A World Health Organization agency declared on Thursday that aspartame, an artificial sweetener widely used in diet drinks and low-sugar foods, could possibly cause cancer.

A second W.H.O. committee, though, held steady on its assessment of a safe level of aspartame consumption. By some calculations using the panel’s standard, a person weighing 150 pounds could avoid a risk of cancer but still drink about a dozen cans of diet soda a day.

The declaration by a W.H.O. agency of a cancer risk associated with aspartame reflects the first time the prominent international body has weighed in publicly on the effects of the nearly ubiquitous artificial sweetener. Aspartame has been a contentious ingredient for decades.

The International Agency for Research on Cancer, or I.A.R.C., said it based its conclusion that aspartame was a possible carcinogen on limited evidence from three observational studies of humans that the agency said linked consumption of artificially sweetened beverages to an increase in cases of liver cancer — at levels far below a dozen cans a day. It cautioned that the results could potentially be skewed toward the profile of people who drink higher amounts of diet drinks and called for further study.

Still, people who consume high amounts of aspartame should consider switching to water or other unsweetened drinks, said Dr. Francesco Branca, director of the W.H.O. Department of Nutrition and Food Safety.

But, he added: “Our results do not indicate that occasional consumption should pose a risk to most.”

Concerns about rising global rates of obesity and diabetes as well as changing consumer preferences have resulted in an explosion of no- and low-sugar food and beverages. Aspartame, one of six sweeteners approved by U.S. regulators, is found in thousands of products, from packets of Equal to sugar-free gum, diet sodas, teas, energy drinks and even yogurts. It is also used to sweeten various pharmaceutical products.

The U.S. Food and Drug Administration, which approved aspartame decades ago, on Thursday issued an unusual criticism of the global agency’s findings and reiterated its longstanding position that the sweetener is safe. In a statement, the F.D.A. said it “disagrees with I.A.R.C.’s conclusion that these studies support classifying aspartame as a possible carcinogen to humans.”

The F.D.A. also said that “aspartame being labeled by the W.H.O. as ‘possibly carcinogenic to humans’ does not mean that aspartame is actually linked to cancer.” The F.D.A. declined to make any of its experts available for interviews to discuss the agency’s specific concerns.

But its salvo against the international organization was sure to ignite further debate in Europe — where the sweetener is still deemed safe — and renew review in the United States. And the dueling global agencies’ pronouncements are likely to fuel confusion among consumers.

The W.HO. has occasionally been out of step with other authorities on potential cancer risks, like glyphosate, and later led the way toward establishing that it was dangerous to human health. The international body’s designation of a cancer link to that ingredient in Roundup, a weed killer, became the stepping stone for lawsuits against the makers of the herbicide.

Around the world, the powerful beverage industry has fought long and hard against any regulatory or scientific finding that tied artificial sweetener use to risks of cancer or other health problems. Aspartame is only the latest battleground for multinational companies to push back against new studies or potential links to health risks.

“Aspartame is safe,” Kevin Keane, interim president of the American Beverage Association, said in a statement. He cited the dueling W.H.O. announcements, singling out the second panel, the Joint Expert Committee on Food Additives, that performed a concurrent review and left its recommended daily intake amount unchanged. It also deemed the evidence for cancer in humans “not convincing,” a W.H.O. summary shows.

“After a rigorous review, the World Health Organization finds aspartame is safe and ‘no sufficient reason to change the previously established acceptable daily intake,’” Mr. Keane said. “This strong conclusion reinforces the position of the F.D.A. and food safety agencies from more than 90 countries.”

Coca-Cola referred questions to the American Beverage Association and PepsiCo did not respond to requests for comment.

The safety of sugar replacements, including the decades-old science dispute over the use of saccharin in the diet drink Tab, has been heavily scrutinized. Once linked to bladder cancer in rats, Congress mandated further study of saccharin. Since then, according to the F.D.A., 30 studies showed the rodent results did not apply to humans; U.S. officials removed saccharin from a list of potential carcinogens. More recently, other sweeteners have come under scrutiny for their ties to possible health risks.

At the center of the dispute over aspartame are rodent studies from 2005-2010 by Italy-based researchers that showed a link to cancer. The F.D.A. has dismissed the long-debated studies as “compromised.”

Dr. William Dahut, chief scientific officer of the American Cancer Society, which led one of the key studies the W.H.O. relied on, said the findings should be considered alongside the W.H.O.’s report earlier this year that indicated artificial sweeteners offered no help in achieving weight loss or protection from other chronic conditions.

