Balanço de massas de mercúrio (Hg) total em duas microbacias da Amazônia Central

Abstract

The effect of soil podzolisation on the biogeochemical cycle of mercury (Hg) was investigated in the Central Amazon through a mass balance study of total mercury in two small stream catchments located near the city of Manaus, Amazonas State, Brazil. The first catchment basin drains the clear water stream Barro Branco in the Adolpho Ducke Forest Reserve and is composed of oxisols in the process of mild podzolisation. The second catchment drains a Campina stream located in the Campina Forest Reserve and is composed of spodosols, which suffer from intense podzolisation. Samples of open rainwater, throughfall, groundwater and surface stream water were collected and monitored for total mercury levels during the period from April, 2005 to March, 2006. Total mercury stocks along margins of both streams were also evaluated by way of transects traversing topographical gradients (upland, slope and riparian terrain). In both basins the fine fraction (< 63 μm) presented larger concentrations of total Hg than the gross fraction (>63 μm). The integrated stocks of total Hg in the top 30 centimeters of soil were 1.3 times greater in the oxisols of the Barro Branco catchment (23 mg/m2) than in the podzols of the Campina basin (18 mg/m2). The forest canopy of the Campina intercepted the Hg deposited from the rainwater more efficiently than the canopy of terra-firme forest of the Ducke Reserve. The total mercury levels in the open rainwater, leaf drip and stemflow water samples increased during the months of June, July, August and September, which directly correlated to the annual peak in forest fires observed for the State of Amazonas in 2005. The groundwater samples from riparian areas in both basins presented the largest concentrations of total mercury. The black water of the Campina stream presented greater total Hg concentrations (13 ng/L) than the clear water stream Barro Branco (9 ng/L). The annual export from the Campina stream (20 μg of Hg/m2/year) was two times more elevated than the Barro Branco stream (10 μg of Hg/m2/year). However, both streams exhibited the highest export flux during the rainy season. The mass balance results demonstrated that the export flux from Barro Branco stream was much less than the atmospheric deposition in its basin (24.4 μg of Hg/m2/year). In contrast, the export flux in the Campina stream closely resembled the basin s atmospheric deposition (21.7 μg of Hg/m2/year). Two hypotheses were proposed regarding the pedological function of the export of mercury: 1) the gradual transformation of an oxisol to a podzol can result in the liberation of old mercury deposits due to chemical and physical intemperism in the soil, or; 2) the retention capacity of recently deposited Hg is simply altered, facilitating its export to the fluvial system. However, it was not clear whether exported mercury in both streams was of recent origin due to atmospheric deposition or of Hg deposits which had accumulated in the soils over a long period of time. The effect of soil podzolisation, influenced by lateral leaching of spodosols in humid systems which possess oblique drainage patterns, appears to be an important controlling factor for mercury dynamics in different soils of the Central Amazon.