Quantification of methylmercury and geochemistry of mercury in sediments from a contaminated area of Descoberto (MG), Brazil

In 2002, metallic Hg was found buried in a rural area of Descoberto city, Brazil. The origin of the Hg was a gold mining explotation plant established nearly one century ago. Although a number of studies have been conducted in order to assess the contamination of the area, none of them investigated the presence of methylated Hg in the hydric system. In this work methylmercury (CH₃Hg⁺) was determined using gas chromatography-pyrolysis-atomic fluorescence detection (CG-pyro-AFS) in material from rain sedimentation boxes and stream sediments near the contaminated area. Total Hg concentration (HgT) along with the chemical speciation by thermo-desorption were performed. HgT in material from the sedimentation boxes was found to be very high, up to 41,580 μg kg⁻¹, even in the rainy season, when in general HgT were much lower than in dry season. The samples from the Grama and Rico streams show a range of HgT from 5.8 to 266 μg kg⁻¹. The thermo-desorption analysis showed predominance of Hg²⁺, possibly linked to organic sulfur, suggested by a good positive correlation between Hg²⁺, HgT, organic mater (OM) and total S. The CH₃Hg⁺ concentration in stream sediment samples ranged from <0.07 to 1.87 μg kg⁻¹ and in the samples of sedimentation boxes the concentrations were 1.33 and 8.0 μg kg⁻¹ during dry season. The sample with the highest percentage of HgT as Hg²⁺ (98%) presented also the highest percentage of CH₃Hg⁺ (0.7%). These are high values, showing that care should be taken to avoid the transport of this material to the hydrological system. Further studies on the transfer through the food chain would be very important.