The Negro river's health
In the Negro river, an unprecedented scientific expedition aboard a floating laboratory brought together researchers from Harvard and Amazonas State University during the basin’s worst drought on record.
By Kevin Damasio, from Santa Isabel do Rio Negro
Photos by Bruno Kelly
Maps by Rodolfo Almeida
In September, 2023, an expedition of scientists embarked on the Rio Negro to create the first Water Quality Index (WQI) for an Amazonian river and to study the mercury cycle in the region
Earth’s largest river archipelago, the Mariuá is a labyrinth of well-preserved islands and waterways—perfect for the development of a WQI
Despite the well-preserved forest and water here, the region still lacks basic sanitation. In Barcelos and Santa Isabel do Rio Negro, raw sewage flows directly into the river
The researchers set out to find out whether the Mercury found at the confluence of the Rio Branco—which receives runoff from mining operations—and the Rio Negro is of natural or anthropic origin
The people living in Vila de Moura use the Rio Negro as their main source of water for drinking, cooking and bathing. The river is also the avenue by which supplies and services reach the village
The water in this preserved leg of the river rated moderate, which was the lowest of the 50 sites analyzed
The water inside Anavilhanas National Park is very clean, important for Novo Airão’s economy, which is based on artisan fishing and ecotourism
The Branco river begins at the confluence of the Uraricoera and Tacutu rivers in the state of Roraima. It then flows 560 kilometers downwards until it spills into the left bank of the Negro river in the municipality of Rorainópolis. The encounter of its white water with the black waters of the Negro is visually striking, but this place is important for more reasons than just its colors: researchers on the scientific expedition that I have joined are especially interested in the preservation of these waters. Why? The history of gold mining in its faraway headwaters.
Amazonia’s beauty is striking there. After collecting water and sediments from the pebbly bed of the Negro river one morning, we tie up on the bank at the Lower Rio Branco-Jauaperi Extractive Reserve. As we enter the flood forests with no sign of humans, the scientists are amazed by the extreme humidity of the soil and the explosion of wildlife – especially capuchin monkeys and a plethora of butterflies on the wing. Later, as we leave that spot and head for the opposite bank of the Negro river, the fading mist gives way to a glaring sunlight as pink river dolphins swim after schools of fish near an igapó, or flooded forest, a few meters from our voadeira, a small aluminum boat. An enormous black caiman slides towards a small sand bank.
“It takes your breath away. Every collection site is a different landscape from the one before,” comments Faiz Haque, a Pakistani researcher from Harvard University. Raised in England, Haque is 25 years old and holds a Master’s degree in analytical chemistry. This is his first field study in Amazonian waters.
It is September, near the peak of 2023’s extreme dry season in the Amazon. Two teams of scientists—one from Amazonas State University (UEA) and the other from Harvard—are leaving Manaus on a 60-foot (18 meter) research vessel equipped with four laboratories. Both groups are pioneers in their studies about the Negro, a river basin home to the planet’s largest fluvial archipelago, riverine communities, Conservation Units and Indigenous Territories. Despite little human activity throughout the basin, the ills of the worst drought recorded in 121 years are affecting daily life: the Negro’s water level drops continually, making navigation difficult and the air in the state capital carries a suffocating amount of smoke from wildfires.
As we push up the river to Santa Isabel do Rio Negro on the first leg of our 12-day journey, the Harvard scientists do their research focused on understanding the mercury cycle in the Negro: collecting samples of water, sediment from the bottom, soils from the riparian forest, and fish consumed by local people. On the trip back down the river, UEA researchers collect samples to develop the first Water Quality Index (WQI) for an Amazonian river – a crucial tool in evaluating water bodies’ state of health to guide public policy and scientific studies. The Negro river was chosen as the starting point for the project because it is the Amazonas basin’s second largest tributary – the main riverbed runs for 2,250 kilometers from its headwaters in Colombia – and due to its low level of degradation.
“The Water Quality Index needs to be based on places without human interference. In Amazonia, the Negro river fits this criteria,” states Sérgio Duvoisin Júnior, head of the Chemical Analyses Center, which is part of UEA’s Chemistry Applied to Technology Research Group (QAT).
Samples were collected at 14-kilometer intervals along the 700 kilometers of the Rio Negro that stretch between Manaus and Santa Isabel do Rio Negro. Despite the extreme drought, only two of the 50 collection points rated moderate—the other 48 rated either good or excellent.
