Chiang rai: Arsenic levels in sediment have exceeded safety standards in the Kok-Sai and Ruak-Khong river basins, reaching critically high levels. According to Thai News Agency, the Pollution Control Department (PCD) has conducted its 12th sediment test, discovering arsenic concentrations that surpass the safe threshold at various sites within the Kok and Sai rivers. One location in the Kok River has recorded levels that are perilous for benthic organisms. Furthermore, two locations in the Ruak River and four in the Mekong River have shown similar alarming levels, with the Mekong River at Chiang Saen district reporting the highest concentration of 80 milligrams per kilogram.
The Pollution Control Department (PCD) from the Ministry of Natural Resources and Environment reported on the 12th sediment monitoring results for the Kok River and its tributaries, the Sai River, the Ruak River, and the Mekong River. Samples were collected from July 7-10, 2026, across 21 points in Chiang Mai and Chiang Rai provinces to investigate abnormal turbidity in the Kok River.
Arsenic sediment quality standards indicate that a level above 10 milligrams per kilogram (mg/kg) is unsafe for benthic organisms, while levels above 33 mg/kg are severely harmful. The tests revealed that the Kok River exceeded these safety limits at six locations, with arsenic levels ranging from 11-23 mg/kg, including the Thai-Myanmar border and Kaeng Tum hamlet at 23 mg/kg, Mae Yao-Doi Hang Friendship Bridge at 11 mg/kg, and several other key sites.
In Mueang Chiang Rai District, at the Chalerm Phra Kiat 1 Bridge, arsenic levels were found at 45 mg/kg, severely dangerous to benthic organisms. Additionally, nickel, lead, and chromium were found in excess of safe levels at this location. Similarly, mercury contamination was detected at Mae Yao-Doi Hang Friendship Bridge and Rim Kok-Wiang Nuea Ruam Jai Bridge.
The Mae Sai River also presented arsenic levels above the safe threshold at Ban Hua Fai and Ban Pa Sang Ngam, Mae Sai District, with measurements of 25 mg/kg and 13 mg/kg respectively. In the Ruak River, extremely hazardous arsenic levels of 49 mg/kg and 55 mg/kg were recorded at Koh Chang pumping station and Wiang Subdistrict, Chiang Saen District, respectively, along with mercury, nickel, and chromium contamination.
In the Mekong River, all four monitoring points showed severely dangerous arsenic levels, with Wiang Subdistrict, Chiang Saen District recording the highest at 80 mg/kg, followed by the Golden Triangle border crossing and other key locations. The Ban Sop Kok area also faced mercury and chromium contamination.
While the tributaries of the Kok River, like the Fang River and Lao River, generally met sediment quality standards, the Korn River reported 38 mg/kg of nickel, exceeding safe levels for benthic organisms.
The Pollution Control Department (PCD) suggested that the elevated levels might be linked to the rainy season, which increases water volume and sediment transport. PCD plans to continue monitoring environmental quality, release inspection results, and coordinate with relevant agencies to ensure public safety.