When Phra Maha Nikhom was a boy, he swam and splashed in the crystalline Kok River. He caught crabs and fish. Today, he is the abbot of a Buddhist temple near Thailand’s border with Myanmar, and monks, with their copious saffron robes, don’t tend toward water play. But even if he wanted to, this tributary of the Mekong, Southeast Asia’s life force, is now a forbidden zone.

Unregulated rare earth mining in Myanmar, directed by Chinese enterprises, is poisoning the Kok and at least three other rivers that flow through Thailand. For months, levels of arsenic and other toxic metals have spiked to dangerous levels in Thai waterways, including the Mekong, government data shows.

The Kok, which normally flows limpid at this time of year, now runs brown, sullied with sediment believed to have been churned up by the mining in Myanmar. People who enter the river complain of skin ailments. The threat of longer-term health problems associated with toxins from rare earth extraction is sobering, including lung, bladder and kidney cancers.

“Seeing the river like this is like seeing your mother in the I.C.U., but you cannot help, and you are separated from her by a glass window,” Phra Maha Nikhom said.

The residents of Thaton, the riverside village where he grew up, were officially informed of the contaminants only in April, months after government data showed unsafe levels. More troublingly, local authorities, who have warned residents to stay away from the water, have not checked for other pollutants, including radioactive ones, that are often released through rare earth processing.

“This is the biggest concern of the people,” said Dr. Suebsakun Kidnukorn of Mae Fah Luang University in the city of Chiang Rai, who earlier this year tested river water only to be chastised by local officials for operating independently. “The government says, ‘Don’t worry, we are taking care,’ but they haven’t tested for what is necessary.”

The toxic legacy of this rare earth industry illustrates the complex trade-offs made to power carbon-free energy. The so-called heavy rare earth elements mined in Myanmar are used in electric vehicles, wind turbines and nuclear power plants. They are not actually that rare but are usually dispersed in other ores, and extracting them can be extraordinarily destructive to the environment. To procure one ton of heavy rare earths, such as terbium or dysprosium, tens of thousands of tons of earth must be moved and treated, often producing a mountain of contaminated waste.

In this remote corner of Southeast Asia, the mining boom is a byproduct of a fierce civil war in Myanmar, where the ruling military’s decades of repression have catalyzed ethnic armed groups and pro-democracy forces to fight back. To fill their war chests, the military and some of the militias rely on a panoply of illegal businesses, from synthetic drugs and opium to wildlife poaching to online scams. Many of these enterprises are directed by Chinese crime syndicates, which have expanded into Southeast Asia after crackdowns at home.

The latest money-spinner is mining. Since the military staged a coup in 2021, the number of rare earth mines in just one state in Myanmar has nearly tripled, to about 370 at the end of 2024, the Institute for Strategy and Policy-Myanmar found, with a combined export value of more than $4 billion.

Myanmar has become a major miner of certain rare earths. Extracted by Chinese companies and their subsidiaries, the minerals are transported from Myanmar directly to neighboring China, which has a stranglehold on the refining of rare earths for the global market — a point of concern in current trade talks between the United States and China.

This extraction occurs in remote reaches of Myanmar, where there are no environmental protections, no worker safety laws, no vows of corporate social responsibility. Since the coup, thousands of acres of forest have been razed in northern Myanmar’s Kachin and Shan states in the quest for rare earths. Hillsides have been reduced to rubble, with toxic byproducts of the mining seeping into the landscape before entering rivers and groundwater. Further refining processes can also release harmful radioactive material and hazardous dust into the environment.

More than a million people living in these riverine ecosystems in Thailand are already affected by the pollution, researchers say. While Thailand is yet to comprehensively test the waters, scientists fear that these toxins could also flow downstream to the cities of mainland Southeast Asia. (The toll in Myanmar is devastating, witnesses say, but largely unrecorded because of the civil war.)

The China Connection

The Kok River enters northern Thailand through a cleft in jungle-clad hills. On the other side, in Myanmar, is territory controlled by the Wa, an ethnic minority that has for decades maintained a secretive and autonomous narco-state supported by China. The Wa, who use Chinese currency rather than Myanmar’s own, boast one of the country’s biggest ethnic militias.

In recent months, Wa representatives have told Thai officials that they have ventured into mining as a new revenue source, meaning they no longer depend on the drug trade. Clusters of new mines have appeared in Wa territory over the past two years, satellite mapping analyzed by the Shan Human Rights Foundation found.

“We saw one and then another and then more,” said Ying Hom, a lawyer and representative of the Shan rights group. “It became hard to count, there were many, many.”

Inside one mine complex near the Thai border, camps are divided into Chinese and Myanmar settlements, according to four people who have observed the enterprise and spoke on the condition of anonymity for fear of retaliation by the Wa, whose soldiers guard the area. Notably, there is no Chinese corporate signage at the mines, they said. But trucks laden with extracted rare earths head north to the Chinese border, they said, passing vehicles coming down from China that are filled with the acid and other chemicals needed to leach the metals on-site.

The witnesses said that the rare earths in Wa territory are mined with technical expertise from employees working for a subsidiary of China Rare Earth Group, a state-owned behemoth. In neighboring Kachin State, where rare earths mining accelerated after the 2021 coup, an arm of the company oversees mines that have been operating without any regulatory oversight, according to the environmental watchdog Global Witness.

Last month, a spokesman at the Chinese Embassy in Thailand said in a statement that “China is highly concerned about the heavy metal pollution incident in the tributaries of the Mekong River in Thailand,” without providing specifics.

The spokesman added that “China has always required overseas Chinese companies to abide by the laws of the host countries and operate legally and in compliance with regulations.”

Wa territory is ungoverned by conventional rule of law. A spokesman for the United Wa State Army did not respond to requests for comment, nor did a representative of China Rare Earth Group.

A Poisoned River

Along the Kok River, fishermen showed blistered rashes that developed after they had waded in the water. While more studies are needed to establish definitive links to the toxic runoff from the rare earth mines in Myanmar, locals said that they had never suffered such skin afflictions before.

Chuthipon Saengtongsi, a 13-year-old studying to be a monk, said he went swimming in the Kok a year ago, before he took his monastic vows. Blisters ravaged his body, and his skin is still itchy.

“If I use a cream, it gets better for a while, but then it comes back again,” he said, scratching his legs.

Downstream, near the city of Chiang Rai, Moonpan, a 34-year-old elephant, blew moisture from his trunk. Normally, he likes to cool off in the Kok River, said his keeper of 10 years, Banleu Dune. But in late March, a few days after a soak in the river, Moonpan’s hide broke out in a constellation of white blisters. Mr. Banleu suffered a rash, too, he said, but his appeared earlier.

“My skin isn’t as thick as an elephant’s,” he said.

In Thaton, Gob Kotkham, a fisherman, picked at his inflamed skin. Three days before, he had to catch a fish for the pollution control department to test. But fish these days, he said, are not plentiful. He spent a long time in the Kok River before he finally speared a specimen. A few days later, he harvested water spinach from the river bank for a meal.

The local government has proposed building a dam to trap the sediment, but scientists are skeptical. The Mekong River Commission, a transnational body, has warned of contamination in the Kok and Mekong and said it would conduct a field visit in late July.

“We all know where the pollution is coming from,” Mr. Gob said, gesturing toward the border with Myanmar. “How can we stop it when it’s in another country in a war?”

Muktita Suhartono contributed reporting from the Kok River and Keith Bradsher from Beijing.