadcaster reported that by November 7, 2026, Mae Sai’s water utility had resumed regular supply. The resumption itself does not confirm that water is safe in every home. Current at time of publication; service recommendations may change.

What is known

According to the information provided in a statement from the regional Pollution Control Department, high turbidity and arsenic in raw water led to a temporary halt of water treatment. The disruption affected 14 villages in 2 subdistricts of Mae Sai district. Specialists took samples at 3 points on the Sai River and sent them for laboratory analysis; the results of these samples are not given in the statement. For advice on choosing food and water, see how to choose safe food and water in Thailand.

Arsenic in rivers: results of raw water checks

During the plant shutdown, raw water turbidity exceeded 10,000 turbidity units (NTU), and arsenic content was 0.30 mg/L. A separate rapid test of Sai River water showed 0.25 mg/L against a stated surface water standard of no more than 0.01 mg/L. These are not measurements of tap water.

In the Huai Fai community on November 5, turbidity was recorded at up to 14,000 units (NTU) with local filtration where possible, around 6,000 units. According to the broadcaster, on November 7 raw water measured 1,164 turbidity units and 0.05 mg/L arsenic. The World Health Organization’s preliminary guideline value for drinking water: 10 µg/L; this is not a local test result. Kok River, Tha Thon Bridge and the Second Thai-Myanmar Friendship Bridge are marked only as geographic reference points, not as boundaries of contamination.

What changed in Mae Sai’s water supply

At the interim stage of restoration, supply was about 60%, then about 80% of water. This is not the current status for all addresses. According to the broadcaster, as of November 7, 2026, the water utility resumed production and supply on a normal basis.

In the 38 surveyed village systems that draw on groundwater in Mae Sai, Chiang Saen and Wiang Chiang Rung, arsenic met the standard. The result applies only to these systems and does not confirm the quality of treated water across the entire Mae Sai network.

Health checks: results for 2025 and 2026

In 2026 (2569 in the Thai calendar), more than 6,000 residents were tested in the province; 546 people from 16 districts submitted urine samples. As reported by Thai PBS, in 31 tests inorganic arsenic exceeded the threshold. In the combined Kok, Sai and Ruak basins, there were 15 such results — these are not statistics for Mae Sai alone.

For 2025 (2568 in the Thai calendar), the same broadcaster indicates 1 exceedance among 322 tests, whereas the Matichon outlet, citing a Ministry of Public Health representative, reports 7 exceedances in a roughly similar sample. Without reconciling methods and geography, these estimates cannot be combined. If arsenic exceeds the threshold at initial screening, further analysis of its chemical form is required to determine whether it is inorganic arsenic; screening is not a diagnosis.

Official measures and further monitoring

At a commission meeting on November 9, 2026 on the Kok, Sai, Ruak and Mekong basins, officials presented the results of the 23rd round of surface water testing and 13th round of sediment studies. Preparations are under way for installing 7 automatic stations; the network has not yet been commissioned. The provincial governor ordered the development of a response and emergency water supply plan for Mae Sai.

Authorities are preparing an emergency groundwater reserve. A project to supply water from the Mekong was already under way; the agency assessed its readiness in 50%. In trials at Mae Fah Luang University, a three-layer system removed up to 96.2% of arsenic; the result cannot be applied to every household filter.

What to do

The earlier warning about untreated Sai River water applied to the subdistricts of Mae Sai, Pong Pha, Si Mueang Chum, Wiang Phang Kham and Ko Chang. Ko Chang here is a subdistrict, not an island; the names serve as reference points, and Si Mueang Chum’s location on the map is approximate.

Do not drink water directly from the Sai River and do not use it for food, ice or beverages without confirmed treatment. Heed local warnings on swimming, washing food, watering food crops, and eating fish caught in the river. For drinking, choose water of confirmed quality; the water provider can tell you the source and the results of testing of the water system. Boiling and ordinary disinfection do not guarantee the removal of arsenic.

If you suspect exposure, contact local medical staff; the helpline is 1422, as listed on the Department of Disease Control’s website. More details: key health risks in Thailand. Recommendations and water supply may change. Check announcements from authorities; this article does not replace medical advice.