Geochemistry of sediment precipitated during acid mine drainage-seawater interaction. Implications for metal mining impacts on estuarine systems

Abstract

Metal-mining pollution impacts estuarine systems, where sediments with high available toxic metal concentrations precipitate. In the Ría de Huelva estuary (SW Spain), the metal-rich acidic water of the Tinto River (headwater pH = 2.2 and electrical conductivity -EC- = 25.8 mS cm-1) interacts with saline-alkaline seawater of the Atlantic Ocean (pH = 7.87, EC = 55.2 mS cm-1). This study analyses the geochemical characteristics of the dissolved and particulate fractions formed when mixing both systems. A pH-controlled test showed that about 2000 L of seawater are necessary to neutralise 1 L of Tinto River headwater. A six-step sequential extraction performed on the precipitated sediments indicated that: a) Fe, Al, Zn, Cu, ∑REE, Co, Pb, and As are highly available in soluble and adsorbed fractions; b) Fe, Al, Cu, Zn, ∑REE, Mo, and Co mostly concentrate in iron oxy-hydroxides and oxyhydroxysulfates; and c) As, V, Mo, U and Pb desorb at pH ≥ 5. Furthermore, Fe and Al precipitates play significant hydrochemical and environmental roles in the studied environment by attenuating pH changes, as sediment precipitation decreases pH and captures chemical elements. The results of this study are useful to better understand natural processes (e.g., disruption of buffers, instability of dissolved elements, sorption/desorption, co-precipitation) taking place in estuaries affected by AMD and to predict geochemical variations in the environment due to climate change (e.g., floods, extreme precipitation, sea level rise).