Mercury accumulation in bottom sediments on Trombetas river biological reserve (REBIO), Oriximiná city, Brazil

Abstract

In the Amazon, mercury (Hg) occurs naturally and by gold mining emissions. Bottom sediments are important for environmental investigations as they provide information about the entry of Hg into the aquatic ecosystem. In Porto Trombetas is situated the bauxite mining area by Mineradora Rio Norte in the Saracá-Taquera National Forest area, near traditional communities and REBIO. Thus, considering the occurrence of impacts, it is intended to evaluate the accumulation of Hg contributing to understand part of its dynamics in this place without gold mining. The objectives are: to evaluate the accumulation of Hg in bottom sediments in the area of the Trombetas River biological reserve; quantify Hg levels in bottom sediment and characterize the environmental quality of sediments according to the Hg levels found with those described in legislation for non-impacted areas; Correlate the levels of Hg in bottom sediments with seasonal variation. The study area is the Trombetas River basin, located in Oriximiná, Pará, Brazil. Surface bottom sediment samples were collected at 30 points across the basin, in four samplings in May and September 2016 and March and June 2017. The samples were stored in plastic bags, preserved and taken to SAMAM. Afterwards, the samples were dried, sieved and subjected to acid opening and analyzed with Cold Vapor Atomic Absorption Spectrometer. Results show statistically non-significant seasonal variation in Total Hg (HgT) concentrations, but it was statistically significant spatial variation. Spatial variation can be explained at the highest concentration points by coinciding precisely where the bauxite mining activities are concentrated. According to the TEL and PEL values and CONAMA Resolution No. 344/2004 all points presented concentrations below level 02 (PEL), when adverse effects on the biota are almost unlikely to occur. Some points presented average concentrations above the range for uncontaminated Amazon rivers. Most of the points fit the Amazonian regional values, with exceptions exceeding regional values. The environmental quality of sediments was within regular standards when compared with international reference values and Brazilian legislation. The environmental quality of the sediment was within the regular patterns when compared with international reference values and Brazilian legislation. It is concluded that activities such as bauxite mining can contribute to Hg remobilization to aquatic environments.