The aim of this project was to investigate the biogeochemistry and transport of metals in a river/estuarine system contaminated by acid mine drainage. The Rio Tinto and Rio Odiel drain a metalliferous mining area in the Iberian Pyrite Belt in the south-west of Spain. The pH values in the rivers were low (≤ 3) and dissolved metal concentrations were extremely high, up to 2.6 mM Zn, 860 mM Cu, 6.0 mM Cd and 72 nM U. The seasonal cycle of low precipitation and flash floods was identified as an important factor in generating the more severe contamination of the rivers with Fe, Al, Mn, Zn, Cu, Ni, Co and Cd observed during autumn and winter, compared to spring and summer.
The estuarine behaviour of dissolved Fe, Mn, Zn, Cu, Ni, Co and Cd was primarily controlled by pH. Apart from an addition of these metals from the sediment in the upper Tinto estuary, conservative mixing was observed up to pH ≈ 5 (at S ≈ 30), above which Fe, Mn, Zn, Cu, Ni and Co were removed from solution. Voltammetric speciation studies showed that Cu complexing organic ligands (logK'CuL ≈ 11.5, CL = 32 - 199 nM) in the estuary were saturated, and thermodynamic calculations indicated that the concentration of Cu2+ reached values (pCu2+ < 9) that are toxic to some marine and estuarine organisms.
The estimation of fluxes indicated that the dissolved metal export from this system to the coastal zone averages 10 t d-1 Zn, 2.3 t d-1 Cu, 180 kg d-1 Ni and 236 kg d-1 Co, with higher contributions during wet, compared to the dry seasons. On-line measurements of Zn, Cu and Ni in the Gulf of Cádiz revealed metal plumes associated with the Tinto/Odiel system and the Guadiana and Guadalquivir rivers. As a result of entrainment by the Atlantic Ocean surface current flowing into the Mediterranean Sea, the metal contamination in waters of the Gulf of Cádiz is transported south and eastward.
© 2000 by Charlotte B. Braungardt. All Rights Reserved
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