The use of oxygen isotopic ratios to identify natural and anthropogenic mercury concentrations in the Lower Brazilian Pantanal

Abstract

Previous studies in the Amazon Basin and the Upper Pantanal show widespread mercury contamination of fish and water as a result of gold mining, and subsequent volatilization of mercury during the recovery process. A study was initiated to assess whether atmospheric transport of mercury to the south has resulted in elevated levels in water and fish in the Lower Pantanal. Water samples were collected from streams and rivers from the Pantanal (Mato Grosso do Sul) down into the southernmost state of Brazil (Rio Grande do Sul) for total Hg concentration and oxygen isotopic analyses. Based on oxygen isotopes and conductivity, surface water samples can be considered as mixtures of three endmembers: groundwater, rain and ''aged'' surface water. Lowest concentrations (\2 ng/L) of total Hg were observed in the northernmost samples, especially those associated with the groundwater sources. Further south, Hg concentrations tended to increase, but higher concentrations ([2 ng/L) likely reflect direct input from more localized watershed sources. Fish collected from streams in the Pantanal had extremely low concentrations of total Hg regardless of their size or feeding habit, although one large carnivorous fish ([2 kg) had concentrations approaching 400 ng/g wet wt. All concentrations, however, were considerably lower than those observed in similar species from the region of the Amazon Basin affected by gold mining.