RESUMO

Mercury (Hg) emissions from forest fires, especially tropical forests such as the Amazonian forest, were shown to contribute significantly to the atmospheric mercury budget, but new methods are still necessary to improve the traceability and to reduce the great uncertainties related to this emission source. Recent studies have shown that the combustion process can result in Hg stable isotope fractionation that allows tracking coal combustion Hg emissions, as influenced by different factors such as combustion temperature. The main goal of the present study was, therefore, to investigate for the first time the potential of Hg stable isotopes to trace forest fire Hg emissions and pathways. More specifically, small-scale and a large scale prescribed forest fire experiments were conducted in the Brazilian Amazonian forest to study the impact of fire severity on Hg isotopic composition of litter, soil, and ash samples and associated Hg isotope fractionation pathways. In the small-scale experiment, no difference was found in the mercury isotopic composition of the samples collected before and after burning. In contrast, the larger-scale experiment resulted in significant mass dependent fractionation (MDF δ202Hg) in soils and ash suggesting that higher combustion temperature influence Hg isotopic fractionation with the emission of lighter Hg isotopes to the atmosphere and enrichment with heavier Hg in ashes. As for coal combustion, mass independent fractionation was not observed. To our knowledge, these results are the first to highlight the potential of forest fires to cause Hg isotopic fractionation, depending on the fire severity. The results also allowed to establish an isotopic fingerprint for tropical forest fire Hg emissions that corresponds to a mixture of litter and soil Hg isotopic composition (resulting atmospheric δ202Hg, Δ200Hg and Δ199Hg were -1.79 ± 0.24‰, -0.05 ± 0.04‰ and -0.45 ± 0.12‰, respectively).

Assuntos

RESUMO

Although high levels of dissolved arsenic were detected in surface and ground waters of Nhecolândia, a sub-region of the vast Pantanal wetlands in Brazil, the possible sources have not been clearly identified and the potential release from the wetland to the draining rivers has not been investigated. In this study we measured the dissolved As content in all the rivers and small streams that supply the southern Pantanal region, as well as in the two main rivers draining the wetland, i.e., the Cuiaba and Paraguay rivers and tributaries. In addition, Arsenic in surface waters, perched water-table, soils and sediments from 3 experimental sites located in the heart of Nhecolândia were compared. On the one hand, the results show the absence of As contamination in rivers that supply the Pantanal floodplain, as well as a lack of significant release from the floodplain to the main drains. The As contents in the rivers are <2 µg L-1, with variations that depend on the lithology and on the geomorphology at the collection point (uplands or floodplain). On the other hand, they confirm the regional extension of As contamination in Nhecolândia's alkaline waters with some values above 3 mg L-1. Arsenic is mainly in the arsenate form, and increases with the evaporation process estimated from sodium ion concentrations. The pH of soil solution and surface water increases rapidly during evapo-concentration up to values above 9 or 10, preventing adsorption processes on oxides and clay minerals and promoting the retention of dissolved arsenic in solution. Solutions from organic soil horizons show higher As contents in relation to Na, attributed to the formation of ternary complex As-(Fe/Al)-OM. In this alkaline pH range, despite high levels of dissolved As, soil horizons and lake sediments in contact with these waters show As values that correspond to uncontaminated environments.

RESUMO

The objective of this work was to assess the flux of atmospheric mercury transferred to the soil of the Amazonian rainforest by litterfall. Calculations were based on a large survey of published and unpublished data on litterfall and Hg concentrations in litterfall samples from the Amazonian region. Litterfall based on 65 sites located in the Amazon rainforest averaged 8.15 ± 2.25 Mg ha(-1) y(-1). Average Hg concentrations were calculated from nine datasets for fresh tree leaves and ten datasets for litter, and a median concentration of 60.5 ng Hg g(-1) was considered for Hg deposition in litterfall, which averaged 49 ± 14 µg m(-2) yr(-1). This value was used to estimate that in the Amazonian rainforest, litterfall would be responsible for the annual removing of 268 ± 77 Mg of Hg, approximately 8% of the total atmospheric Hg deposition to land. The impact of the Amazon deforestation on the Hg biogeochemical cycle is also discussed.

Assuntos