The Water Quality Index (WQI) was created in the 1970s by the United States National Sanitation Foundation. It uses physico-chemical and microbiological characteristics to diagnose the state of a body of water, and was adapted for rivers in the state of São Paulo, Southeast of Brazil, by its state environmental company CETESB in 1975. Nine parameters—each carrying its respective weight—compose a number on a scale from 0 to 100, color-coded to illustrate five classifications from “very bad” to “excellent”. The closer the rating is to zero, the worse the water quality.
It is a reference that is user-friendly for the general population: if the index is in the red zone, it means the water is bad. However, specific indices have never been created for the rivers in the Amazon Basin, the planet’s largest watershed. Determining water quality there has always been carried out by using the WQI for the states of either São Paulo or Rio Grande do Sul, where the water characteristics are very different.
One good example is the pH, the hydrogenionic potential, which indicates the water’s acidity or alkalinity. According to the IQA from the South and Southeast of Brazil, the ideal pH in rivers is more alkaline—between 6 and 9—while the natural pH of the Negro is more acid—from 4.5 to 5.5—because of the organic material in the rainforest that runs off and is dissolved in the river. “If I find a pH of 4.5 in a place with no human activity on the Negro river, the parameters from São Paulo will indicate that the water is bad. But it isn’t. The water is exactly how it should be,” states Duvoisin.
Another peculiarity of the Amazon Basin is that it has three different types of rivers – blackwater, whitewater, and clear water – requiring a different index for each one. The UEA researchers hope to develop WQIs for each of the largest rivers in the Amazonas state, and also maintain regular monitoring of these waters according to the seasons, which are well-defined periods of flooding and ebb throughout the year. The Negro river, for example, oscillates between 13 meters in elevation to nearly 30 meters in the same year.
The main parameters of the index – dissolved oxygen, thermotolerant coliforms, pH and electric conductivity – are directly linked to a lack of sewage treatment. Poor basic sanitation affects public health, as many vectors for disease use water as their vehicle. These researchers’ work will help government decision-makers define relevant actions needed to recuperate a degraded body of water and improve the lives of the human populations nearby.
Efficient remote sensing in Amazonia has already made it possible to identify changing patterns on water surfaces, “but there are limits to what we can understand on the ground, what this data actually means,” observes geographer Ane Alencar, director of science at the Amazon Environmental Research Institute (IPAM).
Now, in addition to guiding public water supply and basic sanitation policy, the WQI will help improve planning for aquatic ecosystem studies on broad and local scales. Without this information, researchers have been “in the dark” as they go into the field where water quality is concerned, says biologist Cecília Gontijo Leal, researcher at the University of Lancaster and at Sustainable Amazon Network (RAS, in the Portuguese acronym), who specializes in Amazonian freshwater ecosystems. “With a local reference, we can identify short-term or permanent impacts,” says Leal, also a scientific consultant for Ambiental Media’s Aquazonia Project.
Exposed sandbars in the Mariuá region showed effects of the severe drought on the Rio Negro—a few weeks later, the river would reach its historic low of 12.7 meters. Mariuá is Earth’s largest river archipelago, composed of 1,400 islands distributed throughout 270 kilometers of the mid-Rio Negro between the mouths of the Rio Urubaxi and the Rio Branco.
Doing science in a year of extreme Amazonian drought
Our research boat left the marina in the Tarumã-Açu igarapé [stream] in Manaus on September 11, headed for São Gabriel da Cachoeira. As the days went by, we watched the Negro’s water level drop frighteningly. This was an extremely hot summer resulted by a combination of warmer waters in both the Equatorial Pacific due to El Niño and in the Tropical North Atlantic. Both phenomena were enhanced by climate changes.
On board were ten scientists – eight from UEA and two from Harvard – six crew members and two Ambiental Media reporters. On a previous trip at the beginning of 2023, the UEA researchers had already defined the sample collection sites, spaced 14 kilometers apart along the route of the Negro river. They had also made all necessary adjustments regarding safety, crew and collection methodology.
During our expedition, the Negro dried up at an average of 28.75 centimeters per day. The waters continued to drop until October 26, when they reached an elevation of 12.7 meters, the lowest ever recorded since measurements began to be recorded in 1902. “It is important to know what the water quality is like during both the flood season and the dry season,” states biologist Fernanda Vieira, who at 26 is working toward her Master’s degree in Environmental Studies inside the Applied Chemistry & Technology research group (QAT) and is this expedition’s field and lab coordinator. “Now we will be able to understand how much influence El Niño has on this basin.”
With the laboratory vessel in the background, Harvard University researchers Evan Routhier and Faiz Haque use an aluminum boat to collect water samples from the Rio Negro with the help of Amazonas State University researchers Adriano Nobre and Clodoaldo Oliveira.
