CHIANG RAI – The Pollution Control Department’s (PCD) latest inspection shows that the closure of the Wa mine due to recent storms has led to a temporary drop in toxic pollution levels in the Kok River. This improvement was seen at every testing site from Mae Ai in Chiang Mai through to Chiang Rai.
Only the water in certain reservoir basins still exceeded safety standards. Even so, the Kok River, which is home to more than ten gold and manganese mines upstream, continues to contain high levels of heavy metals.
Tests have confirmed the presence of heavy metals and arsenic in the Kok River since early 2025. The findings span across Mae Ai and Chiang Rai in Chiang Mai province, as well as the Sai and Mekong Rivers within the Golden Triangle basin.
Towards the end of August 2025, the PCD carried out its ninth round of surface water quality checks in Chiang Rai and Chiang Mai.
Arsenic levels in the Kok River dropped sharply in both provinces, notably along the river from the Thai-Myanmar border into Mae Ai in Chiang Mai. The first eight rounds of monitoring all found arsenic concentrations above safe levels, each time over 0.01 milligrams per litre.
In the ninth round, arsenic at every checkpoint, including the Tha Ton Bridge, Ban Mae Salak Bridge, and Mallika Subdistrict in Mae Ai, fell below 0.01 milligrams per litre.
A similar trend was recorded in Chiang Rai province, where the Kok River passes from Ban Ja De, Moo 6, Doi Hang subdistrict, all the way to Ban Saeo in Chiang Saen district. Tests at 12 points previously revealed arsenic above the safety standard at almost every site.
This time, only four locations saw levels above the threshold: Ban Ja De (0.011 milligrams per litre), Chalerm Phrakiat 1 Bridge in Chiang Rai town (0.011), Chiang Rai Weir (0.011), and Rim Kok-Wiang Nuea Ruam Jai Bridge (0.012).
Asst Prof Dr Suebsakul Kitnugul from the School of Social Innovation, Mae Fah Luang University, who is also part of the Kok River Protection Network, stated that heavy metal levels fell during the ninth testing round.
The likely reason was heavy rainfall after Tropical Storm Wipha crossed mining areas in neighbouring countries. This may have led to the temporary closure of mines to reduce storm risks, resulting in less heavy metal pollution.
The best way to protect the Kok River from heavy metal contamination seems to be the permanent closure of mines in Myanmar. The remaining arsenic at Chiang Rai Weir might be due to flooding that washed sediments and metals into the river. Supporters of sediment weirs should review this information when planning their projects.
Although heavy metal concentrations in the Kok River have dropped, sections of the river flowing in from Myanmar through Mae Sai in Chiang Rai still contain arsenic above health safety standards. At Ban Hua Fai in Mae Sai subdistrict, testers measured 0.029 milligrams per litre.
The Second Thai-Myanmar Friendship Bridge recorded 0.034, and at Ban Pa Sang, Moo 6, Ko Chang subdistrict, Mae Sai district, the reading was 0.019. Manganese was also found in excess at 1.10 milligrams per litre, and lead at 0.074 at the same bridge, with 0.065 at another spot. In other areas, no excessive toxins were found in the Ruak and Mekong rivers.
Reports note there are at least ten gold mines in the Kok and Sai river basins within Special Region 2 (Wa State). These sites are the main sources of arsenic and manganese.
At least two rare earth mines have also been discovered on both sides of the Kok River since 2024, which have helped to reduce turbidity. The PCD has done nine rounds of tests, finding arsenic and other pollutants in the river.
Rare earth mines, at least nineteen, have also been found in the mountains and streams of the Loi basin in Special Region 4 (Mong La) near the National Democratic Alliance Army (NDAA) base at the Myanmar-China border. Water from these sites flows into the Loi River and then the Mekong, about 125 kilometres from the Golden Triangle in Chiang Saen, Chiang Rai.
