Myanmar Mining Suspected in Arsenic Contamination of Northern Thai Rivers

Orange-yellow water in the Kok and Sai rivers has raised alarm among communities in Chiang Rai, where residents depend on these waterways for daily life, farming, and fishing. Thai monitoring found arsenic above the national surface-water limit of 0.01 milligrams per liter at several sites, while sediment testing has also raised concerns. You can read the latest findings in this report on arsenic contamination in the Kok and Sai rivers. Mining in Myanmar’s Shan State remains the leading explanation for the pollution, especially because suspected mining areas sit upstream of the border. However, investigators haven’t fully proven that mining caused the contamination at every location, and other factors, including rainfall, erosion, and disturbed river sediment, may affect test results. That uncertainty matters when assessing health risks linked to drinking water, food, fish, and long-term exposure to arsenic. The Kok and Sai flow into the Ruak and Mekong river systems, carrying the issue beyond a single community or national boundary. This article examines what Thai monitoring has confirmed, why Myanmar mining is under scrutiny, and why cross-border cooperation is needed to protect the rivers and the people who depend on them. The pollution concerns begin upstream, where the Kok and Sai rivers pass through or receive water from areas connected to Myanmar’s Shan State before entering northern Thailand. Once contaminants enter these waterways, currents carry them across the border into Chiang Rai, making the problem shared by communities in both countries. A broader overview of the issue is available in Chiang Rai’s rivers threatened by arsenic contamination. Arsenic occurs naturally in some rock, including ore linked to gold deposits. Under normal conditions, much of it remains trapped underground. Mining can expose and crush that rock, while washing ore and storing waste can allow arsenic-bearing sediment and wastewater to reach streams. Arsenic is dangerous when people consume high levels over time. River contamination can affect drinking water, crops irrigated with river water, fish, and sediment along the banks. However, finding arsenic above a limit does not identify a particular mine. Rainfall, erosion, flooding, and disturbed sediment can also change where contaminants collect. Thailand’s surface-water standard for arsenic is 0.01 milligrams per liter (mg/L). A reading above that benchmark confirms a water-quality exceedance, but it doesn’t prove which mine caused it. Reports have linked the pollution to suspected mining in Shan State, yet investigators have not publicly established one confirmed source for every reading. Thai monitoring reports provide evidence of contamination, not automatic proof of responsibility. Testing shows a mixed pattern rather than one uniform level of contamination. Results change by date, weather, sampling location, and whether officials test surface water or river sediment. A single sample is only a snapshot, so it cannot describe the entire river system. Thailand’s surface-water arsenic standard is 0.01 milligrams per liter (mg/L). The Mekong River Commission reported that some February and March 2026 water readings were only slightly above that limit, while sediment results raised more serious ecological concerns. Its regional update on arsenic contamination explains why both measurements matter. In monitoring conducted from February 24 to 27, 2026, four Kok River sites exceeded the water standard. Samples taken between Tha Ton Bridge in Chiang Mai and Doi Hang Subdistrict in Chiang Rai ranged from 0.013 to 0.016 mg/L. Later testing found a lower exceedance at the Mae Yao-Doi Hang Friendship Bridge, where arsenic measured 0.011 mg/L. Sediment readings at four Chiang Rai sites ranged from 11 to 16 mg/kg, including 15 mg/kg near a Kok River bridge and 13 mg/kg near Chiang Rai Weir. These results show why the latest Kok River water and sediment findings need to be read together. Water may meet the standard during one sampling round while contaminated sediment remains on the riverbed. The Sai River produced the most consistent water exceedances. During the February round, every monitored site measured between 0.011 and 0.015 mg/L. A later round recorded 0.016 mg/L at sites from Ban Hua Fai to the Second Thai-Myanmar Friendship Bridge and Ban Pa Sang Ngam. Sediment arsenic reached 25 mg/kg at Ban Hua Fai and 13 mg/kg at Ban Pa Sang Ngam. Although those figures differ from the Mekong results, they show that contamination can remain in river material after water readings change. All monitored Ruak sites were within the surface-water standard during the February 2026 round. However, sediment testing still found contamination, so a compliant water sample doesn’t prove that the riverbed is safe. The Mekong produced mixed results. Water measured 0.014 to 0.015 mg/L at the Golden Triangle crossing and the Mae Sai waterworks intake in February, although later testing found the monitored Mekong sites within the standard. Sediment readings were much higher, reaching 66, 80, 50, and 68 mg/kg at four Chiang Rai sites. Floods, strong currents, or dredging can disturb that sediment and move arsenic back into the water. That is why repeated testing matters more than any single reading. The link is credible but not yet conclusive. Thai agencies have confirmed elevated arsenic in water and sediment, while the suspected source in Myanmar is based on upstream geography, timing, field reports, satellite imagery, and public statements. Those findings point toward mining, but they don’t prove which operation caused each downstream reading. Thai Pollution Control Department testing found arsenic above Thailand’s surface-water limit in parts of the Kok and Sai rivers. Sediment testing also found elevated concentrations, including in areas where later water samples were closer to or below the legal limit. These results confirm contamination in Chiang Rai, but they don’t identify its source. The geography raises concern because the Kok and Sai river systems receive water from upstream areas of Myanmar’s Shan State. Reports have identified gold-mining activity near Mong Hsat, also spelled Mong Hsai in some accounts, and near the Kok River headwaters. Rainfall, erosion, flooding, and disturbed sediment can also move naturally occurring arsenic, so river chemistry requires repeated sampling and source-specific investigation. Elevated arsenic confirms a water-quality problem. It doesn’t, by itself, prove that a named mine caused it.