He said there was little evidence now to suggest a daily Diet Coke would elevate the risk of cancer, adding that “more research is needed.” Overall, he said, the science was more definitive on reducing cancer risk by avoiding tobacco, alcohol, processed meat and excess body weight.

The I.A.R.C. said it could not rule out the possibility that the studies linking aspartame to liver cancer were a result of chance or other factors associated with drinking diet soda.

The W.H.O.’s cancer agency has four categories: carcinogenic, probably carcinogenic, possibly carcinogenic and no classification. Those levels reflect the strength of the science rather than how likely the substance is to cause cancer.

The other W.H.O. group on food additives recommended that daily consumption should be below 40 milligrams of aspartame per kilogram of a person’s weight — slightly lower than the suggested U.S. level of 50 milligrams.

The F.D.A. said it estimated that a person weighing 132 pounds would need to consume 75 packets of aspartame sweetener to reach the threshold of exposure to a potential risk.

For its review of aspartame, the I.A.R.C. convened 25 cancer experts from 12 nations in Lyon, France, to conduct the review of existing studies. It concluded that there was limited evidence for cancer in humans based on three studies linking artificially sweetened drinks to increases in hepatocellular carcinoma, the most common type of liver cancer.

One study in 2016 was led by W.H.O. officials, who looked at nearly 500,000 people in Europe who were followed for about 11 years. The study tracked participants’ juice and soft drink intake and the relationship to liver and bile duct cancers. It examined those who drank artificially sweetened soft drinks and found that each additional serving of diet soft drink a week was associated with a 6 percent increased risk of liver cancer.

A U.S. study published last year by researchers from Harvard, Boston University and the National Cancer Institute examined sweetened beverage consumption reported by people on questionnaires and cancer case registries. Researchers found an elevated risk of liver cancer in people with diabetes who said they consumed two or more artificially sweetened sodas a day. That study found no increase in liver cancer among diet soda drinkers who did not have diabetes.

A third study, led by the American Cancer Society, examined the use of beverages sweetened by sugar and artificial sweeteners and cancer death data. It found a 44 percent increase in liver cancer among men who never smoked and drank two or more artificially sweetened drinks a day. Even adjusting for high body mass — in itself a cancer risk factor — the men had a 22 percent increase in risk, data in a supplement to the study shows.

The American Beverage Association, which represents Coca-Cola and PepsiCo, has been vocal in saying that the W.H.O.’s food additive panel — not the cancer experts — should be the lead authority evaluating aspartame.

In recent weeks, the beverage industry trade group has financed a new coalition led by Alex Azar, an appointee of former President Donald J. Trump, and Donna Shalala, an appointee of former President Bill Clinton. Both Mr. Azar and Ms. Shalala were former secretaries of the Department of Health and Human Services. In an opinion article in Newsweek earlier this month, the two embraced the F.D.A.’s position on the safety of aspartame, and called the agency “the world’s gold standard for independent regulatory bodies.”

The trade group had previously contested another review of aspartame’s potential links to cancer in California. In 2016, a state committee discussed reviewing aspartame, but it went no further.

California officials said this week that the state could review the latest W.H.O. decision.

Besides aspartame, the W.H.O.’s cancer agency has deemed other possible carcinogens to range from the seemingly benign, like Ginkgo biloba extract and aloe vera leaf extract, to the more concerning, like gasoline exhaust and perfluorooctanoic acid, the most common of the industrial chemicals known as per- and polyfluoroalkyl substances, or PFAS, that has recently been subject to billion-dollar settlements over drinking water contamination.

In deeming aspartame a possible carcinogen, the I.A.R.C. also dipped into one of the central controversies of aspartame research. It concluded that there was some evidence for cancer in lab animals based on studies performed by the Ramazzini Institute in Italy, citing the group’s finding of increased tumors in aspartame studies from the mid-2000s. Based on concerns over the group’s methods and interpretations, though, the findings were deemed limited.

For its part, the Ramazzini Institute said in 2021 that its work on aspartame was validated and that its earlier findings were “savagely attacked by the chemical manufacturing and processed food industries and by their allies in regulatory agencies.”

Dr. Branca of the W.H.O. responded to questions about the need for an I.A.R.C. review during a news conference on Wednesday, saying that 10 million people die of cancer each year. “So there’s a societal concern that our organization needed to respond to,” he said.

He said the results demonstrated a clear need for further high-quality research.

