CHIANG RAI – Heavy rain brought by Storm Wipha led to more water flowing into the Kok and Mekong rivers. This surge helped dilute arsenic contamination, so current levels now mostly meet safety standards. This marks the first time in several months that the rivers are seeing this kind of improvement.
The flooding caused by the storm may have forced mining operations in Myanmar to pause, cutting off a key source of new pollution for now. While this is good news, some risks remain in certain spots.
Heavy metal pollution in the Kok, Sai, Ruak, and Mekong rivers isn’t just an environmental problem. It’s a serious health hazard for people in northern Thailand. The expansion of rare earth mines in Myanmar’s Shan State, right across the border, plays a big role in this contamination.
Reports from the Human Rights Foundation point to a major jump in mining over the past decade, especially since 2015. These mines often use a chemical process called in-situ leaching, where chemicals are injected underground to extract minerals, which can release arsenic into both surface and groundwater.
Pollution Control Department: Twice-Monthly Water Checks
The Pollution Control Department (PCD) under Thailand’s Ministry of Natural Resources and Environment now samples water at over 15 locations along the Kok River (sites KK01-KK15), four tributaries, the Sai River (three sites), the Ruak River (two sites), and the Mekong River (three sites) in Chiang Mai and Chiang Rai.
Samples are collected twice each month, while sediment samples are taken once a month from March through September 2025.
The latest round of testing, done in early August 2025 (ninth for the fiscal year), revealed something encouraging. Where earlier tests found arsenic levels above the standard at every location, this recent round shows most sites are now within safe limits.
Turbidity and heavy metal contamination, especially arsenic, have dropped in the Kok River. Most testing locations now record levels at or below the regulatory limit, likely because floods from Storm Wipha stalled mining activities upstream in Myanmar. With source activity paused, less arsenic is showing up in the river, leaving water quality much improved in many spots.
There’s one exception: at Chiang Rai Weir, just outside Chiang Rai city, arsenic levels still sit above the standard (0.011-0.012 mg/l). Water released to control flooding disturbed riverbed sediment, temporarily raising arsenic concentrations.
The Sai River still stands out as a problem area. Arsenic and lead levels continue to exceed safe standards at every site. Floods and damaged levees have made this worse, lowering water quality and potentially affecting people who depend on this river for their daily needs.
August 2025 Water Quality Test Results
Kok River:
Most locations tested below the 0.01 mg/l arsenic safety limit, except a few near Chiang Rai city:
- KK01 (Thailand-Myanmar border, Tha Ton, Mae Ai, Chiang Mai): less than 0.010 mg/l
- KK02 (Friendship Bridge, Mae Ai): less than 0.010 mg/l
- KK03 (Ban Mae Sak, Mae Ai, Chiang Mai): less than 0.010 mg/l
- KK04 (Ban Cha Doe, Doi Hang, Chiang Rai): 0.011 mg/l
- KK05 (Mae Yao-Doi Hang bridge, Chiang Rai): less than 0.010 mg/l
- KK06 (Ban Pong Na Kham, Doi Hang): less than 0.010 mg/l
- KK07 (Doi Hang bridge): 0.010 mg/l
- KK08 (Mae Fah Luang bridge, Rob Wiang): less than 0.010 mg/l
- KK09 (Chaloem Phrakiat 1 bridge, Rob Wiang): 0.011 mg/l
- KK10 (Chiang Rai Weir): 0.011 mg/l
- KK11 (Rim Kok-Wiang Nuea Ruam Jai bridge, Wiang Chai): 0.012 mg/l
- KK12 (Yonok Nak Nakhon bridge, Mae Khao Tom): less than 0.010 mg/l
- KK13 (Tha Khao Pluak, Mae Chan): 0.010 mg/l
- KK14 (Nong Pa Kor, Doi Luang): less than 0.010 mg/l
- KK15 (Ban Saew, Chiang Saen): less than 0.010 mg/l
Kok River tributaries—Fang (FA01), Korn (KO01), Sruay (SU01), and Lao (LA01)—all showed water quality within safe parameters.
Sai River:
All sites continue to show high arsenic:
- SA01 (Ban Hua Fai, Mae Sai, Chiang Rai): 0.029 mg/l
- SA02 (Second Friendship Bridge, Mae Sai): 0.034 mg/l
- SA03 (Ban Pa Sang Ngam, Ko Chang, Mae Sai): 0.019 mg/l
Ruak River:
Arsenic remains within safe limits:
- RU01 (Ko Chang water pump station): less than 0.010 mg/l
- RU02 (Wiang, Chiang Saen): less than 0.010 mg/l
Mekong River:
All three testing locations show compliance with the standard:
- NK01 (Golden Triangle checkpoint, Wiang, Chiang Saen): 0.010 mg/l
- NK02 (Wiang, Chiang Saen): 0.010 mg/l
- NK03 (Ban Sop Kok, Ban Saew, Chiang Saen): less than 0.010 mg/l
River Routes and How They Connect the Border
Kok River
Originating in Shan State, Myanmar, flows into Thailand at Mae Ai (Chiang Mai), runs through Chiang Rai, and then empties into the Mekong at Chiang Saen. It’s the main river for Chiang Rai, supporting local economies, tourism, and communities that rely on it for water.
Sai River
Also starts in Shan State and acts as a small but important river marking the border between Thailand and Myanmar near Mae Sai, Chiang Rai. It forms a natural boundary for the two countries.
Ruak River
Starts in the hills straddling the Thailand-Myanmar border, passes through Mae Sai and Chiang Saen, and merges with the Mekong at the Golden Triangle. This meeting point is a global tourism hotspot, famous as the junction where Thailand, Laos, and Myanmar come together.
Mekong River
Rises in Tibet, flows through China, Laos, Myanmar, Thailand, Cambodia, and Vietnam. Near Chiang Rai, it marks the boundary between Thailand and Laos—not Thailand and Myanmar.
It’s the lifeline of Southeast Asia, vital for local economies, farming, fishing, and transportation. The recent drop in arsenic is a promising turn for northern Thailand’s rivers, but ongoing checks and action on pollution sources remain critical.
