A transect of mercury concentrations in fish (Black bass, Micropterus salmoides) from the Valdeazogues river, Almadén Hg mining district, South Central Spain.

The Almadén district has been the World’s largest producer of mercury (Hg), for more than 2000 years. The mining activity in the district ceased some 10 years ago; however, the generalized pollution of soils and stream sediments, as well as the atmospheric emissions from these and other sources, still represent possibilities to toxify the human food chain. The Valdeazogues river crosses completely the district, with some 150 km2 extension, and including a huge mine, three mines of median importance, and up to 60 points where cinnabar (HgS) has been recognized.

Largemouth bass (Micropterus salmoides) is a carnivorous freshwater gamefish, very common along the Valdeazogues river. For years it was fished to complement the diet of the local inhabitants, although nowadays is not so common to consume it. We obtained 28 specimens, with sizes between 69 and 335 mm and weight between 11 and 552 gr, in a transect from the El Entredicho open pit to downstream the district (some 36,3 km). The specimens were analyzed using atomic absorption spectrometry with Zeeman effect.

Results show important variations throughout the transect; the largest fish in terms of weight and length had the highest Hg concentration (5246 ng/g), much higher than the fish with the lowest concentration (473.2 ng/g), which was not the specimen with the lowest size. Besides, as we go downstream the Valdeazogues River, moving away from the Entredicho Mine (considered to be the main source of contamination), Hg concentrations drops considerably until stabilizing at approximately 1200 - 1500 ng/g.