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Geologic observation of natural and artificial recovery processes of Brazilian tropical forest destroyed by debris flow and bauxite mining.

Akihisa Motoki & Thais Vargas *

* Department of Mineralogy and Igneous Petrology, Rio de Janeiro State University (DMPI/FGEL/CTC/UERJ), Rua Sao Francisco Xavier 524, Bloco A, Maracana, Rio de Janeiro, Brazil. e-mail: motoki@uerj.br

International symposium on application of natural materials for environmental goetechnology, Tokyo, Oct.29 - Nov.2.

Abstract

Destruction and recovery processes of tropical forests are highly dependent on local geologic condition, especially of regolith, and detailed study of this subject is important, not only for natural disaster mitigation but also for global worming prevention. This paper presents geologic observation of natural and artificial recovery examples of Brazilian tropical forests.

During the pluvial impact in 1996, many debris flows and landslides grooved the Tijuca Forest, State of Rio de Janeiro. The forest is underlain by late Precambrian biotite gneiss and granite. Due to textural and compositional vulnerability against tropical weathering, the gneiss area is covered by thick regolith, up to 30m. The debris flows and subsequent deforesting took place only in the gneiss area. According to the mode of occurrence, the deforested zones are classified in three types: valley bottom; mountain slope; rocky scarp. The first type deforest zones occurred along valley bottom of gentle dip, 15-25 degrees, lower than the stable angle, and are long and narrow, 200-300m x 8-20m. Due to abundance of surface water, remnant regolith, and organic materials, the zones were recovered by natural process within several years. The second type took place on mountain lateral slopes of high angle, 30-35 degrees higher than the stability angle, and are characteristically wide, 200-300m x 40-160m. Because of poor remnant regolith, the forest recovery has not completed yet even the present, 15 years after the deforesting. Therefore, small civil engineering works for soil retention and artificial reforesting are highly desired. The third type occurred on steep slopes near the mountain top, superior to 50 degrees. There, large rock exposures are now cropping out and natural recovery process is not in progress.

Bauxite mining have frequently been classified to be violent and harmful from the viewpoint of natural environment preservation. However, at the CBA Mine (Companhia Brasileira de Alumínio), Poços de Caldas, State of Minas Gerais, the problem is going to solution, at least partially, by means of "A-horizon recycling method". Before the mining, the A-horizon, composed of black surface soil, is removed from the mining area and once transported to deposit area. After the mineral explotation, the A-horizon materials return to the original area and cover there again. After that, artificial plantation is performed on the recycled soil, so rapid forest recovery can be accomplished. During the tree growth, atmospheric CO2 is absorbed by plants and transformed it into bio-mass. After complete recovery, the forest provides raw material to paper industry. The area is reforested again, therefore, the bauxite mine area continues absorbing CO2.

Generally, stable forests, as of the Amazonia, does not absorb CO2. To minimize problems caused by global worming, reforesting must be motivated in world scale. Even some industrial countries do not achieve the reduction objectives of CO2 emission, the remnant amount can be absorbed by reforesting, either in their own country or in the foreign countries. This effect must be considered to be equivalent to the CO2 reduction.


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