Surface water quality and deforestation of the Purus river basin, Brazilian Amazon
In the last years, deforestation constitutes a threat for the aquatic ecosystems. This paper aims to characterize the water quality of the Purus river in the Brazilian Amazon, and investigate the relations between water quality and deforestation of the Purus river basin over a 9-year period, as well as to quantify the Purus river basin’s land cover changes (%) in a 5-year period. Sampling data from upstream to downstream show a decrease in pH-value, dissolved oxygen, electrical conductivity, and total suspended solids. Correlation analysis revealed a significant negative correlation of the accumulated total deforestation values (km2) with the pH-value (in all the study sites), and a significant positive correlation with temperature (only in two sites). However, the deforestation rates (km2/year) did not present, in none of the study stations, any significant correlation with water quality parameters. It seems that the effects of deforestation on water quality are related not with the rate but with the total area deforested. It was estimated that the basin’s forested area decreased by 5.17%. Since similar attributes are common in other basins of the whitewater systems of the Brazilian Amazon, this results may be seen as a warning on the effects of deforestation on water quality (reduction in pH and increment in temperature values), in larger areas than those of our study sites. To maintain the conservation and preservation status of the Purus river basin, it is necessary, the implementation of a transboundary watershed management program that could serve as a conservation model for Brazil and other countries of the Amazonian region.
KeywordsAmazon basin Deforestation Purus Water quality
The Amazon, that presents different characteristics among others Brazilian hydrographic areas, is one of the world’s unique regions containing most of the usable water resources in Brazil. This water potential displays privileged place into the speeches on Amazonian sustainable development. Water chemistry provides important parameters for quantifying biogeochemical cycles and determines management options in river systems and wetlands. The first scientific classification of Amazonian rivers was elaborated by Sioli (1956) who used water color, transparency, pH and electrical conductivity to explain limnological characteristics of the large Amazonian rivers and correlated these characteristics to the geological properties of the river catchments, a landscape ecology approach. Whitewater rivers (such as the Amazon, Juruá and Madeira) are turbid and have their origins in the Andes, from which they transport large amounts of nutrient-rich sediments. Their waters have near neutral pH and relatively high concentrations of dissolved solids indicated by the electric conductivity that varies between 40 and 140 μS cm−1. Blackwater rivers (such as the Negro River) drain the Precambrian Guiana shield, which is characterized by large areas of white sands (podzols), their waters show low quantities of suspended matter but high amounts of humic acids that give the water a brownish-reddish color. The pH values of such rivers are in the range of 4–5 and their electrical conductivity is <20 μS cm−1. Clearwater rivers (such as the Tapajós and Xingu rivers) have their upper catchments in the Cerrado region of the Central Brazilian archaic shield. The transparency of their greenish waters is above 1.5 m, with low amounts of sediments and dissolved solids, electrical conductivity is in the range of 10–20 μS cm−1, and pH that varies between 6 and 7 in large rivers.
Ríos-Villamizar et al. (2014) classified a number of rivers using the combination of several parameters such as alkali (Na, K) and alkali-earth (Ca, Mg) metals, major anions, electrical conductivity, pH, Ntotal, Ptotal, water color and suspended sediment load to distinguish the three classical water types (white, black and clear) and to separate other water bodies with intermediate position, concluding that many rivers and streams have to be considered as mixed waters resulting from the influence of lower order tributaries with different physico-chemical properties of their waters. According to this classification system, the Purus river is a whitewater river but many of its tributaries are of intermediate type (Ríos-Villamizar et al. in preparation) if considering that the geology of the pre-Andean zone is rather heterogeneous with large old sedimentation areas (paleo-varzeas), which sediments are strongly weathered, but still have a higher bioelement content than the tertiary sediments of the central Amazon basin and the soils on the archaic shields.
Materials and methods
Results and discussion
Water quality trends
Relations of the water quality with deforestation variables
Deforestation levels for the municipal districts of the study (PRODES 2007)
Boca do Acre
Correlation coefficient analysis among DR, ATD and water quality variables in the Purus river basin (2000–2006)
Land use changes
Land cover variables percents in the Purus river basin on 2003 and 2007
Land cover variables
The analyzed waters of the Purus river basin showed typical characteristics of Amazonian natural waters. We observed clear trends in changes of physico-chemical properties of water from upstream to downstream of the Purus river such as decrease in pH-value, dissolved oxygen, electrical conductivity, total suspended solids, and increase in turbidity. Only the pH-value presented significant negative relationships with the ATD in all the study sites, indicating that increase in the ATD will contribute to decrease the water pH-value. On the other hand, the ATD values correlated positively and significantly with changes of temperature only in stations 2 and 3, indicating that increase in the ATD will contribute to increase the water temperature. The DR did not present, in none of the study stations, any significant relation with the water quality parameters. Then, the deforestation levels have not yet caused a large effect on the water quality, which is probably most influenced by hydrological and climatic factors such as river discharge, river level, and pluvial precipitations, among others; but this will be addressed in forthcoming papers.
It seems that the effects of deforestation on water quality are related not with the rate but with the total area deforested, and they become apparent after a minimal size of deforested area is achieved.
Therefore, despite the good conservation status for most parts of the Purus river basin, impacts on water quality caused by human activities are evident, especially close to urban areas, at a local scale. Since similar attributes are common in other basins of the whitewater systems of the Brazilian Amazon, this may be seen as a warning on the effects of deforestation on water quality in larger areas than those of our study sites. Actions to control the deforestation in this basin would need to be taken to maintain its conservation status and, in view of that, it would be necessary to implement a transboundary watershed management program for conservation and preservation purposes, and this program could serve as a conservation model for Brazil and other Amazonian countries.
This work was funded by Brazilian National Scientific Council (CNPq), Tropical Forest Protection Program (PPG-7), Grant Number 556899/2005-9. We thank the postgraduate Program of Environmental Sciences and Sustainability in the Amazon (PPG/CASA/UFAM), the postgraduate Program in Climate and Environment at the National Institute of Amazonian Research (INPA/UEA), the Ecology, Monitoring and Sustainable Use of Wetlands Group (MAUA/CDAM/INPA), CAPES/CNPq—IEL Nacional—Brasil, Programa de Apoio à Fixação de Doutores no Amazonas (FIXAM/AM) and Programa de Apoio à Participação em Eventos Científicos e Tecnológicos (PAPE), from the Fundação de Amparo à Pesquisa do Estado do Amazonas (FAPEAM/SECTI/AM) for financial support. We also thank Kyara Martins Formiga by suggestions and elaboration of the study area map.
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