Mercury accumulation in sediment cores and along food chains in two regions of the Brazilian Pantanal

The Pantanal is a 140,000 km2 floodplain wetland stretching across western Brazil and parts of Bolivia and Paraguay. Gold mining with mercury (Hg) amalgamation has thrived since 1980 along its northern rim. We quantified Hg accumulation in sediment cores (N = 5) and food chains in this general region of the northern Pantanal and in a reference region, 200 km deeper into the wetland (Acurizal). Cores were dated with 210Pb and 137Cs using direct gamma-assay. Total Hg was analyzed by cold-vapor atomic fluorescence using a gold-mesh pre-concentration trap. Average pre-1940 Hg accumulation in cores was not significantly different (N = 5, p = 0.14) between both regions and comparable with rates calculated for global reference sites. Post gold-rush Hg (post-1980) deposition averaged 55 ± 11.3 µg m−2 yr−1 in the northern impacted region and was more than 1.5 times higher than the post-1980 rate in Acurizal, implying a regional Hg effect of gold mining. Post-1980 Hg accumulation in Acurizal, in turn, was 2.1 times the rate reported for a global reference during that time period, suggesting an additional basin-wide effect over such reference sites. By combining our core data with assessments of the size of the impacted area and the amount of Hg released to the region since 1980, we estimated that only 2–8 % of this Hg was recovered as a sedimentary signal. The remainder of the Hg was lost to the atmosphere, downstream areas, or stored in biota. Hg concentrations in surface sediments in the northern Pantanal (45.5 ± 5.5 ng gdry−1