Located in the southern half of Spain, Almadén is the world's largest Hg district and mercury has been mined there for over 2000 years. Its slag heaps and old metallurgy plants represent a risk to watercourses that flow through the district and receive the run-off from the mines. This study collected sediment samples along the length of the main watercourses in the mining district. High levels of heavy metals were detected, with Hg levels varying between 1.77 and 255.88 mg kg −1 , Pb levels between 20.59 and 131.07 mg kg −1 and Zn levels between 59.06 and 131.07 mg kg −1 . The dispersion of these heavy metals in rivers as a result of mining activities has caused sediment quality degradation. As such, when measured using reference indexes such as the SeQI and the EF, sediment quality for the entire district is practically poor, due to anthropogenic contributions, with the concomitant threat to the surrounding environment.
IntroductionDecommissioned mercury mines represent a significant source of mercury (Hg) and also occasionally of other heavy metals in the environment as a result of the dumping of waste material originating from mines and metallurgy plants with a high concentration of Hg [1][2][3][4][5]. The metallurgical processing of Hg in mining districts has left a legacy of major mining liabilities as a result of the inefficient calcination of the mineral cinnabar. As this is an incomplete process, other types of soluble and insoluble compounds not originally present at the site, such as metacinnabar or mercury salts, are formed during the processing [6,7].The run-offs originating from the mining liabilities have an adverse effect on the ecosystems of the areas surrounding the mercury mines [8][9][10][11]. Stream-bed sediments and bodies of water located downstream from the mercury mines are characterised by significant levels of Hg and heavy metals, which can persist for decades after mining activities cease [5,7,12].The mining district of Almadén, located in south-central Spain, is the world's largest geochemical mercury anomaly, and it has produced one-third of total global production of this element [13]. The district includes a series of deposits with a range of structures and textures that share a relatively simple paragenesis whose dominant mineral is cinnabar, with pyrite present in lesser quantities. The most important mineralisation type, found in the mines of Almadén and El Entredicho, consists of stratabound cinnabar disseminations in Criadero Quartzite (Lower Silurian) [13]. Other type of mineralisation present in the area is composed of cinnabar filling veins and replacements of volcanic rocks that cannot be associated with specific stratigraphic layers [14,15]. Throughout its history, the district has been the site of various mines and metallurgy facilities. In terms of mines, Almadén is by far the largest and it has been the one most heavily mined, followed by El Entredicho, Las Cuevas and Nueva Concepción. In terms of metallurgy plants, some of them exist along the district, but two are the...
