Heavy metals and As transport under low and high flows in the River Guadiamar three years after the Aznalcóllar tailings dam failure : implications for river recovery and management

Abstract

This paper presents work from an on-going research programme examining the recovery of the Guadiamar River following the April 1998 Aznalcóllar tailings dam failure. Analysis of heavy metal (Cd, Cu, Pb and Zn) and As in surface waters, valley floor alluvium, overbank and suspended sediments are compared between six study reaches downstream of the dam, over five sampling periods. The principal aims of the paper are to examine the distribution of contaminants in sediments and river waters and their delivery downstream under low and high flows, and the implications for river channel recovery and management three years after the tailings dam failure. Results from a survey of Guadiamar alluvium carried out in January 1999 showed that, although clean-up operations were largely successful in restoring the affected area to pre-spill conditions, sediments continued to exceed intervention thresholds for the region. Residual contaminant levels were highest near the spill source, with isolated ‘hotspots’of elevated contamination throughout the affected area, superimposed on a general downstream decline in concentrations. In terms of contaminant mobility, surface water samples taken in May 99, under very low flows, have dissolved Cd, Cu and Zn concentrations which fail EC directive thresholds for surface water abstraction. Element concentrations are significantly lower for waters in January 1999, when controlled discharge from the Agrio reservoir was taking place. Under high flow conditions, sediment-associated contaminant transport is dominant for all elements discussed, accounting for between 91% (Cu) and 99% (Pb) of export yields. This has lead to the remobilisation of significant quantities of contaminants, particularly Pb and Zn, during post-spill floods. Overbank deposits, however, show a fall in contaminant concentrations, following successive flood events. This has been due to sediment mixing of contaminated and “clean” material. Dilution has so far maintained contaminant concentrations below intervention threshold limits in the lower reaches, thereby offering a cautiously optimistic prognosis for the sensitive wetlands of Doñana.