“We’ve in a sense raised a flag here, indicating that we need to clarify much more in the situation,” Dr. Branca said. “It is not something which we can dismiss at this moment.”

Julie Creswell contributed reporting.

The Food and Drug Administration on Thursday approved a birth control pill to be sold without a prescription for the first time in the United States, a milestone that could significantly expand access to contraception.

The medication, called Opill, will become the most effective birth control method available over the counter — more effective at preventing pregnancy than condoms, spermicides and other nonprescription methods. Experts in reproductive health said its availability could be especially useful for young women, teenagers and those who have difficulty dealing with the time, costs or logistical hurdles involved in visiting a doctor to obtain a prescription.

The pill’s manufacturer, Perrigo Company, based in Dublin, said Opill would most likely become available from stores and online retailers in the United States in early 2024.

The company did not say how much the medication would cost — a key question that will help determine how many people will use the pill — but Frédérique Welgryn, Perrigo’s global vice president for women’s health, said in a statement that the company was committed to making the pill “accessible and affordable to women and people of all ages.” Ms. Welgryn has also said the company would have a consumer assistance program to provide the pill at no cost to some women.

“Today’s approval marks the first time a nonprescription daily oral contraceptive will be an available option for millions of people in the United States,” Dr. Patrizia Cavazzoni, director of the F.D.A.’s Center for Drug Evaluation and Research, said in a statement. “When used as directed, daily oral contraception is safe and is expected to be more effective than currently available nonprescription contraceptive methods in preventing unintended pregnancy.”

Since the Supreme Court overturned the national right to an abortion last year, the accessibility of contraception has become an increasingly urgent issue. But long before that, the move to make a nonprescription pill available for all ages had received widespread support from specialists in reproductive and adolescent health and groups like the American Medical Association, the American College of Obstetricians and Gynecologists and the American Academy of Family Physicians.

In a survey last year by the health care research organization KFF, more than three-quarters of women of reproductive age said they favored an over-the-counter pill, primarily because of convenience. Nearly 40 percent said they would be likely to use it. Those most likely to opt for the product included women already taking birth control pills, women without health insurance and Hispanic women, the survey found.

And strikingly, at a time of fierce divisions over abortion, many anti-abortion groups have declined to criticize over-the-counter birth control. Opposition appears to come primarily from some Catholic organizations and Students for Life Action.

In May, a panel of 17 independent scientific advisers to the F.D.A. — including obstetrician-gynecologists, adolescent medicine specialists, a breast cancer specialist and experts in consumer health behavior and health literacy — voted unanimously that the benefits of making a birth control pill available without a prescription vastly outweighed the risks.

The panel cited the long history of safety and efficacy of Opill, which was approved for prescription use 50 years ago. The over-the-counter pill will be identical to the prescription version, which is 93 percent effective at preventing pregnancy with typical use.

Several panelists said there was a pressing public health need for an over-the-counter option in a country where nearly half of all pregnancies are unintended.

“The evidence demonstrates that the benefits clearly exceed the risks,” said one advisory committee member, Kathryn Curtis, a health scientist with the Centers for Disease Control and Prevention’s division of reproductive health.

She added: “I think Opill has the potential to have a huge positive public health impact.”

For proponents of over-the-counter pills, the main issue is affordability.

“If available equitably — meaning that they are priced affordably and fully covered by insurance — over-the-counter birth control pills will be a game-changer for communities impacted by systemic health inequities,” said Dr. Daniel Grossman, director of Advancing New Standards in Reproductive Health, who has led research on over-the-counter contraception.

The Affordable Care Act requires heath insurance plans to pay for prescription contraception, but not over-the-counter methods. Some states have laws mandating coverage of over-the-counter birth control, but most states do not. The KFF survey found that 10 percent of women would not be able or willing to pay any out-of-pocket cost for contraception. About 40 percent would pay $10 or less per month, and about a third would pay $20 or less.

Under a recent executive order by President Biden, the federal government could soon take steps toward requiring insurers to cover over-the-counter birth control. And Senate Democrats have reintroduced legislation to require such coverage.

“We need to make it affordable and available,” Senator Patty Murray, a Democrat from Washington State and the lead sponsor of the bill, said in an interview in May. “Let’s provide women what they need and make sure it’s affordable so there’s equity, and women who are low-income, women who for whatever reason are struggling don’t have to be forced to not have any birth control simply because they can’t afford it today,” she added.

