Pollution Control Department warns agencies to check arsenic levels in sediment from Chiang Rai rivers

The Pollution Control Department has warned government agencies dredging three rivers in Chiang Rai to thoroughly test sediment for arsenic contamination before transporting or disposing of it.

The department issued new guidelines outlining four levels of acceptable arsenic contamination. These apply to sediment dredged from the Kok, Sai, and Ruak rivers, amid growing concerns that toxic heavy metals and arsenic may have entered Thai waterways due to unregulated mining activities in neighbouring Myanmar.

Joint dredging effort raises environmental concerns

Currently, the Harbour Department, the Royal Irrigation Department, the Royal Thai Armed Forces' development corps, and the engineering corps of the Third Army Area are jointly dredging the three rivers to reduce flood risks.

However, environmentalists have raised concerns that the sediment may spread pollution to communities and ecosystems in the areas where it is transported or dumped.

In response, the Pollution Control Department released the following safety guidelines for handling contaminated sediment:

1. Arsenic ≤ 6 mg/kg

The sediment can be used without restriction.

2. Arsenic 7–25 mg/kg

The sediment can be used for general purposes but must not be used for building residences unless the site is fully paved with cement to prevent direct contact.

3. Arsenic 26–500 mg/kg

The sediment should not be used unless the area is paved with cement. If disposed of, the pH must be neutralised with lime, ash, or similar agents. The sediment must then be covered with a plastic sheet and topped with a clean layer of soil.

4. Arsenic > 500 mg/kg

This sediment must be disposed of in a specially prepared hazardous waste facility. The pH must be neutralised with lime, ash, or calcium hydroxide. The site must be sealed with two layers of plastic sheeting and covered with clean soil to prevent contamination.