Metal fractionation in oxic sediments of an estuary affected by acid mine drainage (south-western Spain)

Fractionation of Fe, Cu, Zn, As, Cd, and Pb was studied in 17 surface sediment samples of the Ria of Huelva. In all the cases, total concentration of these elements was high, especially for Fe (11.2%), Zn (1519 ppm), followed by Cu (1413 ppm), Pb (716 ppm), As (442 ppm), and Cd (3.26 ppm). The enrichment factors of these metals follow this order Cd ≫ Cu > Zn ∼ Pb > As > Fe with respect to the regional background. The highest concentrations associated with exchangeable or labile fractions are shown by Zn (85% of total concentration), followed by Cd (76.12%), Fe (58.41%), Cu (40.80%), Pb (37.44%), and As (33.88%). The main sources of metal input are the Tinto and Odiel Rivers, which discharge acidic water (pH < 4), from acid mine drainage (AMD), to the estuary. Acidic water inputs and mixing processes with seawater are the main controls in the spatial distribution of metals and their association with the different target fractions.