In nearly all the municipalities in the state of Amazonas, commercial and passenger transportation depend on boats that come from Manaus. But the Upper Negro is a region with many rocks and rapids that make navigation dangerous. During severe droughts, boats can still reach Santa Isabel do Rio Negro, but beyond there, only lightweight speed boats can pass. For this reason our captain Josimar Pimenta Sanches warned the group that if we were to travel the 250 kilometers upstream to São Gabriel da Cachoeira, we could have problems getting back. The scientists then made the decision to stop in Santa Isabel.
“This river is my home,” 37-year-old Sanches tells me as he zig-zags the boat through Mariuá, Earth’s largest river archipelago, dodging sandbars under the shallow water by steering around eddies. Sanches is a descendant of Baré Indigenous people and migrants from Maranhão. He was born in Santa Isabel do Rio Negro and accompanied his father on trips to transport rubber, local fruits like sorva and piassaba palm tree fiber. When his father stopped working as an extractivist and became a boat captain on the Negro river, Sanches followed in his footsteps.
Santa Isabel do Rio Negro has a population of 14,000 and is the municipality with the highest proportion of Indigenous people in Amazonas (96%) according to the IBGE (Brazilian Institute of Geography and Statistics). Public services and commerce drive this economy that is surrounded by 62,000 square kilometers of primary rainforest that composes 98% of its land area.
However, the precarious state of the local basic sanitation is evident as soon as we arrive. In the morning, fishermen pack onto the port’s small beach to sell tucunarés, aracus, pacus and other fish species. There, raw open sewage runs freely into the Negro, evidence of the lack of water treatment and an inadequate public sewerage system, as reported in the IBGE National Basic Sanitation Assessment of 2017.
Three UEA researchers from Manaus are assigned to collect the water samples, which will be key to study if and how this pollution has affected the river. José Clodoaldo Oliveira is 54 years old and is finishing a Master’s Degree in Water Resources. He keeps an eye on the GPS as he pilots the voadeira in the Mariuá region. Member of the research team since 2008, Oliveira is used to navigating the basins near Greater Manaus like the Tarumã-Mirim, Tarumã-Açu and Puraquequara – a completely different scenario than this “river labyrinth” with treacherous bottoms. He slows down and shuts off the outboard motor so as not to contaminate his samples with fuel.
At the prow, Gabriel Rodrigues takes water samples in eight polyethylene bottles organized by the different parameters – nitrogen, phosphorus, phosphate, chlorides and coliform bacteria, among others. He adds sulfuric acid to some in order to preserve the samples for a longer period. “There is no database on our rivers. So all these samples we are taking here are the first ones,” says Rodrigues, a chemical engineer and QAT laboratory technician.
Amazonas State University researchers Clodoaldo Oliveira (left) Douglas Siqueira (middle) and Gabriel Rodrigues (right) use GPS to locate a pre-determined collection site (left). ASU lab technician Irliane dos Santos analyzes water collected less than an hour before. Parameters requiring immediate analysis like total and thermotolerant coliforms, nitrate, nitrite, biological oxygen demand and chemical oxygen demand were assessed on the research vessel. The remaining parameters were assessed in Manaus.
Meanwhile, Douglas Siqueira dips a probe 30 centimeters below the water surface and, after two minutes, takes it out and writes down the readings on the pH, air and water temperature, dissolved oxygen and electric conductivity. Dissolved oxygen, one of the four main parameters of the Negro river’s first WQI, is vital for the existence of aquatic life, explains Siqueira, a 37-year-old biologist on the research team since 2021. Dissolved oxygen levels can indicate, for example, the context of wastewater discharge, seasonality, water temperature and possible chemical alterations – the lower the concentration, the greater the river’s contamination index. In blackwater rivers, however, dissolved oxygen is naturally low due to the fact that the waters rise and flood the forest soil where organic materials decompose. This is a process that consumes oxygen.
“The water’s pH is 5.14,” adds Siqueira. “In urban areas, the pH is nearly neutral, sometimes even alkaline.” Variations from the pH 4.5-5.5 range in blackwater rivers can indicate anthropogenic activity like wastewater runoff, or even natural occurrences like the influx of white or clearwater tributaries.
About 20 minutes later, the aluminum boat is back and the three scientists hand the samples over to the lab technicians on the research boat. They analyze the parameters with shorter expiration dates while watching through the lab’s windows their subject of study, the Negro river: total and thermotolerant coliforms, nitrate and nitrite, biological oxygen demand (BOD) and chemical oxygen demand (COD). The others like phosphorus, phosphate, dissolved solids and metals will be measured back at UEA once the expedition is over. The team has the expertise and equipment to analyze a total of 161 parameters.
