- In a groundbreaking expedition, researchers from Harvard and Amazonas State University began monitoring water quality and mercury contamination in the Amazon Basin’s largest tributary.
- The Madeira River Basin has been heavily impacted by human actions, such as hydropower plants, deforestation and illegal gold mining, which degrade its ecosystems.
- Initial results from Harvard reveal high levels of mercury in the Madeira, although still below the limit recommended by Brazil’s authorities.
- Predatory fish species showed mercury levels above the recommended limit, while scalefish traditionally consumed by riverine populations were below.
HUMAITÁ, Brazil — When we arrive at the first site in the Madeira River, researchers from Amazonas State University (UEA) and Harvard position themselves at the stern and bow of the voadeira, an aluminum speedboat, to collect samples. The vessel floats 15 kilometers (9.3 miles) upstream of Humaitá municipality, in the Brazilian state of Amazonas, to start an endeavor to monitor the water quality and mercury contamination of the largest tributary in the Amazon River Basin.
Around the boat, river dolphins feed in the water while squirrel monkeys jump from branch to branch in the embaúbas trees and two riverine men fill buckets with water in front of a banana plantation. On the left channel, a ferry passes loaded with cargo and two illegal gold mining dredges go upstream.
The UEA researchers collect their water samples in seven small bottles, which will be analyzed in the boat and inland laboratories, and they take notes of a few parameters measured right away with a probe, such as pH, dissolved oxygen and water temperature.
The Harvard University researchers, besides filling their own water in two 1-liter (2-pint) bottles and two 10-L (21-pt) recipients, also collect riverbed sediments and riverbank soils. Then, we speed up to the next site, 15 km downstream.
The Madeira River is key to the Amazon River Basin both environmentally and economically. From its headwaters in the Bolivian Andes mountain range, it stretches more than 3,315 km (2,060 mi) until reaching the Amazon River, among the 20 longest water bodies in the world. The Madeira spans more than 1.42 million km2 (548,265 mi2) across three countries, home to the largest fish diversity in the Amazon Basin — 60% of the species already described in the rainforest live there. Ten percent of the production moved on waterways in Brazil flows through the Madeira, and it hosts two major hydropower plants that account for roughly 7% of Brazil’s energy.
While navigating the Madeira in Amazonas state, it became clear to the scientists the social importance of their research. Municipalities with fewer than 60,000 inhabitants each spread over 800 km (497 mi), with a riverine population extremely dependent on the resources provided by the Madeira and its tributaries.
“We are concerned about the pollution of the aquatic ecosystem, so we have two goals here,” said biologist Adriano Nobre, a scientist of the UEA’s Chemistry Applied to Technology (QAT) research group. “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.”
That was the second expedition in the UEA and Harvard partnership to investigate water quality and mercury contamination in the Amazon rivers. In September 2023, a group of scientists sailed the Negro River, the second-largest tributary of the grand Amazon to benchmark its waters, and found the river in good shape. The expedition led to the first water quality index (WQI) for the Amazon Basin, aimed at blackwater rivers.
The whitewater Madeira, however, faces a different scenario. With many more communities along its banks than the Negro, the Madeira shows higher levels of fecal coliforms and ammoniacal nitrogen, according to initial results from the expedition, although still way below the maximum limits established by authorities. All points also exceeded the safe levels of total phosphorus, probably resulting from agricultural fertilizers.
And then there’s the mercury issue.
The river has been an illegal mining hub for many years. Miners use quicksilver mercury to extract gold from the riverbed sediments, which is usually discarded directly into the river. Although this type of mercury is not highly soluble in water and tends to deposit on the riverbed, over time, it can mix with the sediments, be released into the atmosphere and enter the food chain through fish.
The ongoing mining operations, usually done with floating dredges that stir up the riverbed sediments, can accelerate the process of mercury contamination. This poses a serious threat to the health of the river ecosystem and the local communities that depend on it for their livelihoods.
While the first data from the Harvard study reveal that total mercury levels in the Madeira River water are still below the limit recommended by Brazil’s Ministry of Health, it is considered elevated when compared with other rivers globally.
“The Madeira has the added complexity of gold mining mercury inputs, so we would like to see if there are significant impacts on the levels in the water and if that contamination translates in the fish that people are eating,” said biochemist Evan Routhier, a researcher at Harvard’s Sunderland Lab, a group dedicated to the study of global contaminants, such as mercury and PFAS.
The Harvard scientists found that predatory fish in the Madeira, such as pirarara (Phractocephalus hemioliopterus), have mercury levels above the safe limit for human consumption. In other species, like jaraqui and pacu, these levels were found to be below the recommended, but still in an amount that demands the attention of public authorities as well as the need for continuous monitoring.
On board an 18-meter (60-foot) research boat, Mongabay joined this expedition through the Madeira River in April 2024, navigating more than 1,800 km (1,118 mi) over 12 days between the cities of Manaus and Humaitá.
