Temporal and spatial accumulation of heavy metals in the sediments at Paiva Castro Reservoir (São Paulo, Brazil)

Abstract

The analysis of heavy metals in sediments, associated with geochronology, can identify the historical accumulation of these contaminants that occurred over time as a result of anthropogenic activities. A study was conducted at Paiva Castro Reservoir, a tropical reservoir in Brazil, to (1) establish baseline values for the metals Cd, Cr, Cu, Ni, Pb, Zn, Mn, Al and Fe; (2) investigate the history of heavy metal accumulation through geochronology using 210Pb; and (3) identify the main variables that differentiate the sampled regions. A gravity corer was released twice in three reservoir areas. Each core was sliced at intervals of 2 cm and stored in a sealed plastic bag. One core was used for the determination of metals and for dating using the 210Pb technique. The other core was used for the determination of organic matter and particle size. The baseline values for heavy metals were calculated according to the mean bottommost samples of the three core analysed. In terms of the levels of heavy metals, the greatest impacts in the region occurred during periods prior to the period of reservoir operation. In surface sediments, Cu exceeded several times (97.0 mg kg−1) the background (24.0 mg kg−1). This result may be a consequence of the application of copper sulphate for controlling algal blooms. A canonical discriminant analysis indicated that the variables that significantly differentiated the three sampled areas were related to the elements Al, Fe, and Mn and to particle size. These data indicate that the main differences are due to lithological variations and to the dynamic operation of reservoirs that provide a larger particle size fraction in the area where the uptake of water for the public water supply is located and in the area upstream from Paiva Castro, where there is higher flow energy. Evidence of the degradation of Paiva Castro Reservoir is already being reflected in the sediments. It is necessary that public policies are actually applied to counteract the degradation of this important ecosystem; otherwise, the reservoir’s ‘good quality’ will be at risk.