Chiang rai: The Pollution Control Department continues its rigorous inspection of the Kok, Sai, Ruak, and Mekong rivers after detecting arsenic levels that exceed safety standards in several areas. The department's fifth water quality test, conducted between June 9-13, 2025, revealed that arsenic concentrations above the standard of 0.010 milligrams per liter were found at numerous points, particularly near the Thai-Myanmar border and at the confluence of the Sai, Ruak, and Mekong rivers.
According to Thai News Agency, the examination of the Kok River from the Thai-Myanmar border through Mae Ai District, Chiang Mai Province to Mueang District, Chiang Rai Province, showed that arsenic levels surpassed the safety standard at all nine initial testing points. Notably, Ban Ja Der in Doi Hang Subdistrict (KK04) recorded the highest concentration at 0.047 mg/l. Although most downstream locations showed arsenic levels below the standard, KK11 at Rim Kok Bridge-Wiang Nuea Ruam Jai was an exception, recording a level of 0.011 mg/l.
The Sai and Ruak rivers, significant tributaries of the Mekong, also recorded arsenic levels exceeding the standard across all five testing sites. Particularly at SA02, near the 2nd Thai-Lao Friendship Bridge, arsenic was found at 0.044 mg/l. Additionally, the Mekong River's checkpoints, including NK01 at the Golden Triangle border crossing, showed above-standard arsenic levels.
The Pollution Control Department's analysis suggests a correlation between high turbidity values and elevated arsenic levels in these border areas, which aligns with the hypothesis of mining activities in Myanmar. It was observed that arsenic tends to bind with suspended particles in the water rather than existing in a dissolved state. The department also noted that increased rainfall and river dredging might exacerbate sediment disturbance and turbidity, especially in Doi Hang, Chiang Rai Province, compared to the Mae Ai District, where no rain preceded the sample collection.
In contrast, other tributaries feeding into the Kok River, such as the Fang, Lao, Korn, and Suai rivers, maintained water quality within safe standards. The Pollution Control Department affirmed its commitment to monthly water quality assessments until September 2025 to monitor changes and develop strategies to mitigate the impact on local water use.