Hydrogeochemistry of a small catchment in Northeastern Amazonia: A comparison between natural with deforested parts of the catchment (Serra do Navio,Amapa State, Brazil)

To undertake a comprehensive study of the disturbance effects due to mining activities (manganese ore exploitation) on the chemical composition of dra inage waters, a 164 ha catchment in North-eastern Amazonia tropical forest was sampled weekly for a year period (June 1993 to June 1994) to determine the contrasts in rainwater, throughfall and stream water chemistry between the upper undisturbed area and the lower part affected by deforestation and mining works. During the 3 dry months season only about 15% of the total a nnual input-output of chemical species to and from the catchment occurs. In the three wettest months, about 30% of the flux occurs, except 60% of the potassium and nitrate release from the catchment occurs in these three mont hs. The rain waters are slightly acidic (pH approximate to 5.2) with low al kalinity, while the stream water is nearly neutral (pH approximate to 6.9): alkalinity is generated within the soil and soil water system. The rainfal l-throughfall relationship of the chemical species in the natural areas exh ibits strong internal recycling and little export from the catchment. In th e area changed by deforestation and mining works almost all chemical specie s show a net export in surface waters: these are mainly derived from the de forested area of the basin, where most probably due to enhanced weathering processes because the vegetation cover has to re-establish itself.