Rare-earth-element fractionation patterns in estuarine sediments as a consequence of acid mine drainage: a case study in SW Spain.

The Tinto and Odiel river basins drain the volcanogenic massive sulphide deposits of the Iberian Pyrite Belt and water from the estuaries flows into the northwest of the Gulf of Cadiz. The effect of seawater dilution and acid neutralisation on REE fractionation was investigated based on analysis of sediment samples from traps placed along the estuaries. The REE fractionation in the sediments could be grouped into three North American Shale Composite (NASC) normalised patterns depending on the environmental conditions prevailing in the areas where they were deposited. Fluvial estuarine sediments deposited at pH less than 4 and Cl concentrations below 2 g/l showed a slightly middle REE-enriched NASC normalised pattern. The estuarine mixing zone involves steep gradients characterised by pH values of 5-7 and chlorinity of 2-10 g/l and under these conditions the sediments exhibited light REE depletion and an almost flat trend for middle and high REE. Once acid neutralisation was finished at the outer estuary the pH increased to more than 7 and the chlorinity to above 10 g/l and the sediments showed a light and medium REE enriched pattern with a slightly negative La anomaly typical of marine conditions. The close relationship between pH and REE content indicates that pH is a key variable controlling REE geochemistry in estuarine systems affected by acid mine drainage.
The Tinto and Odiel river basins drain the volcanogenic massive sulphide deposits of the Iberian Pyrite Belt and water from the estuaries flows into the northwest of the Gulf of Cadiz. The effect of seawater dilution and acid neutralisation on REE fractionation was investigated based on analysis of sediment samples from traps placed along the estuaries. The REE fractionation in the sediments could be grouped into three North American Shale Composite (NASC) normalised patterns depending on the environmental conditions prevailing in the areas where they were deposited. Fluvial estuarine sediments deposited at pH less than 4 and Cl concentrations below 2 g/l showed a slightly middle REE-enriched NASC normalised pattern. The estuarine mixing zone involves steep gradients characterised by pH values of 5-7 and chlorinity of 2-10 g/l and under these conditions the sediments exhibited light REE depletion and an almost flat trend for middle and high REE. Once acid neutralisation was finished at the outer estuary the pH increased to more than 7 and the chlorinity to above 10 g/l and the sediments showed a light and medium REE enriched pattern with a slightly negative La anomaly typical of marine conditions. The close relationship between pH and REE content indicates that pH is a key variable controlling REE geochemistry in estuarine systems affected by acid mine drainage.