Opill is known as a “mini pill” because it contains only one hormone, progestin, in contrast to “combination” pills, which contain both progestin and estrogen. A company that makes a combination pill, Cadence Health, has also been in discussions with the F.D.A. about applying for over-the-counter status.

The F.D.A. analysts who evaluated the data Perrigo submitted in its application for a nonprescription Opill had raised concerns about whether women with medical conditions that should preclude them from taking birth control pills — primarily breast cancer and undiagnosed vaginal bleeding — would follow the warnings and avoid the product. The F.D.A. analysts also raised questions about whether younger adolescents and people with limited literacy could follow the directions.

But in a memo explaining the approval decision on Thursday, Karen Murry, deputy director of the F.D.A.’s office of nonprescription drugs, wrote, “For an individual consumer of the product, the risk is very low, and almost nonexistent if they read and follow the labeling.”

“Overall,” she continued, “the total public health impact of the potential harm related to incorrect use by people with progestin-sensitive cancer is likely outweighed by the probable larger public health impact of prevention of a large number of unintended pregnancies with all their attendant harms.”

Several advisory committee members said patients with breast cancer, the main medical condition that precludes taking hormonal contraception, typically have doctors who would advise them to avoid birth control pills. They also said that Opill might actually be safest for adolescents because they are very unlikely to have breast cancer. And because young people often start off with contraception they can buy over-the-counter, it is especially important for them to have easy access to a method more effective than condoms and other birth control products available in retail stores, the panelists said.

Perrigo reported that participants in a study took Opill on 92.5 percent of the days they were supposed to take it. Most participants who missed a pill reported that they had followed the label’s directions to take mitigating steps, such as abstaining from sex or using a condom, Dr. Stephanie Sober, the company’s U.S. medical liaison, said at the advisory committee hearing. She said that among 955 participants, only six became pregnant while using Opill.

Most people who said they had missed doses attributed that to running out of pills before they could get to one of the study’s resupply sites, results that, Dr. Sober said, “illustrate precisely the barriers to adherence that could be lessened” by making the pill available over the counter.

PABLO, Mont. — In the mountain streams of southern British Columbia and northern Montana, a rugged part of the world, fish with misshapen skulls and twisted spines have been caught over the years.

Many scientists attribute the malformed creatures and declines in certain fish populations to five enormous open-pit coal mines that interrupt this wild landscape of dense forest flush with grizzly bears and wolves.

For decades, these mines owned by Teck Resources, a multinational mining company based in Canada, have been the subject of environmental concerns because of chemicals like selenium, a mining waste product, that leach into mountain rivers flowing through Indigenous land and across the border into U.S. waterways.

Selenium is a naturally occurring chemical important in the environment as a trace element. But selenium pollution has long been recognized as an extremely hazardous byproduct of coal mining. In larger concentrations, the chemical accumulates in the eggs and reproductive organs of fish and birds, and can cause a variety of detrimental effects, including lowered reproduction, deformities and death. The risk to human health from eating contaminated fish is not well understood.

Teck has repeatedly disputed various state and federal regulatory standards over what should be considered as safe levels of selenium in waterways. And those limits differ for lakes and rivers and between countries, complicating oversight efforts.

The latest case involves Montana and Idaho, where environmentalists’ lawsuits are waging a campaign over levels set by Montana for Lake Koocanusa in 2020. Its state standard is being challenged as a debate rages over cross-border pollution of the waterways, part of the war among regulators, tribal nations and scientists against Teck over whether the levels pose a hazard to aquatic life.

In a letter in the journal Science in 2020, a group of scientists warned of the cross-boundary pollution from Canadian mines and criticized what they and others attributed to a lack of regulatory oversight. “Mine assessment and permitting do not require incorporation of transparent, independent and peer-reviewed science,” they wrote. In Canada, they said, “Teck’s Elk Valley permit allows contaminant discharges up to 65 times above scientifically established protective thresholds for fish.”

They urged the Canadian and U.S. governments to begin bilateral negotiations, through the International Joint Commission, although previous appeals had gone unheeded.

In March, President Biden and Prime Minister Justin Trudeau of Canada pledged to reach an agreement by this summer to reduce and mitigate the water pollution in the Elk-Kootenay watershed. U.S. and Canadian officials say they are engaged in talks to set up a bilateral process in the coming months.

On the Montana standard, Teck has challenged the state’s levels, which are more restrictive than U.S. standards. “Significant, ongoing monitoring confirms selenium concentrations in the Koocanusa Reservoir are safe, not increasing and have been stable since 2012 and do not pose a risk to aquatic or human health,” said Chris Stannell, a spokesman for Teck.