Crisis on traditional fishing
The Sunderland Lab scientists study the mercury cycle in the Negro and the Madeira rivers in the water, riverbed sediments, soils of the riparian forest and fish most consumed by local populations. The two largest tributaries of the Amazon River have opposite chemical characteristics that can influence the behavior of mercury, such as pH and dissolved organic carbon concentration.
“The Madeira has a few more communities, there are dams upriver and more deforestation in the basin, which could also cause differences in the amount of mercury that’s coming into the system,” Routhier said.
The Madeira expedition will also help the UEA scientists develop the first WQI for whitewater basins. “This index is mainly used for public supply, and it needs to be adjusted to take into account the chemical, physical and biological characteristics of the basin,” Nobre said.
In the six days navigating upstream the Madeira, the researchers could observe its complexities: the vast diversity of fauna and flora in the water and in the forest, the pastures of buffaloes and cattle in the mouth of the river region, the traditional floodplain agriculture of the riverine communities, the fishing in the main river watercourse and in the blackwater tributaries, several clusters of gold mining rafts.
On April 21, the research boat docked in Humaitá, Madeira’s most populous municipality in Amazonas state. Fishing is the source of income for 3,700 of the 60,000 inhabitants, both in urban and rural areas. But in the municipal market, the researchers came across stands full of nursery tambaquis (Colossoma macropomum) and little diversity.
“The most missing ones are the scalefish — pacu, jatuarana, sardines. They used to be the most traditional species here,” Osvaldo de Araújo, a 63-year-old fishmonger, told Mongabay. “Before, I would catch 500 kilos [1,102 pounds] in my canoe within five days. Nowadays, the fishers spend 12 days in order to bring that much.”
In 2012 and 2013, two large hydroelectric plants began operating in the Madeira River, in the neighboring state of Rondônia. According to scientists, the dams generated hydric pulses, with abrupt daily variations in the river level that affect the fish’s migratory patterns. Thus, fishing productivity was lost, and many fishers shifted to illegal activities that degrade the forest and aquatic environments: deforestation, land-grabbing, gold mining.
“We used to drink this water from the Madeira,” José Pessoa, a 58-year-old fisher from Humaitá, told Mongabay. “No one does it anymore. The people who live on the riverbank suffer a lot from this because they have no other resource. They must travel far away to buy water in the city.”
The Paraizinho community sits 10 km (6.2 mi) upstream from Humaitá. The daily life of 33 families in this community revolves around the Madeira River, which serves as transportation, a source for consumption and personal hygiene and irrigation for floodplain plantations.
Small-scale farming supports this community, whose production is acquired by government programs. However, fishing has been hindered by the unpredictable water level variations caused by hydroelectric plants. Nowadays, the fishers need to travel to more distant locations if they want to maintain production.
“People are abandoning fishing because they can’t afford the expenses to search for fish,” said João Mendonça, a 50-year-old community health agent and president of the Paraizinho farmers association, which also represents the local fishers. “Many people have shifted to mineral extractivism. The government doesn’t let them, because of the impact it causes, but they have to support their families,” he told Mongabay.
The impact of quicksilver mercury
Sailing between Humaitá and Manicoré, the research boat passed a few meters away from a huge mining raft in operation. The dredge was immersed in the river and scraped sediments off the riverbed. The material was sent to mats on the second floor of the vessel, where the gold, if collected, would be retained in a carpet with the use of the quicksilver mercury.
“One thing that surprised me was just how much discharge was being dumped just right into the river,” Olivia Pietz, a 24-year-old PhD student in environmental science and engineering at Harvard, told Mongabay. The researchers collected samples in sites above and below mining operations.
Illegal gold mining ostensibly occurs in the Middle Madeira region, which covers the municipalities of Humaitá, Manicoré and Novo Aripuanã. In the months following the expedition, two Federal Police operations in the region destroyed hundreds of mining rafts.
Initial data from the Harvard study revealed high levels of mercury in the Madeira, although still below the limit of 0.2 micrograms per liter (µg/L) recommended by Brazil’s Ministry of Health. When comparing its data with 446 rivers around the world, Routhier found that the Madeira falls in the upper third of these global rivers examined in an article published by Nature. “So it’s in the range of what we would consider elevated.”
The chemist Elsie Sunderland, leader of the research group, notes that some elemental mercury used in gold mining may still be in the pure liquid phase. When discarded, the metal tends to seep into the riverbed and sometimes into the soil of the surrounding areas. “My guess is that some mercury used is still quite patchy in its distribution and hasn’t fully diffused into the surrounding environment. In a way, this is good news because you can’t see a widespread impact yet, even though you know that activity is occurring,” said Sunderland, a professor of environmental chemistry and planetary sciences at Harvard.
However, this doesn’t mean the Madeira River is free from a severe problem. “Over time, that mercury can work its way into the river and mix with the sediments and enter the atmosphere as a gas, and then you’ll have an issue that is more widespread. Mercury dissolved in river water makes it easier to get into the food web as well as the fish,” she said. Dredging moves sediments in the riverbed, causing them to float in the water, potentially accelerating this process.
