Trace metal partitioning over a tidal cycle in an estuary affected by acid mine drainage (Tinto estuary, SW Spain)

The Tinto River estuary is highly polluted with the acid lixiviates from old sulphide mines. In this work the behaviour of dissolved and particulate trace metals under strong chemical gradients during a tidal cycle is studied. The pH values range from 4.4 with low tide to 6.9 with high tide. Precipitation of Fe and Al is intense during rising tides and As and Pb are almost exclusively found in the particulate matter (PM). Sorption processes are very important in controlling the mobility (and hence bioavailability) of some metals and particularly affect Cu below pH6. Above pH~6 Cu is desorbed, probably by the formation of Cu(I)–chloride complexes. Although less pronounced than Cu, also Zn desorption above pH6.5 seems to occur. Mn and Co are affected by sorption processes at pH higher than ca. 6. Cd behaves conservatively and Ni is slightly affected by sorption processes. [Display omitted] •The Tinto estuary shows strong pH gradients and high trace elements concentrations.•PM has a hysteretic relationship with tides and high contents of Fe, Al, As and Pb.•Co and Mn are controlled by river and sea water mixing and sorption processes.•Sorption processes strongly affect Cu below pH 6, above this value Cu is desorpted.•Cadmium behaves conservatively along the pH range studied (4.4–6.9).