When a part of the Amazon forest is invaded and an area of animal pasture is established, man does not only produce an impact on the climate and the biodiversity. He causes another alteration in the ecosystem, still little understood: rivers of small and medium size begin to have stretches unquestionably changed, tending to become polluted due to the substitution of the forest for cattle ranches. Studies carried out in the state of Rondônia reveal that the electrical conductivity – the quantity of ions, groups of atoms with positive or negative electrical charge, present in the water, a parameter used to infer the general quantity of dissolved substances – of some of the rivers of the Ji-Paraná river basin, the largest in the state, has already reached levels similar to that of some contaminated rivers in the interior of São Paulo.
The data indicates that in the areas without abundant vegetation, part of the carbon and of the nutrients – above all phosphorus and nitrogen – that normally would be absorbed by the forest and by the soil end up getting into the rivers, washed down by rains and by the erosion of the river banks. In high concentrations, these compounds work like river pollutants and can be detected even in stretches situated out of deforested areas. There are not scandalously visible impacts yet, such as dead fish and large floating dregs, but the contamination of the rivers is a reality at some points.
In the tributaries Rolim de Moura, Urupá and Jaru, situated in the middle portion of the Ji-Paraná basin, in the heart of the most devastated zone in Rondônia, the conductivity levels vary from 50 to 100 µS/cm3 (microsiemens per cubic centimeter of water), values up to twenty times higher than those in stretches less impacted in the river basin. The highest conductivity levels happen during the dry season (September), when the ability of the rivers to dilute the pollutants with the assistance of rain is smaller.
“The highest levels probably reflect man’s action in the region and are of the same order of those verified in fairly polluted rivers such as the Atibaia, one of the tributaries of the Piracicaba river, in the state of São Paulo”, says Reynaldo Victoria, of the Nuclear Energy in Agriculture Center of the University of São Paulo (Cena/USP), in Piracicaba, the coordinator of the biogeochemistry of the waters of the LBA (Large Scale Experiment on the Biosphere-Atmosphere in the Amazon) and of a thematic project financed through FAPESP about the impact of the changes in the use of the soil on the biogeochemistry (biological, geological and chemical processes) of the rivers in the Ji-Paraná basin.
The values of the conductivity in the above mentioned three tributaries is both worrying and predictable. They have a slow flow rate, are close to towns and consequently accuse more easily the ions presence in their composition. It is not by chance that exactly in this group of rivers the levels of chloride, an indicator of possible pollution by sewage or industrial waste, were the highest in the basin, again getting close to the numbers of the Atibaia river. For this reason, the electrical conductivity in this stretch of the basin is not a surprise. What really caused a shock were the levels found all the way along the Ji-Paraná basin at five measuring points the conductivity reached average levels of between 25 and 50 µS/cm3. What does all of this mean?
Let’s take it one step at a time. In three points of intermediate portion, where the river had not reached its maximum in terms of water volume yet, the poor result doesn’t scare. After all, in these sectors, highly modified by human action, the Ji-Paraná snakes through zones in which the forest has been frequently cut down, giving way to pastures and, on a smaller scale, urbanization has occurred (such as the city of Ji-Paraná). Consequently, it was to be expected that this stretch would clearly register the effects of the brutal change in the occupation of the soil. “To a large extent, a river is a reflection of the countryside through which it flows”, commented the researcher.
At the other two sampling collection points, in the final portion of the river basin, the story is different. By having the same the measurements verified in the three previous collection points, the levels of conductivity in these stretches are a signal that the river is really in a bad state. This is because at this stage the Ji-Paraná river basin runs through preserved forest, having already achieved its maximum flow rate, some twenty to thirty times greater than at its starting headwater. Due to all of this, the researchers believed that the river – at this very voluminous point and presumably with a greater capacity for recovery – had the conditions to easily absorb the impacts of human action, in the previous intermediate portion.
Through this thinking, close to its mouth the Ji-Paraná should have a conductivity of between 5 and 25 µS/cm3, the same as its cleaner tributaries, equally situated in areas of low impact – the Machadinho, Preto and Comemoração rivers and the final stretch of the Pimenta Bueno. “The problem is that the contamination sign by ions persists, a piece of evidence that the river no longer totally supports the impact of the change”, comments Reynaldo Victoria. If one day the signal becomes so strong as to be detected in the Madeira, a river about to twenty five times more voluminous than the Ji-Paraná, possibly the river basin will have been totally altered.
In 1999 in order to evaluate the impact of the change in the use of the soil in each sector, the researchers set up fourteen water sample collection points along the basin. They chose areas with low, medium and high levels of alteration in the use of the land. In this manner the Cena scientists could then compare what happens to a river in a highly deforested zone, to another fairly devastated and finally in an area of preserved forest.
The ideal situation would be to collect samples monthly from January to December. This would have permitted a practically uninterrupted monitoring of the waters during a water cycle, minimizing seasonal phenomena and effects. For example, the same concentration of a chemical component means something during the rainy period and another during the dry season. However, since twenty days of field work are needed to collect samples at the fourteen points of the basin and the main researchers live in the interior of the state of São Paulo, thousands of kilometers away, this option was unfeasible.