Fernanda Fraga, 24, begins the work with analyses of nitrite and nitrate – compositions used to preserve processed foods and stabilize color and flavor. “They are unhealthy for both human health and aquatic environments,” observes the biologist. The presence of large quantities of these chemicals indicates activities like illegal waste disposal, but in the well-preserved Negro river their levels were very low.
In the afternoon, Fraga processes the coliform samples. In a polyethylene flask holding 100mL of the water, she places the reagent that will help the bacteria proliferate and shakes the flask until it is diluted. Once it is soluble, she transfers it to a tray where the water, now yellow, is divided into small and large squares. The trays are then sealed in a machine and left to incubate for 24 hours. After this period, Fraga will have up to four hours to read the result for total coliforms. If bacteria are detected, the liquid turns into shades of pink. If it is clean, it will remain yellow.
Harvard University researchers Faiz Haque and Evan Routhier collect soil samples in a riparian forest to gain understanding on the movement of mercury, which may move from the ground surface to deeper soil and then to the river (left). In the laboratory on the research boat, Routhier organizes containers with water collected at parallel sites on the Negro river: the water is lighter in color on the left bank, at the mouth of the Branco river; in contrast, water collected on the right bank of the Negro is much darker without the influence of water from the Branco.
Next, Fraga reads each tray in a dark box to check for the presence of thermotolerant coliforms – an indicator of bacteria like Escherichia coli, which can cause diseases like diarrhea and urinary infections. “Thermotolerant coliforms are the biggest indicator of raw sewage runoff. They reveal the state of water and are vehicles for disease,” tells Vieira, the campaign coordinator.
Vieira waits until all of the day’s collections are done to prepare the biological oxygen demand (BOD) samples. “We calculate how much oxygen the bacteria present in a certain sample consumes,” explains Vieira. She places 400mL of the material into a beaker and adjusts the pH – either with sodium hydroxide if it needs to be raised, or sulfuric acid if it needs to be lowered. She then places it in the system with an amber flask and a BOD reader, dripping in four drops of potassium hydroxide, which will stimulate the growth of the oxygen-consuming bacteria only.
Finally, she takes the processed samples to the incubator where they are maintained at 20ºC for five days on supports with magnetic bars that cause the bacteria to move around and consume oxygen. “The result should be below 90, depending on the environment. A higher number means the water is polluted,” says Vieira.
In another lab, 30-year-old chemist Andreza Guimarães filters the samples for total and dissolved metals. She prepares two types of samples for each collection site: one filter with particulate matter and three flasks of filtered river water. They are kept under refrigeration because the analysis will be carried out in the UEA lab through a spectrometry technique with an argon plasma that runs the samples at around 10,000ºC – nearly double the temperature of the Sun’s surface – and reads for total metals present.
On the next workbench, biologist Irliane dos Santos, 24, prepares test tubes for analysis of chemical oxygen demand, which would indicate industrial waste. With a pipette, she places 3.5mL of concentrated sulfuric acid mixed with silver sulfate (a transparent liquid) in a flask to help the reaction. She then adds the digestion reagent – 1.5 mL of red-orange sulfuric acid with 2.5 mL of water taken from the Negro river. She repeats the process until 39 bottles are finished. There are three for each of the 13 collection sites that day – a method called triplicate. The liquid inside the flasks undergo an exothermic reaction that raises its temperature. Next, they are placed in the digester, which spins them for two hours and mixes the previously heterogeneous liquids. The next morning, the samples will be analyzed under a spectrometer. The boat must be moving during this process.
Novo Airão’s local economy is driven by artisan fishing and tourism. It is located at the entrance to Anavilhanas National Park, which harbors Earth’s second largest river archipelago with 400 islands and 60 lakes.
Signs of conservation in the riparian routine
Biologist Adriano Nobre is 35 years old and coordinator of field and laboratory for the Negro river expedition. For him, this cooperative project between UEA and Harvard is special because it is not a one-time study, but rather the start of a continuous monitoring of the aquatic environment’s quality during both dry and wet seasons. Nobre is from Autazes, a municipality in the Amazonas state. Growing up surrounded by rivers led him to study water resources. “Those who live by the river depend on the forest and the waters for extractivism, agriculture and fishing. The quality of these environments is a part of their public health,” he points out.
