Water, sediment and biota were sampled at different seasons during a 2-year study from the Valdeazogues River, which flows east–west from the mining District, to La Serena Reservoir. Simultaneously, a comprehensive study was undertaken to determine the influence of some major physico-chemical parameters that potentially influence the fate of Hg within the watershed.
Concentrations of dissolved Hg in water were below 0.14 µg/L, whereas particulate Hg ranged from 0.1 to 87 µg/g, with significant seasonal variation. Total Hg concentrations varied from 7 to 74 µg/g in sediment from the Valdeazogues River, while in sediments from La Serena Reservoir were below 2 µg/g. On the other hand, methyl-Hg reached concentrations up to 0.3 ng/L in water and 6 ng/g in sediment from La Serena Reservoir, whereas maximum concentrations in Valdeazogues River were 5 ng/L and 880 ng/g in water and sediment, respectively. The distribution of Hg species in the Valdeazogues River–La Serena Reservoir system indicated a source of Hg from the mine waste distributed along the river. Total Hg in water was strongly correlated with total dissolved solids and chlorophyll a concentrations, whereas organic carbon and Fe concentrations seem to play a role in methylation of inorganic Hg in sediment. Total Hg concentrations were low in fish from Valdeazogues River (0.8–8.6 ng/g, wet weight) and bivalves from La Serena Reservoir (10–110 ng/g, wet weight), but most was present as methyl-Hg.