Chiang Rai finds heavy metals above limits in five crop samples as river pollution threatens farms

Chillies and aubergines fail safety checks in three districts

Five of 185 crop samples collected in Chiang Rai between October 2025 and September 2026 contained heavy metals above permitted limits, adding a food safety concern to the contamination affecting the Kok, Sai and Ruak rivers. Cadmium was the main problem in chillies and long aubergines from Wiang Chai and Mae Chan districts. Lead exceeded the limit in chillies from Mueang Chiang Rai district.

The five samples represent about 2.7% of the crops tested under the monitoring programme, which focused on plants exposed to water sources in the three river basins. The findings give authorities specific crops and locations to investigate as farmers prepare for another planting season.

Tests of 20 freshwater fish flesh samples also detected heavy metals, although all remained within the limits under Ministry of Public Health Notification No. 414 of 2020. Soil results raised a broader concern: of 52 samples collected in February 2026, 20 were classified as at risk, 22 required surveillance and 10 were within standards.

Chiang Rai authorities plan to collect soil samples again between December 2026 and January 2027. Local agencies will also use MSense equipment to monitor metals in water and use the findings to guide measures protecting public health and agricultural production.

The immediate pressure extends beyond testing. Mae Sai temporarily stopped producing tap water after arsenic exceeded the river water standard on November 5, 2026. Farmers using the same river system need decisions about irrigation before rice planting begins in December, while communities are asking for faster testing and alternative supplies.

Why crop results from different surveys vary

An earlier agricultural monitoring programme tested 119 produce samples, including 77 from Chiang Rai and 42 from Chiang Mai. In the Chiang Rai group, arsenic was detected in 21 samples, cadmium in 33 and lead in 25, with all reported concentrations within the applicable limits. The later monitoring total of 185 samples included five exceedances.

Duangsamorn Prueththikun, director of the Plant Protection and Soil and Fertiliser Management Promotion Division at the Department of Agricultural Extension, described testing across seven Chiang Rai districts. Crops included rice, chillies, tomatoes, cucumbers, bamboo shoots, aubergines and leafy vegetables. Samples were sent to the Department of Medical Sciences for analysis.

Mae Ai district in Chiang Mai presents another concern. In an open letter dated November 13, 2026, Mae Fah Luang University academic Dr. Suebsakul Kitnukorn said sampling conducted since December 2025 had found arsenic, lead or cadmium above food safety limits across 24 types of vegetables and fruit from Tha Ton and Mae Ai subdistricts.

Suebsakul said produce from those areas supplies local markets and export destinations including Malaysia, Singapore and Japan. The earlier 42 samples from Mae Ai had recorded metals within standards. The programmes covered different sampling groups and periods, making dates, locations, crop types and measured concentrations essential information for farmers and buyers.

Soil monitoring identifies land needing closer attention

The February survey placed 20 of 52 soil samples in the risk category, about 38.5%, and another 42.3% under surveillance. Together, those groups accounted for 42 samples, or about four fifths of the total. Repeat testing is intended to help authorities decide what preventive or corrective action is needed.

A separate preliminary soil assessment released on October 10 put about 20% in the red category, 50% in yellow and 30% in green. Parinyanon Thongkham, director of the Chiang Rai Land Development Station, said that survey collected more than 200 soil and water samples with funding from the Chiang Rai Irrigation Office.

Under the Department of Land Development classification described for that assessment, soil with less than 14 milligrams of arsenic per kilogram is green. Concentrations from 14 to 25 milligrams per kilogram are yellow and require monitoring. Levels above 25 milligrams per kilogram are red and exceed the standard.

Survey teams sampled soil 15 to 20 centimetres below the surface, around the depth where roots take up nutrients, at locations between 100 metres and two kilometres from riverbanks. The planned schedule called for red areas to be checked every two to three months and yellow areas every six months.

Parinyanon said limited budgets and unclear rules for stopping irrigation intake when water carries heavy sediment complicated the response. His station was working with agricultural extension officials to advise farmers in areas of greater risk about alternatives to food crops, including crops grown for energy.