In its annual report for 2022, the company said it was continuing “to engage with U.S. regulators to work toward the establishment of appropriate science-based standards for the reservoir.” And the company has also pointed out that there are other lakes in Montana with high levels of naturally occurring selenium.

Scientists in Montana, however, do not accept Teck’s assessments or assertions of levels in the lake, which spans the international border. Selenium levels in the Kootenai River have not decreased, environmentalists said.

At the site where the Kootenai River empties into Lake Koocanusa in Canada, levels have steadily increased well above those metrics, said Erin Sexton, a senior scientist at the Yellow Bay Biological Station, operated by the University of Montana on Flathead Lake.. “It’s a hockey-stick graph, it just goes up and up,” she said.

Provincial government officials say there are “robust monitoring and assessment programs in place” that have not detected any effects.

The environmentalists’ lawsuit is seeking to preserve the more restrictive Montana standard, which has come under fire from Republican-led lawmakers and some state agency officials. In its company report, Teck questioned whether the lower limit was in force, presumably because of the internecine feud among state authorities.

The U.S. level for selenium in rivers is 1.5 micrograms per liter in lakes and 3.1 in flowing rivers. Montana’s standard for Lake Koocanusa, after six years of research, was set at 0.8. The level for protection of aquatic life in British Columbia is 2.

Tracing the waterway chemicals

When rain falls or snow melts, waste rock from the mines leaches selenium into waterways. The level of selenium in the Fording and Elk Rivers in British Columbia near the mines have at times reached levels many times higher than provincial standards. A population of genetically pure cutthroat trout in the Fording River was decimated, at least in part, by selenium levels. In 2021, Teck was fined $60 million, a record, under Canada’s Fisheries Act for release of selenium into the Fording River.

The Elk River flows 140 miles from its source until it enters Lake Koocanusa, created by the damming of the Kootenai River, which straddles the border. The lake becomes the Kootenai River again below the Libby Dam in Montana and Idaho, and it flows back north into Kootenai Lake in British Columbia. Eventually it empties into the Columbia River.

Unlike in an oil spill, the effects of high selenium levels do not result in large kills of fish that suddenly appear belly up in the water. Instead, selenium poisoning reduces fish numbers by causing mortality in the larval stage.

“It’s a really nasty contaminant because it causes deformities in reproductive organs,” Ms. Sexton said. “They call it an invisible contaminant because they fail to thrive. You don’t find eggs that don’t hatch.”

Indigenous land concerns

U.S. and tribal officials argue that the mining-related presence of chemical pollution violates the 1909 International Boundary Waters Treaty. Tribal leaders in the United States claim it may breach their treaty rights of 1855, which guarantee “taking fish at all usual and accustomed places.” They want the International Joint Commission, a U.S.-Canadian body that oversees trans-boundary disputes, to study the issue and prepare recommendations for cleanup.

But the government of British Columbia has continued to oppose efforts to remedy the situation, Montana officials say.

Part of the problem, Ms. Sexton said, is that the province of British Columbia allows industries, including mining companies, to largely monitor themselves and offer evidence produced by their own scientists.

Mr. Stannell pointed out that the company had spent $1.2 billion on wastewater treatment near the mines, and planned to spend $750 million more to improve water quality in the coming years.

A region rich in coal

Coal mining in this Canadian province began more than a century ago, though the methods changed in the 1980s, when underground mining was largely abandoned for open pits.

High-grade metallurgical coal is mined using a method called cross-valley fill, similar to the mountaintop-removal technique employed in West Virginia and some other states. Explosive charges blast away the top of the mountains, removing whole sections of a range, to expose rich deposits of coal. Giant shovels and massive 550-ton dump trucks mine the coal, which is transported by rail to Vancouver and then by ship to Asia, where it is essential for steel manufacturing.

Other mining-related pollutants, partly from the extensive use of explosives in the blasting, include cadmium, sulfates and nitrates, experts say.

Teck’s open-pit mines produce more than 21 million metric tons of coal a year. An analysis issued last year by the British Columbia Chamber of Commerce estimated that the company provided nearly 13,000 jobs in the province and $4.6 billion to its gross national product.

The company is seeking to expand one of its five Elk Valley mines, and it has applied to open two new ones.

Calvin Sandborn, the legal director of the Environmental Law Center at the University of Victoria and one of the authors of a 2021 report, accused the governments of British Columbia and Canada of deliberately failing to regulate Teck.

