Assessment of trace element pollution and its environmental risk to freshwater sediments influenced by anthropogenic contributions: The case study of Alqueva reservoir (Guadiana Basin)

Abstract

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 and Davutluoglu et al., 2011). In fact, one of the most serious environmental issues concerning trace elements, which distinguish them from other toxic pollutants, is that they are resistant to biodegradation and have potential to bio-accumulate and become biomagnified, increasing the exposure of aquatic communities and human populations through the trophic chain (Gao and Chen, 2012 and Subida et al., 2013). As the principal compartment of trace element accumulation, the assessment of sediment 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 specifically 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 and Yuan et al., 2014). Consequently, the preservation of this compartment is an important step to maintain the full quality of the water body. The environmental risk of sediments and its quality could be assessed through the following: (i) pollution indexes, such as the enrichment factor (EF) and the geoaccumulation index (Igeo), that characterized the trace element anthropogenic contributions and the contamination levels, comparing the metal enrichment and the unpolluted reference concentrations (background levels) ( Christophoridis et al., 2009, Delgado et al., 2010 and Mil-Homens et al., 2007); (ii) sediment quality guidelines (SQGs), which relate the pollution status of sediments, considering the total trace element content, with their adverse effects in aquatic organisms, for instance using the threshold effect level (TEL) and the probable effect level (PEL) ( Caeiro et al., 2005, Díaz-de-Alba et al., 2011 and Sallem et al., 2013); (iii) and methodologies that evaluate the ecotoxicological risk, taking into consideration the mobility/availability of the pollutant, such as, the risk assessment code (RAC; classification that correlates the percentage of the metal more available, with the risk to the aquatic species) ( Delgado et al., 2011, Passos et al., 2010 and Yuan et al., 2014). Hence, despite the use of total trace element content as a criterion to assess their possible risk to the aquatic ecosystem, it provides insufficient information about the mobility, bioavailability and consequently toxicity, of these hazardous substances to the aquatic and human populations ( Gu et al., 2014, Hooda, 2010 and Sundaray et al., 2011). Accordingly, the speciation of metals in sediments is therefore a critical factor in assessing their potential environmental impacts ( Peng et al., 2004) and can be determined with the use of sequential extraction procedures. Of the many existing schemes, the most widely accepted standardized method was proposed by the European Community Bureau of references (BCR sequential extraction procedure) ( Ure et al., 1993) and improved in subsequent works ( Rauret et al., 1999 and Sahuquillo et al., 1999). The BCR sequential method presents advantages compared with other methods, as high reproducibility and high recovery percentages ( Coung and Obbard, 2006 and Pueyo et al., 2008). Further, this procedure has already been applied to assess metal mobility in several types of solid samples, such as sediments ( Davutluoglu et al., 2011, Díaz-de-Alba et al., 2011, Delgado et al., 2011 and Passos et al., 2010), soils ( Martley et al., 2004, Pérez-López et al., 2008 and Rao et al., 2008), and sewage sludge ( Alvarenga et al., 2007). This work was developed in the sediments of the Alqueva reservoir (the biggest artificial lake of the Iberian Peninsula) in the Guadiana Basin, which is one of the most important rivers of the Iberian Peninsula and drains the western part of Iberian Pyrite Belt (IPB), one of the world's most important metallogenic sulfide provinces, where mining dates back to the Third Millennium B.C. (Delgado et al., 2011 and Nocete et al., 2005). Associated with these mining areas are acid leachates containing metals, metalloids and sulfates, which constitute the acid mine drainage (AMD). Presently, the mining activity is limited to a small number of active mines (Neves Corvo and Aljustrel), but the environmental impact of the AMD still exists, due to the some abandoned mines which need to be rehabilitated (Matos e Martins, 2006), and several authors have reported AMD as an important anthropogenic source of trace elements in the Guadiana Basin (Fernández-Caliani et al., 2009, Guillén et al., 2011 and Nieto et al., 2007). On the other hand, the intensive agricultural activity and the discharge of untreated or inefficiently treated domestic wastewater may constitute other sources of metals in this water body (Palma et al., 2010, Palma et al., 2014a and Silva et al., 2011). Previous studies developed at the Alqueva reservoir have characterized the textural structure and organic content of the sediments, the levels of nutrients, the total contents of As, Cd, Cr, Cu, Ni, Pb, Zn, Fe and Mn; and some ecotoxicological effects (Palma et al., 2014a and Palma et al., 2014b). Currently, it is important to understand, which amounts of trace elements are arising from anthropogenic sources, as well as their mobility and bioavailability in the sediments, and try to correlate these results with the toxicological risk for the reservoir and for the populations. In this scenario, the main aims of the present study were as follows: (1) to characterize the status of pollution of the sediments, in terms of potentially toxic trace element contents, based on the specific geochemical regional backgrounds, and using as pollution indexes, the EF (which estimates the anthropogenic impact, on sediments, of each of the element analyzed) and the Igeo (based on geochemical data, that makes possible to map the areas according to their pollution degree); (2) to investigate the mobility and the bioavailability of the most potentially toxic elements (As, Cd, Cu, Cr, Pb and Zn); (3) to assess the environmental risk associated with the total and bioavailable trace element contents in the sediments, using available SQGs (TEL and PEL) and the RAC; (4) to identify, among the trace elements analyzed, which are those with higher risk for the aquatic communities. The results of this study, in combination with the outcomes obtained in previous ones, are intended to help the water resource managers and regulatory authorities to establish priority actions aimed at achieving the chemical and ecological status objectives, outlined in the Water Framework Directive. Further, this research intended to demonstrate the usefulness of diverse tools to estimate the real environmental risk of pollutants, which could become an innovative and useful approach to be applied in risk assessment of water environments.