CHIANG SAEN, Thailand -- Perched on the bow of his long-tail fishing boat, 75-year-old Sukjai Yana untangled a handful of small fish from his net, disappointed by his catch and fretting over whether he can sell them.

Some days Yana earns nothing: demand for fish is falling due to worries over contamination of the Mekong River and its tributaries by toxic runoff from rare earth mines upstream that is threatening millions who rely on those waters for farms and fisheries.

Chiang Saen, a fishing hub in northern Thailand, has been Yana's family's home for decades. “I don’t know where else I’d go,” he said.

Yana is one of 70 million people in mainland Southeast Asia who depend on the nearly 5,000-kilometer (3,100-mile) Mekong River. Rising demand for rare earth materials is driving an unregulated mining boom centered in war-torn Myanmar, to the west, that is spreading to Laos, in the east.

The Mekong has long faced mounting pressures, from plastic pollution to hydropower dams hemming it upstream and sand mining devouring its banks. But experts warn that the toxic runoff from the mines could pose an existential threat.

Exposure to heavy metals such as arsenic, mercury, lead and cadmium raises risks of cancer, organ failure and developmental harm, especially for children and pregnant women.

Thailand is bearing the brunt of the mining boom as such toxins imperil its global food exports — from bags of rice in U.S. supermarkets to edamame snacks served in Japan and garlic used in Malaysian kitchens. Responses remain local and limited, while smuggling and Myanmar’s civil war complicate regional fixes, raising concerns for downstream Cambodia and Vietnam.

Agriculture is the backbone of Southeast Asia’s economies, said Suebsakun Kidnukorn of Mae Fah Luang University in northern Thailand's Chiang Rai, warning that rare earth mines are destroying “the world’s kitchen.”

While cutting banana bunches on a farm in the hilly Thai village of Tha Ton, 63-year-old Lah Boonruang taps his fingers to count the toxin-exposed crops he harvests — rice, garlic, corn, onion, mangoes and bananas.

He irrigates his fields with water from the Kok River, a Mekong tributary that flows into Thailand from Myanmar and is laden with toxins.

“Everyone is afraid of the toxins," he said. “If we can't export, a farmer is the first to die.”

Thailand is one of the world's top rice exporters along with India and Vietnam. It exported over $10 billion worth of rice and fruits in 2024, according to trade figures that rank the U.S. as the top rice importer.

“Our worry is that toxins accumulate in the rice we export. This would make our rice farming industry, which is our culture, collapse,” said Niwat Roykaew, founder of the environmental institute The Mekong School in northern Thailand's Chiang Khong.

Thai scientists have found elevated heavy metal pollution in other Mekong tributaries, like the Sai and Ruak rivers.

The Mekong starts in China and flows through five Southeast Asian nations before emptying into the sea. Millions rely on fish from the Mekong Basin for protein.

Warnings to ethnic minorities in the hills of northern Thailand to avoid using river water are painful for the Lahu, who are famed as fisher people, said Sela Lipo, 56, a Lahu elder.

“The Lahu’s way of life is always with a river," he said. "The contaminated river has cut off our lifeline.”

Thailand's government says it has little leverage against mining operations across the border in strife-torn Myanmar and Laos. The Thai response has also been constrained by limited expertise, information and funds, said Aweera Pakkamart of Thailand's Pollution Control Department.

Instead, public universities, local governments and regional organizations like the Mekong River Commission, have mainly focused on monitoring levels of heavy metals and educating communities about risks.

Recent water, fish and sediment samples from Mekong tributaries had high levels of dangerous heavy metals, such as arsenic, mercury, lead and cadmium, from rare earth mining, said Warakorn Maneechuket, a researcher at Thailand’s Naresuan University.

In a lab, she uses a scalpel to point out tell-tale signs of contamination — tumor-like growths, discolored scales, and unusual eye coloration — before dissecting a catfish caught from the Kok River.

The accumulation of heavy metals is insidious. Arsenic can cause organ failure. Mercury damages the nervous system. Lead impairs cognition and cadmium harms the kidneys.

To raise awareness of health risks, Tanapon Phenrat of Naresuan University helped develop a smartphone fish safety app, training fishers in Chiang Saen to use it to identify and upload images of suspicious fish. Building a citizen-science database for northern Thailand can help quantify the scale and spread of contamination, he said.

“Each and every sample is very important,” he said.

The ubiquity of rare earth elements means demand keeps rising.

Rare earths are vital to modern technology, from smartphones and electric vehicles to missiles and jets. Despite the name, they are common. It is the costly mining and complex refining process, concentrated in China, that makes them scarce.

The U.S.-based Stimson Center has used satellite photo analysis to identify nearly 800 suspected unregulated rare earth and other mining sites along Mekong tributaries in Laos, Myanmar and Cambodia.

Many in Myanmar are in areas of active fighting. The war has driven a "diversification of mines” geographically, according to Regan Kwan of The Stimson Center, who has tracked expansion of mining to 26 sites along rivers in Laos.

Rare earths are mined by digging up rock or washing chemicals through soil to extract the minerals, creating toxic waste. The physical footprint of this process is recognizable in satellite data, Kwan said.

Myanmar is China’s leading supplier of heavy rare earths, exporting more than $4.2 billion worth of such materials to China between 2017 and 2024, mostly after a miliary takeover in 2021.

U.S. President Donald Trump made securing America's supply of critical minerals and rare earths a key foreign policy objective. Used in fighter jets like the F-35, submarines, Tomahawk missiles, radar systems and smart bombs, according to the U.S. government, the need for more supplies is growing as the U.S. replenishes and expands military stockpiles drawn down by the wars in Iran and Ukraine.

This is bad news for the river that sustains mainland Southeast Asia.

Conflicts in last century — which include the Vietnam War and the Khmer Rouge genocide — were the most devastating for the Mekong region, but toxic runoff ranks a close second, said Brian Eyler of the Stimson Center, who called it an "atomic bomb” for river basin.

It's far more damaging than other threats like large dams and "it is not stopping.”

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Ghosal reported from Hanoi, Vietnam. Freelance reporter Ladawan Sondak contributed to this report.

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