Zinc contamination in the bottom and suspended sediments of the Guadalquivir estuary after the Aznalcollar spill south-western Spain/ . Control of hydrodynamic processes

A toxic spill of 5 million t of mud from a pond of pyrite mining waste in the ‘Los Frailes’ mine affected the Guadiamar river, a tributary that borders on the Do˜nana Natural Park and discharges into the Guadalquivir estuary. The suspended particulate matter and bottom sediment were analysed in order to study the impact of the spill on the estuary. This accident caused a major anomaly in the Zn content of the estuarine solid phase. Acid waste-water with very high concentrations of dissolved Zn reached the estuary probably before the construction of temporary dams, and the drastic pH increase in the confluence with estuarine water caused the precipitation of Zn. This resulted in an 80-fold increase of the Zn content in the bottom sediment of the lower Guadiamar river and a 20-fold increase of Zn concentrations in the sediment and SPM of the Guadalquivir estuary compared to the background values in unpolluted sediments. In this estuary, the Zn content of the SPM showed great variability which was controlled by the tidal cycle. Zn-contaminated particles tended to be temporarily trapped in the area of maximum salinity gradient associated with the turbidity maximum. This retention favoured the deposition of particles in the estuary, which took place mainly during the high tide period. These Zn-contaminated particles were periodically resuspended during high speed flow peaks. In the outer part of the estuary, the Zn content of the SPM increased due to coagulation, whereas the Zn content of the sediment decreased because Zn contaminated particles were transported offshore by wave-induced currents instead of being deposited on the seabed.