Rice findings create urgency before December planting

Suebsakul's November letter identified 58,723 rai of rice fields in the Kok basin and approximately 60,500 rai in the Sai and Ruak basins. Together, that is 119,223 rai, or about 19,076 hectares, relying on irrigation systems serving the affected river areas.

The letter described contamination above standards in testing for the 2025/2026 rice season. It cited arsenic in grain from seven of 20 Kok basin sampling points and 16 of 20 Sai and Ruak points. Those figures correspond to 35% and 80% of the respective sampling points. Lead was also reported in grain at one point in each area, while arsenic was reported in soil at 11 of 20 Sai and Ruak locations.

Kallaya Boonsanga, director of the Chiang Rai Rice Research Centre, gave a different assessment during the November 15 response. She said rice produced in Chiang Rai remained within consumption standards, citing analyses by a medical sciences centre that had found no arsenic exceedances. The relationship between those tested batches and the locations listed in Suebsakul's letter was not specified.

Kallaya called for water entering fields to be checked before the next planting cycle, alongside continued soil and water sampling and advice to farmers about reducing contamination risks. For growers preparing to plant in December, the practical decision is whether their irrigation source and land are suitable for another crop.

Fish tests and household water checks cover different risks

The 20 fish flesh samples in the Chiang Rai update sit alongside a broader fisheries programme covering more than 400 fish samples. Amphon Sakses, Chiang Mai provincial fisheries officer, said arsenic and mercury were detected on some occasions in Kok River fish, at low concentrations within consumption standards. Testing also covered lead and cadmium.

From April to September 2025, fisheries teams sampled twice a month. Surveys at six Kok River points recorded 12 species and 49 specimens. Two native catfish had abnormal nodules; subsequent checks at 22 sites on the Kok, Sai and Mekong rivers found no further abnormalities.

The Department of Health has advised residents in the Kok and Mekong basins to avoid fish internal organs and aquatic animals living near the riverbed, including shrimp and shellfish, and to avoid eating the same fish species repeatedly over long periods. The advice addresses the possibility of metals accumulating through repeated consumption.

Household water monitoring produced more reassuring results during 2025. Nonthaphat Theerawatthanasiri, a senior public health technical officer at Health Centre 1 in Chiang Mai, described six rounds of testing covering 51 drinking and domestic water samples between April and September. No tested parameter exceeded its applicable standard. Five rounds of urine screening among residents at risk also found arsenic within the reference level, and health surveillance had identified no illness attributed to accumulated arsenic or lead exposure.

A tap water stoppage brings the problem into homes

Mae Sai's November 5 production stoppage followed very cloudy river water and an arsenic reading above the river standard. On November 11, representatives from Sai Lom Joy and Tham Pha Chom communities joined irrigation users in submitting nine demands to Prime Minister Anutin Charnvirakul through Mae Sai district chief Warayut Khomboon.

Warayut said eight water trucks were ready if another interruption lasted more than 24 hours. Backup sources included Ko Chang, Huai Khrai, Mae Chan and groundwater supplies, with additional help to be requested from military, disaster response and local charitable organisations if necessary.

Residents requested mobile water treatment equipment, faster development of the Mae Kham reservoir in Mae Chan, groundwater for 60,500 rai of farmland and a Chiang Rai centre able to test soil, aquatic animals and crops. They also sought joint Thai and Myanmar water monitoring with public participation, restrictions on specified mineral imports from Myanmar and permanent flood barriers without relocating existing communities.

During his November 15 visit, Deputy Prime Minister and Minister of Higher Education, Science, Research and Innovation Yosschanan Wongsawat drank water supplied by the Provincial Waterworks Authority's Mae Sai branch and said the water distributed from its service vehicle was safe. Continued checks of river intake water and treated supplies remain central to the provincial response.

