Is the Negro River Basin (Amazon) impacted by naturally occurring mercury?

In order to investigate the major sources and cycling of mercury in the Negro River Basin (Amazon), total metal measurements were carried out in soils, in river and lake waters, in the atmosphere, and in bulk precipitation during the period 1995 throughout 1998. Median values of 1.3 ng m(-3) in the atmosphere, 172 microg kg(-1) in soils, 4.6-7.5 ng l(-1) in three different lakes, 4.5 ng l(-1) in 17 different Negro River tributaries and 20.3 microg m(-2) year(-1) in bulk precipitation were found. Mercury concentrations in rivers and lakes waters, as well as in soils and bulk precipitation were high, considering the scarcity of anthropogenic point sources in the region. Mercury leaching from soil, the largest regional reservoir of this metal, was considered to be the major pathway to mercury enrichment in the region.