Distribution, speciation and risk assessment of Hg in sediments from the Castilseras reservoir (Almadén, Spain)

The Castilseras reservoir is geographically located in the south-central part of the Almadén Hg mining district (Ciudad Real, South-Central Spain). This reservoir collects rainwaters, mine water drainage and leachate from spoil heaps from four of the most important decommissioned Hg mines of the district (Las Cuevas, El Entredicho, Nueva- and Vieja Concepción). As consequence of its special hydrological conditions, artificial reservoirs can be considered important barriers, which significantly alter the natural solid phase transport. These basins promote sedimentation processes becoming significant repository of sediments and of heavy metals associated to them. Moreover, the specific physico-chemical conditions found in these reservoirs, as compared to flowing waters, may induce the development of biologically mediated transformations of inorganic Hg into different and more toxic Hg species for the environment such as methylmercury. Therefore, sediments of the reservoir can be seen as sink and/or source of contaminants and their ecological risk assessment should be a priority in order to know the environmental quality of the fluvial compartment. In this work, we report the results of the studies of contents, species and potential risk of Hg in the Castilseras reservoir sediments, determined using different techniques to assess its role in the aquatic environment of the reservoir. Results showed that Total Hg (THg) values range between 2.49-17.30 µg g-1, decreasing from the upper reservoir area to the dam and significantly exceed legal toxic reference values such as EPA severe effect level (2 µg g-1). Moreover, Organic Hg values up to 10.30 ng g-1 increase their contents in the flow direction, where the highest value was measured from the deepest sampling point near the dam, suggesting methylation processes associated with the deep. On the other hand, a four steps Sequential Extraction Procedure (SEP) was accomplished to determine the availability of the mineral phases to incorporate Hg species into the aquatic ecosystem. Results showed that the non-residual fraction of Hg, which can be seen as indicator of the potential environmental risk, range between 19 and 58% suggesting an important anthropogenic origin of Hg. Considering only the extracted percentages in the most labile step of the SEP, recovery percentages range from 5 to 27%, showing that the risk varies from low to moderate. These results suggest that Castilseras reservoir is heavily contaminated by mercury, and this structure may play an important role as sink and source of Hg in the aquatic environment, generating a potential environmental risk.