Distribution of metals in sediments of the Guadiamar river basin 20 years after the Aznalcóllar mine spill: Bioavailability and risk assessment.

In April 1998, 6 million m3 of acid water and sludge were accidently spilled on to the Guadiamar riverbed (South of Spain). In this study, the long term distribution of Al, Cd, Cu, Fe, Mn, Pb, and Zn in the sediments of the Guadiamar river basin is described using fractionation analysis. Changes in the availability of metals from 2002 to 2018, covering a period of 20 years after the dam collapse, have been evaluated and their potential environmental risks have been examined. A substantial decrease in the concentrations of all the metals studied was observed, except Pb. However, the concentrations of Cu, Zn, and Pb remained higher than the background concentrations, which indicates a high influence of the mining activity, even in 2018. An increase of the residual fraction was observed from 2002 (47%) to 2018 (67%), which was mainly due to the mobilisation of metals from their oxidisable fraction to their residual fraction. Environmental risk assessment revealed a significant decrease in the risk associated with metals from 2002 to 2018, mainly due to the decrease of the metals concentration in the sediments over the year and to the lower availability of these metals in 2018. In 2002, the most challenging metals were Pb and Zn, whereas in 2018, Zn and Cd were the most problematic due to their toxicity and availability.