On April 21, 2026, the Office of Environment and Pollution Control Region 1 (Chiang Mai) reported the results of its third round of water quality monitoring in the Salween River. Water samples collected between March 24–27, 2026 found arsenic (As) levels exceeding the standard at all monitoring sites, from Ban Tha Ta Fang in Mae Yuam subdistrict, Mae Sariang district, to Ban Sop Moei in Sop Moei district. Recorded levels ranged from 0.049 to 0.060 mg/L, compared with the standard of 0.01 mg/L.
The third round of testing showed higher arsenic concentrations in surface water than the previous two rounds. In the first round, which sampled from Pha Tang along the Thai–Myanmar border in Mae Kong subdistrict, Mae Sariang district, Mae Hong Son province, to Ban Sop Moei, arsenic levels ranged from 0.023 to 0.029 mg/L. In the second round, levels exceeding surface water quality standards were recorded from Ban Tha Ta Fang, Mae Yuam subdistrict, Mae Sariang district, to Ban Sop Moei Village 4 in Mae Sam Laep subdistrict, Sop Moei district, with arsenic concentrations ranging from 0.034 to 0.038 mg/L.