We could verify this perspective in loco when we docked in Moura, a district 150 km downstream from the urban center of Barcelos. Two hundred families in this small village live from subsistence fishing, farming and hunting with some jobs at three local quarries. For them, the Negro is the route by which their supplies arrive and also their source of drinking water. Two pumps installed on the riverbank serve the community. There is also a privately-owned well at the quarry owned by the Brazilian Air Force’s Amazon Region Airport Commission (COMARA), to which the locals have limited access.
“For drinking, we get it from COMARA, where they have a well with filtered water. But to cook and clean, that we get from the river,” tells 60-year-old José Farias on the front porch of his wooden house at the entrance to the village. “We use chlorine and drink it. It hasn’t made us sick.”
The analyses done in Moura could be an instrument for the local community to certify the quality of the water they drink and petition for changes like the drilling of artesian wells, which are already under negotiation with City Hall in Barcelos.
Community members play volleyball in a riverside court in Vila de Moura, Barcelos. In the towns and villages along the banks of the Negro river, water from the river is used for drinking and day-to-day activities. Because of the diagnostic it provides on the water’s quality, the WQI serves as a tool to help local communities request public policy on basic sanitation.
The Negro river expedition is one of 15 projects in the Amazonas State Water, Air and Soil Monitoring Program (ProQAS/AM), a QAT initiative financed by the Amazonas Institute of Environmental Research (Ipaam), the State of Amazonas Research Foundation (Fapeam) and by the State Secretariat of the Environment (Sema/AM). For five years, another of the program’s projects has been monitoring Greater Manaus’ five main river basins every three months and making the data available through an online platform.
On March 23, 2024, the UEA research team presented its WQI for Amazonian blackwater rivers at an event for scientists and public policy makers. With this new index, the researchers were able to recalculate the monitoring data from the basins around Manaus to gain a more accurate picture, because the parameters and weights are now ideal for this type of river. The result was that the WQI of the Educandos and São Raimundo igarapés, as the small streams are called in the Amazon, in Greater Manaus, changed from 32 to 44 – still a “bad” rating. The others were bumped up to “acceptable” based on readings from last November: Tarumã-Açu with WQI 70, Tarumã-Mirim with 68 and Puraquequara with 62. The scientists aim to expand their work to include all the micro basins surrounding Manaus, but the existing indices were already serving as a basis for public policy.
One such case is PROSAMIM (Manaus’ Igarapés Social and Environmental Program), created by the Amazonas State Government in 2003. After the WQI was developed and the Educandos and São Raimundo igarapés that cut through Manaus city began being monitored, the water quality was found to be bad at most of the collection sites because of human occupation along the riverbanks and wastewater release. By the time the program had finished in 2021, 29,000 people had been relocated, urban transport works had been completed and surface water and sewage systems had been created.
In Tarumã-Açu, over 1,000 houseboats popped up along the waterfront, especially for recreation and as rentals, intensifying the pollution already caused by houses, restaurants, condominiums and two garbage dumps on nearby canals. At the request of the Amazonas Environmental Court of Justice, the research team worked on a technical report identifying a worrisome rise in pollution levels. Judge Moacir Pereira Batista then ordered that all houseboats be removed and demolished by December 31, but the decree did not affect condominiums and garbage dumps in the surrounding areas.
Reilson Dias da Costa and Irlane Mendes Bibiano in their fish shop in Novo Airão. Pacu-galo, matrinxã, jaraqui and aracu-camanaru are the species most consumed by the people living here. “I catch new fish every day, nice and fresh,” he says. “Fish are my life.”
Villain of the waters: the mercury cycle in rivers
The UEA research boat was designed so that other partnering groups could use it to develop their own projects. During the September’s expedition, researchers from Harvard University’s Sunderland Lab –which studies the biochemistry of global contaminants – worked on the first phase of a study on the mercury cycle in the Negro and the Madeira rivers, their natural and anthropogenic origins, contaminated environments and the possible exposure of local communities.
The researchers collected samples of water, riverbed sediment, soil from riparian forests and fish consumed by urban and riverine populations. “We want to understand how mercury and dissolved organic carbon can interact with the river and eventually result in mercury methylation,” states 26-year-old biochemist Evan Routhier.
Methylmercury is the organic form that bioaccumulates in river organisms and causes health problems, especially when fish are eaten. Measurements of its levels in water, in particulate matter and in fish, indicate the level of risk to local populations. The total mercury analysis investigates the inorganic types of this chemical element that don’t bioaccumulate but measuring them help to calculate the stock that could potentially become methylmercury. The dissolved organic carbon can in turn bond with the mercury and “is an important factor in monitoring the amount of mercury available for methylation, and the methylation rates within the system”, continues Routhier.
