Arsenic contamination detected at 11 sites in Kok River, PCD warns

The samples were collected from 15 locations in the Kok River and its tributaries across Chiang Mai and Chiang Rai provinces, including the Sai and Mekong rivers, between May 12 and 16. 

This is part of a monitoring plan that includes water sampling twice a month and sediment sampling once a month from March to September 2025.

Based on the results of the analysis against surface water quality standards, the findings are as follows:

• Kok River: Out of 15 sampling points, 11 were found to have arsenic levels exceeding the standard of 0.010 milligrams per litre (mg/L). In particular, three locations in Chiang Mai (KK01-KK03) and eight in Chiang Rai (KK04-KK11) recorded arsenic levels ranging from 0.011 to 0.030 mg/L. The highest concentration, 0.030 mg/L, was detected at KK01, near the Thai-Myanmar border in Mae Ai district, Chiang Mai.
• Tributaries: The Fang, Korn, Lao, and Sruay rivers were found to meet the established water quality standards.
• Sai River: Arsenic levels exceeding standards were recorded at all three sampling points, ranging from 0.017 to 0.024 mg/L. The highest concentration, 0.024 mg/L, was detected at SA02, near the Second Friendship Bridge over the Sai River in Mae Sai district, Chiang Rai.
• Mekong River: Both sampling points in the Mekong River recorded arsenic levels exceeding standards, ranging from 0.025 to 0.026 mg/L.

Based on an initial analysis, the PCD hypothesises that:

• High turbidity and arsenic levels in border areas: Water quality results from the Kok and Sai rivers near the Myanmar border indicate high turbidity and elevated heavy metal contamination, specifically arsenic, which suggests significant mining activity in the upstream areas.
• Impact on the Mekong River: Arsenic detected in the Mekong River may have originated from the Sai River, which converges with the Ruak River before flowing into the Mekong. However, the PCD intends to examine water quality from sources in Laos to complete its assessment.
• Health and ecological risks: Areas with excessive arsenic levels pose potential health risks if water is consumed without treatment. Additionally, high arsenic concentrations may harm aquatic life, making fishing and water-related activities unsafe.

The PCD has already conducted a fourth round of water sampling between May 26 and 30. The results will be announced once laboratory analyses are completed.

Residents and water users in affected areas are advised to exercise caution and closely follow official updates, the department stated.