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Chemistry

Sea of mercury

Soil and water characteristics favor high concentrations of this metal in the River Negro basin

 

maurício de paivaAn anti-mercury diet: fish and Brazil nut-based food protect riverbank dwellers from the toxic effects of this heavy metalmaurício de paiva

The thousands of inhabitants of the basin of the River Negro, one of the largest and important in the Amazon region, live on top of a gigantic reservoir of mercury. Along almost all of the 1,700 km of its dark waters, it runs through the dense, closed forest, without the least sign of industrial activity or mining. Yet the soil’s mercury levels are almost four times higher than the world average and very close to those found in the contaminated land of a thermometer plant in China. The oddest thing is that the surprisingly high concentration of this silver colored heavy metal, at least in the basin of the River Negro, has nothing to do with panning for gold, which is considered the origin of the contamination in other Amazon region rivers, such as the Tapajos and the Madeira.

Over the last ten years, Wilson de Figueiredo Jardim’s team, from the Chemistry Institute at the State University of Campinas (Unicamp), has been mapping each step of the mercury in living beings and in the different ecosystems of the River Negro, studying everything from the air to strands of hair taken from people living on the banks of the river, who depend for their survival on the fish from its waters. Extensive analyses show that the high levels of mercury in the River Negro and its tributaries are natural. “The area’s geological characteristics created a soil that is naturally rich in mercury”, says Jardim. More important, there is no reason to be concerned about the health of these river-bank dwellers, at least for the time being. Generally toxic (associated with health problems such as a weakening of the muscles, loss of sight and even neurological damage) the mercury in the basin of the River Negro appears not to have affected the health of those who live in the region, judging from the medical monitoring that the team from Campinas has been doing there. The most reasonable explanation for this paradox is in the fish and Brazil nut-based diet, which protects the riverbank dwellers from the harmful effects of the mercury.

Jardim’s interest in studying the River Negro basin arose in the early nineties, when the first studies were published showing high levels of mercury in the fish from this region, historically little affected by mining. Until then, research aimed at tracking the levels of this heavy metal in the rivers of the Amazon region had focused on areas close to gold prospecting sites, such as the basins of the Rivers Tapajos and Madeira, where mercury was considered an important factor in this sort of contamination; in these rivers, the manual extraction of gold frequently uses mercury. Found in its liquid form at environmental temperature, mercury is added to the mixture of sand and gravel from the rivers because it makes it easier to separate the gold. The problem is that this amalgam is subsequently heated for the mercury to evaporate into the air, in order to obtain pure gold. The outcome is that water, soil, plants, animals, gold prospectors and riverside dwellers near gold mining areas are all contaminated.

Medical literature is full of worrying reports about the effects of mercury on human health. The most famous case is that of the neurological problems and congenital defects that affected the inhabitants of the Bay of Minamata, in Japan. Between the thirties and the sixties, the Chisso chemical industry discarded into the sea in this area large quantities of methyl-mercury, the most toxic form of this metal, which is easily absorbed by fish and other aquatic animals. As it is difficult to eliminate it from living organisms, it tends to become increasingly more concentrated in the tissue of carnivorous animals that feed on other mercury-contaminated animals.

Faced with this potentially very dangerous threat, Jardim decided to study further what happened in the basin of the River Negro. The first step was to map out the region’s mercury levels. From September 1995 to November 1998, he and chemist Pedro Sérgio Fadini, now working at the Pontifical Catholic University of Campinas, collected samples from the soil, the air, rivers and lakes from approximately two thirds of the basin of the River Negro, stretching from Santa Isabel do Rio Negro, a town near to the border with Venezuela, to near Manaus, where the Negro joins the Solimões to form the Amazon River. Detailed in a series of scientific articles published over the last few years, the results confirm the presence of abnormally high levels of mercury in the basin of the River Negro.

In the case of the rivers and lakes, for example, Jardim and Fadini calculate that the metal’s concentration is three to four times greater than that expected for such remote areas of the planet. It ranges from 4.6 to 7.5 nanograms per liter of water in the lakes and reaches an average of 4.5 nanograms per liter in 17 of the Negro’s tributaries. These are levels close to those of Lake Michigan and of the waters of the Adirondack Mountains, in the Great Lakes region in the United States, areas with a high population density and major industrial activity. This similarity with industrialized regions in the Northern hemisphere was also maintained when the researchers analyzed the amount of mercury in the air: in the basin of the River Negro there are 1.3 nanograms of mercury per cubic meter of air. In the North American state of Wisconsin, in the Great Lakes area, this rate is 1.6 nanograms per cubic meter of air.