Meanwhile stable mercury isotopes serve as a tool for monitoring the element’s movement within the environment. It is a sort of “fingerprint”, says Routhier, from which the mercury’s signature can be identified and therein its origin—the river, the groundwater or the forest. “In the same way, we can measure isotopes in fish and compare them with the water and other mediums isotopes in order to understand the mercury cycle in the environment.”
Frequently, the quest for scientific answers like these come with a dose of adrenaline. An hour after we left Manaus on September 11, researchers Clodoaldo Oliveira, Adriano Nobre, Evan Routhier and Faiz Haque (the last two from Harvard) boarded the small aluminum boat and headed out at top speed to the so-called “Rio Negro strait”, which lies 22 kilometers from the state capital. This site is where the banks of the Negro are closer together, only 1.62 kilometer apart, which means the river is at its deepest here – 120 meters.
The laboratory boat headed up behind, planning to meet up with the others as they entered the strait. Suddenly, a storm began to form with heavy clouds and strong winds, while there was no sign of the researchers in the voadeira. Douglas Siqueira tried to contact them via radio with no success, then headed out after them in another aluminum boat with captain Michael Christian just as the torrential rain started to fall. They returned an hour later without having found the others. We finally came across the researchers as we later headed toward the Anavilhanas archipelago towed by a family after three hours adrift.
“We gathered our water and sediment samples, and suddenly the weather turned completely”, said Haque. “We got soaked by the rainstorm. But it was a cool experience. None of us panicked. Definitely an adventure.”
Keeping a cool head and being brave are important for anyone willing to do research in the Amazon. Being disciplined also is key. Just to illustrate: in the cities along our route from Manaus to Santa Isabel do Rio Negro, the Harvard researchers needed to gather fish samples of the most-eaten species – and extremely fresh ones. “Preferentially pulled from the water and frozen on site,” said Routhier. This, because methylmercury can degrade in the heat and mask its real levels present in the sample. “Total mercury doesn’t degrade, but if the muscle begins to decompose because it’s exposed to heat for a long time, it can be lost.”
Evan Routhier (foreground) buys tucunarés from a local fisherman at the port of Santa Isabel do Rio Negro (left). The fish is measured for analysis in the boat laboratory. It was important to collect a sampling of the most-consumed fish. “The location on the river where people catch the fish they eat can have an effect on methylmercury concentrations,” says Routhier.
Early one Tuesday morning, things are just starting to get active in Novo Airão as we walk around to find fish shops. The town lies in front of Anavilhanas National Park, which protects the second largest river archipelago on Earth. Small-scale fishing and ecotourism drive this economy, a town in the Negro river’s right bank, with 16,000 inhabitants lying 115 kilometers from Manaus and which only gained access to the state capital when the Rio Negro bridge was built in 2011. With the UEA researchers as translators, Routhier and Haque learned from the fishmongers which species were the most sought-after: jaraqui, pacu, sardinha, tucunaré, and aracu.
“It is important to sample a variety of the fish that people eat locally. The spot on the water column where they eat could impact the methylmercury levels,” explains Routhier. “We got some surubim, which is a catfish bottom feeder and therefore a source of potential bioaccumulation rates.”
At their first stop, the researchers buy aracu-camanaru and aracu-embaúba. Next, at a fish shop called Habitart de Deus, they purchase jaraquis, a very popular fish in the region. Routhier asks me to question the fishmonger if the locals worry about mercury contamination. The shop owner says no. “Mercury is more common where there’s gold. There’s no mining on the Negro,” says 35-year-old Reilson Dias da Costa.
Costa was raised in Manaus’ Compensa neighborhood at a time when the city’s blackwater basins “were good enough” to swim in the streams. Later he moved to an rural area upstream, near the Jauaperi river, a tributary of the Negro on the state border with Roraima. There, he worked as a commercial fisherman until the activity was prohibited when an extractive reserve was established there. He then migrated to Novo Airão in 2009, “an ecological paradise” as he describes it. When a herniated disk forced him to retire from fishing, he opened a fish shop in which he stocks with the catches of local artisanal fishermen.
Before noon, the researchers get in the aluminum boat to begin collecting their samples. Routhier and Haque begin with the surface water samples, which will be used for mercury, methylmercury and dissolved organic carbon analysis. They then fill 20-liter jugs from which they will extract particulate matter for stable mercury isotope analysis. Then, they tie up at a small sandy shore and walk in the forest to gather soil samples. Routhier digs his shovel 20 centimeters into the soil and lifts out the surface material. Haque collects dirt from the first centimeter and then from the layer 10-11 centimeters below the surface. Their aim is to study the downward movement of the mercury, which is “the soil process that leaches mercury from the top layer into the lower layers”, explains Routhier. “We will compare the mercury isotopes between the soil and water to better understand the movement to the water from soils.”
