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 t he 'Los Frailes' mine affected the Guadiamar river, a tributary that border s on the Donana 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 ma jor anomaly in the Zn content of the estuarine solid phase. Acid waste-wate r with very high concentrations of dissolved Zn reached the estuary probabl y before the construction of temporary dams, and the drastic pH increase in the confluence with estuarine water caused the precipitation of Zn. This r esulted 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 th e sediment and SPM of the Guadalquivir estuary compared to the background v alues in unpolluted sediments. In this estuary, the Zn content of the SPM s howed great variability which was controlled by the tidal cycle. Zn-contami nated particles tended to be temporarily trapped in the area of maximum sal inity gradient associated with the turbidity maximum. This retention favour ed the deposition of particles in the estuary, which took place mainly duri ng, 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 co ntent of the sediment decreased because Zn contaminated particles were tran sported offshore by wave-induced currents instead of being deposited on the seabed. (C) 1999 Elsevier Science B.V. All rights reserved.