Researchers make alarming discovery after expedition through the Amazon River: 'Exposure at high levels can harm the brain, heart, kidneys, lungs ...'

The Madeira River is the largest tributary flowing into the Amazon River and is home to many people living on the banks. These residents drink and eat fish from the river, so its water quality directly impacts them, as well as the many people living downstream. Sadly, a recent expedition discovered high levels of mercury in the water, boding ill for future generations, Mongabay reports.

What's happening?

Researchers from Harvard and Amazonas State University teamed up to monitor the water quality of the Madeira River, starting over nine miles upstream from Humaitá municipality. They used an aluminum speedboat to collect water samples and tested them for a range of factors including pH, dissolved oxygen, and water temperature. They also collected sediment and soil and examined multiple species of fish.

"We are concerned about the pollution of the aquatic ecosystem, so we have two goals here," explained biologist Adriano Nobre, per Mongabay. "The creation of a water quality index for whitewater rivers and understanding the effects on environmental and public health regarding the presence of illegal miners and, consequently, the mercury contamination in the water and in the fish that the population consumes."

According to Mongabay, the researchers found high levels of mercury — not high enough to exceed the limits set by Brazil's authorities, but still high enough to be of concern, as it ranks among the top third of rivers globally.

"So it's in the range of what we would consider elevated," said biochemist Evan Routhier, per Mongabay.

The excess mercury comes at least in part from illegal gold mining operations along the Madeira. Miners use the mercury to extract gold from the river sediment, and then simply let the contaminant flow away into the water.

Thanks to the higher levels of mercury in the water, researchers also found higher levels in the fish. The species that residents generally eat were below legal thresholds, but the carnivorous species — which tend to accumulate the largest amount of a variety of pollutants — showed elevated mercury levels that would put them above the legal limit.

Why are these findings important?

Mercury is a dangerous pollutant. According to the U.S. Environmental Protection Agency, "Mercury exposure at high levels can harm the brain, heart, kidneys, lungs, and immune system of people of all ages. High levels of methylmercury in the bloodstream of babies developing in the womb and young children may harm their developing nervous systems, affecting their ability to think and learn."

This isn't just a threat to people living along the Madeira, but potentially to everyone downstream, which includes much of the Amazon Basin and the Atlantic Ocean. It also affects the Amazon ecosystem, including both plants and animals — and that ecosystem holds an irreplaceable part of the world's biodiversity. Among other effects, it impacts the population of fish that local residents rely on for food.

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What's being done about the elevated mercury?

Monitoring the situation is the first step in resolving it. This is the second expedition by Harvard and Amazonas State University to test the Amazon's tributaries, after the first in September 2023 to examine the Negro River. The work will continue with a third expedition during the dry season, Mongabay revealed.

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