Chiang mai: Thailand's Pollution Control Department (PCD) reported on Monday that arsenic levels have exceeded safety standards for the 19th consecutive time across multiple northern border rivers, signaling persistent heavy metal contamination.

According to Thai News Agency, the May 19-22 assessment of 22 surface water sites along the Kok, Sai, Ruak, and Mekong rivers in Chiang Mai and Chiang Rai provinces showed most heavy metals within safe limits. However, arsenic levels surpassed the regulatory threshold of 0.010 milligrams per liter in several areas.

The Sai River in Chiang Rai's Mae Sai district remains the main area of concern, with arsenic and lead exceeding limits at all three testing sites. Arsenic peaked at 0.041 milligrams per liter at Ban Hua Fai, while manganese also surpassed safety standards at one site.

Elevated arsenic levels were also recorded at four locations along the Kok River and two points on the Ruak River. In contrast, the Mekong River meets all water quality standards.

The PCD stated that the findings align with previous monitoring trends of ongoing contamination in northern border waterways, adding that it will continue monitoring to protect local ecosystems and public health.

While independent environmental monitors attribute the contamination to toxic runoff from unregulated upstream mining operations, regional authorities state that a direct transboundary link has not been officially confirmed.