November response and the next deadlines

Chiang Rai Governor Chucheep Phongchai joined a November 10 online meeting chaired by Nirat Phongsitthithawon, director general of the Department of Disaster Prevention and Mitigation. Monitoring discussed there found arsenic above standards at some points on the Kok and Ruak rivers, and both arsenic and lead above standards in the Sai.

Nirat assigned provincial disaster officials to coordinate pollution control, improvements to wastewater treatment, monitoring of aquatic animals and checks of raw water used for tap water production. The key dates now connect emergency supplies with the next agricultural season:

• November 5, 2026: Mae Sai temporarily stopped tap water production after an arsenic exceedance in river water.
• November 10: Provincial and national agencies met to coordinate protective measures.
• November 11: Community and farmer representatives submitted their demands through the district chief.
• November 13: Suebsakul dated his open letter calling for a national response.
• November 15: Yosschanan visited Mae Sai and reviewed water testing and sediment proposals.
• November 16: Senior officials were scheduled to inspect Ko Chang's water production station and hear public proposals.
• December 2026 to January 2027: Authorities plan further soil sampling, as farmers enter the next rice planting period.

Fast water screening and a proposal to trap sediment

Yosschanan's visit included a demonstration of M-Sense at the first Mae Sai Friendship Bridge. Developed by the National Nanotechnology Center under the National Science and Technology Development Agency, the equipment was described as detecting arsenic, lead, mercury and cadmium in water within five minutes.

Suebsakul said confirmation from central laboratories can take one to two months, sometimes extending beyond harvest. Rapid water screening and proposed provincial laboratories address different parts of that delay: field equipment can provide quick water readings, while local laboratories could shorten the wait for analyses of soil, crops and aquatic animals.

Yosschanan also proposed studying small weirs to slow water and allow sediment carrying arsenic to settle before removal. He said an earlier study could be revisited. Construction on an international river would require further technical detail before a formal proposal.

Assistant Professor Sathian Chanta of Chiang Rai Rajabhat University and Suebsakul have also called for scrutiny of proposals involving the Pollution Control Department and Siam Cement Group to incorporate contaminated sediment into construction materials. Their concerns centre on testing, intended uses and environmental safety when handling material removed from rivers or water treatment systems.

Farmers seek maps, compensation and action upstream

Sathian has urged authorities to publish detailed contamination maps and give farmers clear instructions for each category of land. He said green areas could continue normal cultivation, yellow areas needed guidance on suitable crops, and red areas required treatment and a halt to food crop production. He also called for consideration of compensation when restrictions remove farmers' usual income.

Suebsakul's letter described farmers facing lower purchase prices and some restrictions on growing export crops such as sweetcorn and okra. His proposals included a national committee involving farmers and consumers, a plan lasting at least five years, a shared public database, alternative irrigation supplies and testing centres in Chiang Rai and Chiang Mai.

Upstream mining in Myanmar has been identified as a concern in the river pollution crisis. Yosschanan said laboratory work would support comparisons between contaminants upstream and downstream and provide evidence for international discussions. Chucheep said Chiang Rai had written to Tachileik's governor, Tin Wai Tong, and was awaiting a response.

Farmers' immediate needs connect those diplomatic efforts to daily decisions: an identified source of usable water, results tied to their own fields and crops, and financial support if planting must change. Public maps linked to dated laboratory results would give residents and buyers a clearer basis for those decisions.

Key Points

• Five of 185 Chiang Rai crop samples exceeded heavy metal limits, mainly involving cadmium in chillies and aubergines and lead in chillies.
• All 20 freshwater fish flesh samples in the provincial update remained within food standards.
• Of 52 soil samples collected in February 2026, 20 were classified as at risk and 22 required surveillance.
• An academic's letter cited rice contamination above standards at specified sites, while the Chiang Rai Rice Research Centre said its referenced tests found rice within consumption limits.
• Mae Sai prepared eight water trucks for interruptions lasting more than 24 hours after a temporary production stoppage on November 5.
• Further soil testing is planned for December 2026 to January 2027, alongside demands for alternative irrigation water, public contamination maps and faster local testing.