Thailand faces new environmental threat from arsenic contamination in northern rivers

The primary source of the contamination is believed to originate from the headwaters of the Kok River in Shan State, Myanmar, where extensive gold and heavy metal mining is conducted. Hazardous chemicals, including arsenic, are commonly used in the extraction of metals from raw ore in the area.

During periods of heavy rain or soil erosion from mining activity, these toxic substances are washed into upstream tributaries, eventually flowing into rivers that cross into Thailand, such as the Kok and Sai rivers. The absence of clear regulatory controls at the source further exacerbates the problem.

In response, the Pollution Control Department (PCD) has developed a systematic environmental monitoring plan. The initiative focuses on collecting water and sediment samples from rivers in Chiang Mai and Chiang Rai provinces.

Sampling is currently underway at 15 sites along the Kok River, three sites along the Sai River, and two sites on the Mekong River, as well as additional monitoring on the Fang, Korn, Lao and Sruay rivers.

The operation for the 2025 fiscal year is scheduled to run from March to September. During this period, water samples are collected twice a month to monitor quality, while sediment samples are taken once a month.

The results from the fourth round of sampling, conducted between May 26 and 30, revealed that arsenic contamination at all 15 sampling points along the Kok River exceeded the safety threshold of 0.010 milligrams per litre (mg/L).

The detected arsenic levels at each location were as follows:

• KK01 (Thai–Myanmar border, Mae Ai district, Chiang Mai): 0.016 mg/L
• KK02 (Nawang–Thaton Friendship Bridge, Mae Ai district, Chiang Mai): 0.017 mg/L
• KK03 (Ban Mae Salak border bridge, Mae Ai district, Chiang Mai): 0.015 mg/L
• KK04 (Ban Cha Der, Moo 6, Mueang district, Chiang Rai): 0.022 mg/L
• KK05 (Mae Yao–Doi Hang Friendship Bridge, Mueang district, Chiang Rai): 0.022 mg/L
• KK06 (Ban Pong Na Kham, Mueang district, Chiang Rai): 0.023 mg/L
• KK07 (Bridge over the Kok River, Mueang district, Chiang Rai): 0.016 mg/L
• KK08 (Mae Fah Luang Bridge, Mueang district, Chiang Rai): 0.018 mg/L
• KK09 (Chaloem Phrakiat 1 Bridge, Mueang district, Chiang Rai): 0.020 mg/L
• KK10 (Chiang Rai Weir, Mueang district, Chiang Rai): 0.022 mg/L
• KK11 (Rim Kok–Wiang Nuea Ruam Jai Bridge, Wiang Chai district, Chiang Rai): 0.020 mg/L
• KK12 (Yonok Nakhon Bridge, Mueang district, Chiang Rai): 0.015 mg/L
• KK13 (Tha Khao Pluak subdistrict, Mae Chan district, Chiang Rai): 0.015 mg/L
• KK14 (Nong Pa Kor subdistrict, Doi Luang district, Chiang Rai): 0.015 mg/L
• KK15 (Ban Saeo subdistrict, Chiang Saen district, Chiang Rai): 0.013 mg/L

Water quality in the Kok River’s tributaries — namely the Fang, Korn, Lao, and Sruay rivers — remains within the prescribed safety standards.

Arsenic levels in the Sai River also exceeded safety standards:

• SA01 (Ban Hua Fai, Mae Sai district, Chiang Rai): 0.027 mg/L
• SA02 (Second Sai River Friendship Bridge, Mae Sai district, Chiang Rai): 0.038 mg/L
• SA03 (Ban Pa Sang Ngam, Moo 6, Mae Sai district, Chiang Rai): 0.031 mg/L

Similarly, arsenic contamination in the Mekong River was found to be above the safety limit:

• NK01 (Wiang subdistrict, Chiang Saen district, Chiang Rai): 0.018 mg/L
• NK02 (Ban Saeo subdistrict, Chiang Saen district, Chiang Rai): 0.014 mg/L

Three key findings emerged from the analysis:

• High arsenic levels and turbidity near the Thai–Myanmar border

Sampling points along the Kok and Sai rivers near the border consistently showed elevated turbidity and high levels of arsenic — a heavy metal commonly linked to mining operations. These indicators align with known mining activities across the border in Myanmar.

• Possible indirect impact on the Mekong River from the Sai River

Preliminary analysis suggests that arsenic levels detected in the Mekong River may be linked to inflows from the Sai and Ruak rivers, which merge before reaching the Mekong. However, further investigation is needed, particularly upstream monitoring in Laos, to systematically confirm the source of contamination.

• Rainy season increases contaminant runoff and dispersion

The fourth sampling round took place in early rainy season (after May 15), when river flows were high and turbidity had increased. Water gates at the Chiang Rai Weir had also been opened for irrigation and flood control, accelerating the wash-off of sediment and contaminants from upstream sources. As a result, arsenic levels in both the Kok and Sai rivers exceeded the safety threshold at every monitored location.

PCD is continuing its monitoring efforts and has already collected samples during the fifth round, conducted between June 9 and 13. These will be analysed and compared with earlier results to track changes and better understand the situation.

Once laboratory analysis is complete, the department will publicly release the findings. The data will be used to inform measures to address the contamination and mitigate its impact on local communities that rely on river water for daily use.