Distribuição, comportamento e definição de valores de background do mercúrio em sedimentos da bacia hidrográfica do rio Tapajós, Pará, Brasil

Abstract

Geochemical mapping and background determination help identify anomalies in the environment and distinguish natural or anthropogenic sources. Mercury (Hg) is a worrying element because it is conservative and can bioaccumulate and biomagnify. In the watershed Tapajós River, miners apply Hg to recover gold, this Hg can be released into the environment together with Hg from the soil, resulting in contaminated areas, associated with forest suppression, exposing the soil to erosion. In the study carried out, 343 samples of river sediments were collected and analyzed by SGB/CPRM in 2005/2006, with a sampling density of 1/135 km². The Hg content in the samples was analyzed by ICP-OES with a hydride generator. Hg values ranged from 0.01 to 4.46 mg.kg-1, and the geochemical background established for the area was 0.48 mg.kg-1, close to the probable effect level, 0.486 mg.kg-1. When classifying the samples by lithology, namely, alkaline granitoids, calcium alkaline granitoids, sedimentary rocks and gneisses and volcanic rocks, the mMAD showed the respective results, 0.69; 10.19; 0.03 and 0.77 mg.kg-1 for the upper thresholds and 0.01, 0.00, 0.02 and 0.00 mg.kg-1 for the lower threshold. The use and occupation classification, namely, Pasture and Forest presented mMAD values of 6.10 and 0.10 mg.kg-1 respectively for the upper thresholds and 0.00 and 0.01 for the lower thresholds. It should be noted that such a calculation for the mining area cannot be carried out due to the significantly low percentage of samples above the lower detection limit of the analytical method, suggesting high concentrations of Hg only in specific samples. The Enrichment Factor (EF) revealed a significant increase in mercury (Hg) in areas with greater human influence in the interpolation maps, identifying iron manganese oxyhydroxides and clay minerals as the main components responsible for Hg adsorption. The Contamination Factor Maps (CF) identified the same areas as contaminated that the FE identified as enriched. The studied region is naturally enriched in Hg in relation to other biomes, due to the following factors: 1° geogenic, associated with rock erosion and the process of tropical weathering, which leads to the formation of duricrust and lateritic soil, causing Hg to accumulate efficiently in the soil; 2° the local anthropic factor, acting on soil degradation, promoting the conversion of forests into pastures and mining areas, remobilizing and dispersing Hg. Considering the scale of approach of this work, a greater impact related to land use change was observed. A more detailed approach is suggested, such as a greater number of samples on a larger cartographic scale to define local impacts.