Mercury sink in Amazon rainforest: soil geochemical data from the Tapajos National Forest, Brazil

Atmospheric mercury sink in Amazonian rainforest was tested in this work. Forest soil was analyzed for 11 soil profiles at depths of 0–40 cm in the northern portion of the Tapajos National Forest, Brazilian Amazon. Major oxides and Hg contents in soil and rock were determined by XRF and by Zeeman AAS, respectively. A mercury content of 146 μg kg−1 was found in the bedrock, whereas the mercury content in soil averaged 240 μg kg−1. The soil pH varied from 4.4 to 5.2 and the organic matter contents varied from 1.4 to 7.5%. Enrichment factors of Hg for soil were calculated in relation to the parental bedrock assuming aluminum as immobile element. The lower soil layers presented mercury enrichment of 29–98% in comparison to the bedrock. The upper soil layers showed mercury enrichment of 4–24% in comparison to lower soil layers. Mercury in soil was highly correlated with Al, Fe, and Ti; however, no significant correlation was observed between Hg and organic matter contents. All results pointed to mercury retention in forest soils as a result of the combined action of rock weathering processes and continuous input of metal from the atmosphere, estimated to be between 31 and 46% of the total Hg in the upper soil layers. These results are consistent with the hypothesis that the Amazon rainforest is a sink for atmospheric Hg and they also highlight the role of rainforest as a barrier for a number of substances associated with suspended atmospheric particulate.