Abstract
The Guadiana Basin (SW Iberian Peninsula) is affected by acid mine drainage (AMD), a consequence of ancient mining activities in the Iberian Pyrite Belt (IPB). Consequently, the sediments at the Alqueva reservoir (SE Portugal) in the Guadiana Basin are potentially contaminated by trace elements, which make important: (i) to characterize the status of trace element pollution of the sediments; (ii) to evaluate the mobility and the bioavailability of As, Cd, Cu, Cr, Pb and Zn; and (iii) to assess the environmental risk associated with the total and bioavailable concentrations of trace elements, using the sediment quality guidelines (SQGs) and the risk assessment code (RAC). Metal enrichment factors (EF) and geoaccumulation indexes (Igeo), determined taking into account the regional background levels, revealed that, among the metals analyzed, Cd contributed the highest to pollution levels followed by Pb and As. Despite the trace element contamination of the Alqueva sediments, the sequential extraction showed that most of them are found in the oxidizable and residual fractions, which indicates that they are sparingly bioavailable, with exception of Cd (acid-labile fraction) and Pb (reducible fraction). Based on the RAC, Cd was the only metal that presented a high risk, while Pb, As and Zn showed a medium risk. Moreover, the SQGs revealed the existence of certain areas of extremely high risk, particularly related to high concentrations of total As and, in less extent, of Pb and Cd, associated with AMD, wastewater discharges and runoff of plant protection products from agricultural fields located near the reservoir.
Highlights
Pollution of the natural environment by trace elements is a world-wide problem
To evaluate the anthropogenic influence on the trace elements quantified in the Alqueva sediments, the enrichment factors (EF) values were determined
The results allowed the characterization of the status of metal pollution within Alqueva and the identification of the elements with higher bioavailability, which would probably induce higher ecotoxicological effects in the aquatic species, and are responsible for the bioaccumulation processes along of trophic chain
Summary
Pollution of the natural environment by trace elements is a world-wide problem. With origin in natural sources (e.g. weathering of soil and rock, erosion, forest fires and volcanic eruptions) and anthropogenic activities (e.g. industrial effluents, mining and refining, agriculture drainage, domestic discharges and atmospheric deposition), these elements continuously enter to the aquatic systems, posing serious threat due to their hazardous characteristics (Carman et al, 2007; Chon et al, 2010; Davutluoglu et al, 2011). As the principal compartment of trace elements accumulation, the assessment of sediments quality plays an important role in the good ecological and chemical status of water (Borja and Heinrich, 2005), which is the principal goal of the European Water Framework Directive (WFD) (ECC, 2000). Notwithstanding, this legal document does not mention the sediments as a compartment to be investigated, this matrix constitutes one of the most important source of water contamination by trace elements, as well as, an important carrier of these hazardous substances within the rivers, reservoirs and other waters (Sekabira et al, 2010; Yaun et al, 2014). The preservation of this compartment is an important step to maintain the full quality of the water body
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