“If they had acted on the warnings of their scientists years ago, they would have dealt with this problem,” Mr. Sandborn said. “And they didn’t because it’s a corporation that’s too big to fail.”

According to the Elk Valley Water Quality Plan, British Columbia allows Teck to continue to operate its mines as long as it stabilizes selenium levels and eventually reduces them after 2030.

Scientists worry that the existing mines could pollute the rivers for centuries. And some do not believe the technology exists to remove enough selenium from flowing rivers or groundwater to reach safe levels. Ms. Sexton said Teck could do more to seal the contaminants in the waste rock.

Critics of government policies point out that when John Horgan stepped down from his post as premier of British Columbia in 2022, he became a member of the board of Elk Valley Resources, a spinoff of Teck Coal, created to manage the mining resources. According to BIV, a publication that covers business in British Columbia, board members are paid at least $235,000 annually.

The Canadian Ministry of the Environment and Climate Change Strategy did not respond directly to questions seeking a response to accusations that the government’s oversight of the mining giant was inadequate or lax.

“We are pleased that Canada and the United States have committed to working together to reduce and mitigate the impacts of water quality concerns,” said David Karn, a spokesman for the agency. “Protecting and enhancing water quality is a key priority, and through our regulatory activities, we continue to undertake and oversee projects to improve and protect water quality in the Elk River Valley and Koocanusa Reservoir.”

New policies cannot come soon enough for Indigenous people and conservationists in both countries.

The mining on the ancestral lands of the Kootenai people, (known as Ktunaxa in British Columbia), has become a longstanding issue. “Over a century of mountaintop-removal mining has laid waste to the traditional territory of the Ktunaxa Nation, contaminating the Kootenay River and fish that depend on it,” the six governments of the tribal nation said.

“Our native fishery is extremely important to us,” said Tom McDonald, chairman of the Confederated Salish and Kootenai Tribes at their headquarters in Montana and a fisheries biologist. “Water to us is almost considered holy water. It’s very sacred.”

“You catch a fish and its gill plate is missing or its jaw is all malformed, are you going to eat it?” Mr. McDonald said. “No, you are not going to. When you lose that ability to fish, it disconnects you from your culture. It takes a whole thing away from the people — their society, their sense of place, their community and their family. It’s an extreme taking.”

The Kootenai/Ktunaxa tribes have worked to protect water quality and fisheries in their territory. The Kootenai band in Bonners Ferry, Idaho, has a long-term program to restore burbot to the Kootenai River. The long, eel-like fish, known for its flaky white flesh, is important for subsistence fishing, and it almost became extinct before the tribe built a hatchery to rear fish for introduction back into the river. Now, selenium has been found in the fish there.

Whitefish populations below Libby Dam, which created Lake Koocanusa, have declined considerably in recent years. Monitoring in 2018 found that populations, which are usually 700 fish per 1,000 feet, were down 53 percent in 2018 and 55 percent in 2023. High levels of selenium, above state and U.S. limits, have been found in fish eggs and ovaries.

The selenium from the mines “is likely causing the decline,” said Jim Dunnigan, a fisheries biologist for the Montana Department of Fish, Wildlife and Parks who is studying the contamination. “It’s cause for serious concern.”

Wyatt Petryshen of Wildsight, the Canadian environmental group that monitors Teck’s operations, said environmentalists were worried about recent moves by Teck that split its operations into Teck Metals Corp. and Elk Valley Resources, which will own the coal-mining operation.

“There are very real concerns that Teck is trying to spin off the company to avoid paying for environmental damages while maintaining cash flow to their metal mining business,” Mr. Petryshen said.

Sheila Murray, chairwoman of Teck’s board, defended the change, saying it would be more profitable for shareholders and would “support a sustainable future for the benefit of employees, local communities and Indigenous peoples.”

U.S. officials and advocates said the International Joint Commission, the bilateral body, would be the best authority to seek ways to contain and reduce the mining pollutants. “We need a scientific advisory panel of both U.S. and Canadian scientists,” Mr. Sandborn said. “We need to get this to the International Joint Commission so that we have a proper watchdog.”

Jennifer Savage, a spokeswoman for the U.S. State Department, which oversees the United States’ role in the commission, said it wanted the international body to take the matter up soon.

“Indigenous communities along the watershed depend on these waters for cultural survival and for their survival,” said Ms. Savage, director of the department’s office of Canadian affairs. “We’re impatient. We are definitely eager to find a solution.”