Open cast metal mining in the Iberian Pyrite Belt (IPB) in soutwestern Spain has left a legacy of large acidic pit lakes. Mine waters have caused severe pollution of acidity, dissolved metals (Fe, Al, Mn, Cu, Zn, Cd, Pb), arsenic, and sulfate to the neighboring river systems. The majority of the pit lakes are meromictic and show a permanent stratification with a chemocline separating an upper, oxygenated, lower-density layer from a lower, anoxic, higher-density layer. Only acidophilic microbes (both eukaryotic and prokaryotic) can survive in the water column of the pit lakes because of the extreme geochemistry. However, less acidic porewaters have been found in the bottom sediments of these lakes due to the activity of Fe(III)- and SO4-reducing microbes. We carried out sediment incubations to identify biogeochemical/mineralogical processes occurring naturally in Cueva de la Mora pit lake near the sediment/water interface. Intact sediment cores were taken from shallow depths. Sediments were dominated by silicates, and ferric iron (16-20 wt.% FeT) in the form of shwertmannite and jarosite, and variable goethite [1]. The sediments contained a high organic carbon content due to the abundance of benthic green algae [2]. Another core was shaken to destroy the original sediment stratigraphy and produce homogeneous starting conditions for laboratory incubation. The core was monitored weekly during three months to study the evolution of pH, Eh and metal concentrations within the sediments (pore waters) and in the overlying water. As the incubation proceeded, significant chemical changes occurred throughout the column but especially within the sediments. Marked pH shifts across the sediment/water interface developed (from pH 2.5 to 5.5 in the upper 4-6 cm of the sediments). The formation of a thin layer of a milky, gel-like, Al-containing precipitate occurred in this pH transition zone. Al precipitation significantly affected the concentration of contaminants in the sediments. Results from these studies will be presented and implications for the management of acidic pit lakes will be dicussed. [1] Diez-Ercilla, et al., Biogeochemistry (2014), 121-3, 519-536. [2] Wendt-Potthoff et al., Limnologica (2012), 42-3, 175-188.

Contaminant dynamics in waters and sediments of acidic pit lakes

253rd National Meeting (2017)

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