In the Brazilian case, the origin of these high levels of mercury is now clear: the region’s soil. Each kilo of soil contains, on average, 172 micrograms of mercury, almost four times more than the levels considered normal for soils elsewhere. This is a rate very close to the 200 micrograms per kilo of soil found in a polluted area close to the mercury thermometer plant in China.

The other possible source of the mercury in the basin of the River Negro – its use in gold prospecting or industrial activities – has been practically discarded by the researchers, because it is absurdly high. Considering just the most superficial layer of soil, around 1 meter deep, Jardim and Fadini calculate that there are approximately 126,000 tons of mercury in the region. “It’s improbable that all this mercury has accumulated there because of human activity”, says Jardim. To get an idea of what all this mercury represents, it is estimated that over the last 30 years of mining activity in the whole of the Amazon region, some 3,000 tons of the metal have been released into the environment. What is found today in this particular area of the Amazon corresponds to much more than all the mercury used in mining by Spanish-speaking countries over four hundred years, from the sixteenth to the twentieth centuries.

Bioaccumulation
Just the presence of mercury in the soil, however, does not explain the high concentration of this metal in fish and in the organism of river-bank dwellers. “The composition of the waters of the River Negro creates a unique effect on the mercury cycle in nature”, explains Jardim. This process also depends on sunlight, as found by Gilmar Silvério da Silva, who did his doctorate under Jardim. Every year during the rainy season, when the river overflows and floods the forest, its dark acid waters, rich in chemically degraded organic material, receive a huge amount of young organic material. With the action of sunlight, this young organic material produces hydrogen peroxide, which helps oxidize the mercury and transform it into a more reactive form that remains in the water and escapes into the atmosphere. By the action of bacteria, this form of mercury generates methyl-mercury, which easily enters the aquatic food chain and starts accumulating in the bodies of fish, a phenomenon known as bioaccumulation.

In collaboration with biochemist José Doréa, from the University of Brasília (UnB), Jardim proved this bioaccumulation. They studied fish that represent the whole of the river’s food chain hierarchy: fish that eat just plants, fish that feed on detritus and fish that eat a bit of everything and also eat other fish. As expected, of the 951 examples analyzed, those with the highest levels of mercury were the fish that eat other fish (piscivorous), such as the piranhas, tucunarés and mandis, which are at the top of the food chain and more susceptible to the concentration of mercury in their organisms. They contained, on average, 690 nanograms of mercury per gram of weight, a figure far higher than what is regarded as safe for human consumption (500 nanograms per gram).

The consequence is that the inhabitants of the basin of the River Negro have a mercury level in their bodies, identified from hair samples, that is greater than among those who eat fish from the sea. For Jardim, however, there is no reason for panic or to avoid eating fish from the River Negro. So far, no harmful effects resulting from this consumption by humans has been irrefutably detected. “The results of the cognitive tests that seemed to indicate this harmful effect are very closely linked to the level of formal education of the population and, therefore, their reliability is low”, says Jardim. Furthermore, the Amazon diet itself offers a mercury antidote. The river-bank dwellers eat a lot of Brazil nuts; like the fish, these are rich in the chemical element selenium, which protects against the harmful effects of mercury. Another reason not to eliminate fish from the diet of these people is that it is their staple source of protein. “It would be a lot worse for their health if fish was eliminated from their diet”, Jardim states. “We even tried suggesting to people that they avoid eating piscivorous fish, but the reply normally is “Look, whatever I catch I’m going to eat.” Until concrete evidence of the harmful effect of mercury is found in this region, the best alternative, in Jardim’s opinion, is to monitor the population’s health.

The Project
Mercury flows in the basin of the River Negro, in the Amazon (nº 00/13517-1); Modality Thematic Project; Coordinator Wilson de Figueiredo Jardim – IQ/Unicamp; Investment R$ 592,059.79