Finally, we navigate another 1.5 kilometer to an island in Anavilhanas to draw sediments from the bottom at two locations. “The sediment contributes to the mercury cycle. Either as a source when it is suspended in the water column, or as a sinkhole, if the particles are fixed in the riverbed,” says Routhier.
Of the 25 collection sites along the river, the Harvard researchers found the mouth of the Branco river, the Negro’s largest tributary, the most intriguing. “There has been a history of gold mining at its headwaters. And because it is a whitewater river, there could be a different signature in terms of the isotopes that are being introduced there,” explains Routhier. Whitewater rivers like the Madeira and Solimões carry more sediments and nutrients and have a higher pH.
“The water in the Branco river is relatively clear, but there is particulate in it, whereas the Negro has a lot of dissolved organic matter, so that is why it is so dark, but very low suspended particulates — most of it is dissolved in the water”, completes the biochemist.
In the Negro river, the organic material that leaks in from the forest is acid, so when it dissolves into the water it lowers the river’s pH. The differences between the samples on the two riverbanks are significant: at the mouth of the Branco, it’s 7-7.09 and in the Negro, it is 5.2-5.22. “The pH is measured on a logarithmic scale, so it is raised to the tenth power. A drop of 1 point is a huge difference, and this is nearly 2,” expresses Routhier. “In the process, it certainly affects the mercury bonds, or if it dissolves in water, and maybe its availability for microorganisms to make methylmercury.”
Villain of the waters: the mercury cycle in rivers
More than once during our trip, people living along the river warned the scientists about pirates on the Negro. A common risk in basins where much illegal mining takes place like the Madeira or along drug trafficking routes and border zones like the Solimões, bandits who steal ships and fuel are a new phenomenon on the Negro river.
It is a threat that lurks over the Barcelos region, which deems itself “the international capital of sport fishing”. During the fishing season — which runs from the end of August to January — one is constantly coming across rafts, aluminum boats and speed boats full of hopeful tourists awaiting their first tucunaré catch. This is the city’s mascot fish. Fishing here is catch-and-release after the traditional snapshot and a chance to weigh and measure.
Near the Mariuá archipelago, we even hear of pirate attacks nearby the spot we had anchored the day before—a village called Dom Pedro II with just a few homes. To be safe, the researchers decide to abandon their afternoon collecting schedule in order to spend the night safely in the port in Barcelos.
But danger still lingers nearby. The following morning as we are preparing to take samples at a spot called Sítio Caioé, a speed boat suddenly appears at top speed. Apprehension. “Hands on your heads!” yells one of the men at the prow of the bullet-proof boat, with their rifles up, as they quickly approach us. They are Civil Police. A second speedboat pulls up immediately afterward.
After verifying that we are researchers and journalists, the captain warns us about violence. Things are risky here: the previous night, pirates had killed a person in the area. In an operation coordinated with the Military Police, they were planning to ambush the criminals. “We’ve been waiting for a few days now,” says the captain (a couple of days later, they found the pirates in the same region. In January, a militar fluvial base was installed in the Branco river mouth in order to scare away the criminals).
But the UEA researchers didn’t let this frightening encounter with the police stop them from finishing their work in the voadeira: they managed to collect samples at 18 sites in just one day, completing a total of 50 sites sampled by the UEA during the expedition. Before nightfall at a spot in the Rio Negro’s straits near a village called Tuyuka, they pulled up on a beach and waited. Meanwhile, the laboratory boat went downstream between the islands of Anavilhanas in the middle of a lightning storm. The sky over the river was already dark when the two groups met up—exhausted by the tiring field work but also pleased with the expedition and the outcomes they had already seen. The preliminary results had met their expectations: few sites produced evidence of water quality abnormalities. “It is a healthy body of water,” observes biologist Fernanda Vieira.
Well-preserved water even in extreme drought
The UEA team’s work produced positive results. The new WQI shows that 18 sites on the Rio Negro have “excellent” water quality, 30 are classified as “good” and two rank as “acceptable”. The average score of the 50 sample sites is 84 points, the maximum of the “good” category.
Based on the data collected on the Negro river, the scientists were able to adapt the WQI so it is ideal for this type of water body. They defined that four of the nine parameters for a blackwater river would have equal weight in the index’s total value, or 15%: pH, dissolved oxygen, thermotolerant coliforms and electric conductivity. Conductivity substituted water temperature, which showed to have little influence on the water quality.