Contaminated fish
On a Friday afternoon, many boxes full of jaraquis, a traditional species in the diet of the Madeira’s riverine, arrived at Manicoré’s local market. “This mercury thing has never affected us,” 59-year-old fisher Ancelmo de Menezes told Mongabay. “It has never resulted in health problems for us, thank God. The water is treated here in the city, and we take it when we go fishing. We end up having to drink from the river, but it’s not harmful.”
Antônio Veiga, president of Manicoré’s fishers association, however, sees the Madeira as already polluted due to the incessant traffic of ferries, in addition to mining operations. “In the past, we used to treat the Madeira’s water and drink it, but this water is no longer consumed here in the city,” he told Mongabay. “We have artesian wells with top-quality drinking water to consume.”
Harvard researchers collected 29 fish in Humaitá, Manicoré and Borba. Predatory species, such as pirarara and candiru, a flesh-eating fish not consumed locally, showed mercury levels above 1.0 mg/kg, the maximum safe limit established by Brazil’s authorities. Ten jaraquis purchased in Manicoré had an amount of mercury within the limits for non-predatory fish. Scientists will investigate the reasons for these results — whether they occur due to mining activity or to fish ecology, or even a combination of both.
“None of them are really low. The predators biomagnify the mercury and are too high for people to eat regularly. But the other ones look OK for regular consumption, and it would be interesting to craft advice depending on how much people eat,” Sunderland said. “Monitoring helps identify which species can be consumed more frequently than the others without exceeding health-based guidelines for safe mercury exposure. That’s the kind of advice that the government can provide.”
The next step of the Harvard study is to analyze the other elements of the mercury cycle, including levels of methylmercury, the organic form of this metal that bioaccumulates in fish and can cause health problems for humans.
“Understanding the mercury cycle is really important for the region, especially the propensity for methylation and exposure for communities,” Routhier said.
The fight for safe water
One morning, fishers were waiting for a school of matrinxãs swimming down the Aripuanã River, while illegal miners were dredging the bottom of the Madeira searching for gold beside an island right in front of the city.
Allan de Barros, president of Novo Aripuanã’s fishers association, said he believes the mining activity contributed to the decline of fish stocks, adding to the impact of hydroelectric plants, because it worsened the water quality. “Many of our fishermen went mining, especially the young ones,” he said. “But now, with these [Federal Police] operations, many are coming back.”
Water safety is a priority issue for Novo Aripuanã, the municipality with the second-highest number of diarrhea hospitalizations in Amazonas state – 1,045 per 100,000 inhabitants. Only 13.7% of the city sewage is treated before being disposed of in the Aripuanã River, a tributary of the Madeira, according to official data.
Every Thursday, Silvana Cabral travels from the Madeira Sustainable Development Reserve to Novo Aripuanã to sell her produce from floodplain agriculture: banana, watermelon, pumpkin, cassava, pepper. This sustainable management conservation unit has 96% of its area composed of forest from which residents extract Brazil nuts, cupuaçu (Theobroma grandiflorum) and açaí.
Besides moving goods, the Madeira is used for people’s basic needs, such as bathing and washing dishes. Whenever they can, the riverine residents buy water in the city, but they often consume it from the river. “The Madeira’s water is used for everything,” said Cabral, who is president of the Santa Maria do Uruá community association. “I don’t think the water is good. I believe it is very contaminated.”
In front of her community, Cabral said that every year, about 100 mining rafts gather to extract gold. “The water still has the same color and the same taste. But we hear a lot about mercury in water, which causes various types of diseases.” She said she believes the water monitoring initiated by UEA and Harvard is important to bring “an improvement to our people.”
The water quality index is usually composed of nine physicochemical and microbiological parameters. In the case of Madeira, the researchers will add a 10th: mercury. “Inserting this variable would be an important strategy to establish acceptable limits within the WQI,” biologist Nobre told Mongabay.
To develop the Madeira River’s index, the UEA research group needs at least one more expedition during the drought season. But the data from the first campaign already give clues about the condition of this water body. With the exception of mercury levels, Madeira is healthy — it has been able to handle the multiple human pressures that surround it, the scientists concluded.
As soon as the laboratory boat passes through the municipality of Nova Olinda do Norte, we embark on the voadeira for the campaign’s last sampling sites. The night begins to fall when we reach the final point, surrounded by an aquatic horizon where the Madeira discharges 34.4 million L (9 million gallons) per second into the Amazon River.
UEA researcher Christopher Hildemberg states that the journey through these white waters has brought to light the effects of human actions on the Amazon Basin’s largest tributary. “I still hope that the impacts that have already been caused can be mitigated in some way. In order to do that, we need to monitor, understand and study.”
Banner image: Harvard researchers Evan Routhier collects water from the Madeira River near illegal mining rafts between the municipalities of Humaitá. Image by Bruno Kelly.
Study to benchmark water quality finds key Amazon tributary in good shape
Citation:
Liu, M., Zhang, Q., Maavara, T., Liu, S., Wang, X., & Raymond, P. A. (2021). Author correction: Rivers as the largest source of mercury to coastal oceans worldwide. Nature Geoscience, 14(12), 956-956. doi:10.1038/s41561-021-00839-5