Siltation processes and metal sediment profiles in a hydroelectric power plant reservoir in the Paraíba do Sul river Basin, Southeastern Brazil

The middle Paraíba do Sul River region represents Brazil’s most industrialized area, where the largest steel industry in the country, the National Steel Company ( Companhia Siderúrgica Nacional —CSN) is located. The Light hydropower complex is located downstream from the CSN and, in the present study, four sediment cores samples were collected at the Vigário Reservoir, one of the seven reservoirs existing in this complex. The reservoir region has suffered huge transformations since its implementation and CSN construction, enhancing reservoir siltation. Despite this, bathymetric surveys are scarce, with only two carried out in seventy years, making siltation process changes along this period unknown. To obtain this information, 210 Pb dating was applied. Based on porosity profiling, three cores were selected for 210 Pb dating, only the core sampled near the water inflow was considered not suitable for 210 Pb dating. The same results in the three sediment cores were noted, with an increasing sediment accumulation rate (SAR), from 0.6 cm y −1 originally to 2.9 cm y −1 currently. Chromium and zinc, both associated to CSN activities, presented a trend toward lower values during the last twenty years, attributed to the implementation of the ISO 14001 policy at the CSN. Chromium reached concentrations higher than the probable effect level (PEL), before 2000, although actual values were close to the threshold effect level (TEL). Mercury concentration variations were attributed to sediment granulometry changes due to land use transformations, and not to eventual illegal gold mining activities along the middle of the Paraiba do Sul River.