Electric conductivity is an important indicator of pollution, says physical chemist Sérgio Duvoisin Júnior from UEA. In blackwater rivers, the levels are very low because there are very few dissolved ions – result of the natural lack of nutrients. The average score for conductivity was 9 microsiemens, while the healthy limit is 60. “The results for conductivity were very homogenous, which shows us that it is a characteristic parameter for blackwater rivers.
The project has brought scientists, the population and the government a better understanding of the Rio Negro, which “is very healthy,” says Duvoisin. Still, individual analysis of the physical-chemical and microbiological parameters can identify locations where more attention should be paid to basic sanitation on stretches of the river near cities and villages. It is an early diagnosis that could support public policy action at the beginning of the problem. This could result in more effective and less costly solutions. “Now this knowledge should be used so that decision-makers in these small communities can begin taking preventive measures to keep it the way it is—to preserve something that is perfect.”
“From a scientific point of view, we have an unprecedented reference on severe drought,” continues Duvoisin, who points out the importance of continuing the monitoring expeditions. “Even in the strongest drought, the river endured the struggle and maintained its characteristics. It really didn’t affect the water quality so much.”
The Harvard researchers in turn are already planning to continue the project in the region in order to better understand the chemistry of mercury and its dynamics in different seasons, as well as the ways in which carbon and mercury interact.
On our way back to Manaus, biochemist Evan Routhier summed up the expedition. Understanding how local communities are exposed to mercury is vital for public health management, he says, recalling the incredible experience he had in Amazonia.
“This is just the beginning of what could be a long-term study. I hope these data are useful and can guide public health projects for the Rio Negro and local communities,” completes Routhier.
Drought left a longer stretch of white sand exposed by the Negro’s dark waters than usual on the beach in Barcelos. The atypical dry spell was result of an extreme climatic event involving warmer waters in the Pacific due to El Niño together with warming of the North Atlantic driven by the climate crisis.
How this report was produced
Ambiental Media reporters traveled to the Rio Negro with support from Harvard University’s São Paulo office, which is connected to the Brazil Office of the David Rockefeller Center for Latin American Studies at Harvard University.
The scientists listed below are part of the research project headed by Amazonas State University and Harvard University, but were not involved with producing the content developed by our team of journalists according to the ethical directives of independent and transparent journalism produced by Ambiental as detailed in our site’s terms of use (item 6).
Teams
Ambiental Media
Thiago Medaglia – Project director
Ronaldo Ribeiro – editor
Kevin Damasio – text and reporting
Bruno Kelly – photography
Rodolfo Almeida – maps
Luís Lima – front end developer
Laura Kurtzberg – data visualization and maps consultant
Sofia Beiras – design editor
Collaboration: Fernanda Lourenço and Miguel Vilela (editing and coordination)
Amazonas State University (UEA)
Sérgio Duvoisin Júnior – physical chemist, UEA professor and leader of the Applied Chemical Technology (ACT) research team
Rafael Lopes e Oliveira – biologist, adjunct professor at UEA and ACT team researcher
Adriano Nobre Arcos – biologist, post-Doctorate researcher on the UEA ACT team, expedition team and field coordinator
Fernanda Vieira Mattos – biologist, Master’s Degree candidate on the UEA ACT team, expedition field and laboratory coordinator
Douglas Siqueira – biologist and UEA ACT team researcher
Gabriel Rodrigues – chemical engineer and UEA ACT team laboratory technician
José Clodoaldo Oliveira – geographer, Master’s Degree candidate in hydric resources, UEA ACT team researcher
Andreza Guimarães – chemist and UEA ACT team laboratory technician
Fernanda Fraga – biologist and UEA ACT team laboratory technician
Irliane dos Santos – biologist and UEA ACT team laboratory technician
Harvard University
Evan Routhier – biochemist and Sunderland Group PhD candidate
Faiz Haque – Master of Analytic Chemistry and Sunderland Lab researcher
Research team members who did not travel on the expedition
Elsie Sunderland –Sunderland Lab professor of Environmental Chemistry and Earth and Planetary Sciencesa
Scot Martin – professor of Environmental Chemistry and Earth and Planetary Sciences; director of Harvard University’s School of Engineering and Applied Sciences Environmental Chemistry Lab
Other scientists interviewed by journalist Kevin Damasio
Ane Alencar – geographer, scientific director at the Amazon Environmental Research Institute (IPAM), coordinator of MapBiomas Fogo
Cecília Gontijo Leal – biologist and researcher at the University of Lancaster (UK), managing board member of the Sustainable Amazon Network (RAS)