“Therefore we considered another alternative”, comments the biologist Alex Krusche from the Cena. The solution found was to set up a timetable for four years of work, from 1999 until 2002: during this period the researchers programmed twelve trips to Rondônia, always during different months. In this manner, they would have data on a hydrological hybrid year. For example, the data on February was with respect to February of 2000, and of May referred to the collection of 1999, those of December to 2001, and so on. Nine trips have already taken place.
When they find some parameter apparently altered, such as high levels of conductivity in stretches of the basin, a doubt comes up: do these numbers really reflect a change in the working of the soil, a change in the areas from forest to pasture and towns, or are they a natural phenomenon? To form an opinion, one firstly needs to know in details the area and to understand how the components of the system interact. Parallel to the collection and analysis of the water samples, it is essential to prepare a complete radiography of the region.
This has been done. Producing new data and organizing old information, the scientists determined the physical characteristics of each stretch – its geomorphology (depressions, lakes, deltas, plateaus, valleys, etc.), relief (altitude for each stretch), drainage network and chemical composition of the soils. They also accounted for the marks left by man – mapped the number of inhabitants of the towns, the use of the soil and the vegetation cover. In general, the soils of the Ji-Paraná system are poor, with few ions. Subsequently, one expects to find low levels of electrical conductivity in the waters. When this does not occur, such as in the case of some tributaries, then some human activity must be behind the anomaly. In Rondônia, the origin of the excess of ions seems to be associated with pasture land, not the forest.
Besides measuring the conductivity, the chemical composition of the waters was analyzed. Preliminary results show a relevant statistical correlation between the percentage of pasture areas of each sector and the concentration in the water of dissolved chlorides, sulfates, sodium, calcium, magnesium , organic and inorganic carbon, as well as carbon dioxide. It has been observed that the increase in the pasture areas made the levels of these dissolved compounds grow, but it cannot be affirmed what is the proportional growth of each compound yet. There is an intriguing confirmation: the levels of dissolved oxygen in the rivers of Rondônia, even those most impacted, keep themselves relatively high when compared with the polluted rivers of the interior of the state of São Paulo. “We still have to study and understand lots of biogeochemical processes in the region”, observes Reynaldo Victoria.
The main goal is to understand how the substitution of the forest areas by pasture and towns alters the distribution of carbon and nutrients in tropical rivers. Another goal is to follow the change in the signals emitted by these substances as they pass from a micro scale water system (small rivers or streams) onto a medium scale (a river basin such as the Ji-Paraná). The studies on the micro scale are carried out on the rivers that cut through the Nova Vida Ranch, close to the town of Ariquemes, in the northeast of the state. Its area of influence is around to seven times smaller than that of the Ji-Paraná river basin. Here the researchers verified, as it was expected, the existence of different carbon isotopes in the waters that cut through areas deforested to make way for pasture and through stretches of preserved forest.
On crossing cattle zones, the river reveals carbon of isotopic composition characteristic of the plants of the photosynthesis cycle C4, of tropical grasses (pasture). In sectors with preserved forest, the predominant carbon through the photosynthesis cycle proved to be C3, due to the forest trees. “We’re trying to understand why the carbon indicator for the grasses disappears in mixed stretches where there’s forest next to pasture”, adds the researcher. In these stretches the influence of the pasture area appears to be neutralized by the proximity to the native forest, but as yet no one knows why this is occurring.
A river basin of ideal size
The Cena researchers handpicked the forward point of the Ji-Paraná river basin to study alterations in the biological, geological and chemical cycles of the Amazon rivers. In the 80’s, Reynaldo Victoria had participated in similar studies in the main channel of the Amazon and its tributaries, which, in some stretches, as in the vicinity of the city of Manaus, pass through areas clearly modified by human action. The results produced close to one hundred scientific articles, but were not conclusive in the detection of alterations in the rivers through the action of man.
The immense size of the hydrographic system – 20% of the fresh water of the planet passes through the Amazon basin – inhibited the perception of signs of contamination. The solution was to look for a basin of medium size covering deforested areas, where the consequences could be more easily measured in the waters of the rivers.
With more than 75,000 km² of extension, the Ji-Paraná river basin has peculiarities that make it ideal for the study in question. It cuts through all of the state, covers forty five cities and it is inhabited by 560,000 people, two thirds of them in the intermediate portion of the basin. The river rises in the southeast of Rondônia, winds through the central-west portion and ends in the northeast. According to 1999 data, almost 30% of the area of the basin has been turned into pasture land, the main fate to the sectors of the forest that were cut down.
The tropical rain forest covers a little more than 60% of the basin, the reforested areas almost 5%, and there is a small stretch of cerrado (wooded savanna) 0.7%. The remainder is occupied by urban areas (0.5%), exposed soils, water streams and plantations. The main urban occupation of the state is formed around the BR 364 highway, which cuts across the State. “The Ji-Paraná is didactic”, points out Reynaldo Victória: “It exhibits all of the levels of the alteration in the use of soil, from the lowest to the highest”.
Alterations in the Dynamics of Organic Material in Rivers of Micro and MediumScale in the State of Rondônia, as a Function of Changes in Soil Usage (nº 99/01159-4); Modality Thematic Project; Coordinator
Reynaldo Victoria – Cena/USP; Investment R$ 565,456.97 and US$ 427,226.95