The effect of former mining activities on contamination dynamics in sediments, surface water and vegetation in El Avenque stream, SE Spain

This work aims to identify and characterize heavy metal contamination in a fluvial system from Cartagena-La Unión mining district (SE Spain). In order to assess the dynamics of transport and the accumulation of heavy metals, sediments, surface water and vegetation, samples along "El Avenque" stream were collected. The former direct dumps of wastes and the presence of tailing ponds adjacent to the watercourse have contributed to the total contamination of the stream. Total Cd (103 mg kg ^-1), Cu (259 mg kg ^-1), Pb (26,786 mg kg ^-1) and Zn (9,312 mg kg ^-1) in sediments were above the limits of European legislation, being highest where tailing ponds are located. Bioavailable metals were high (3.55 mg Cd kg ^-1, 6.45 mg Cu kg ^-1, 4,200 mg Pb kg ^-1 and 343 mg Zn kg ^-1) and followed the same trend than total contents. Metals in water were higher in sampling points close to ponds, exceeding World Health Organization guidelines for water quality. There is a direct effect of solubilisation of sediment metals in water with high contents of SO ₄ ^2-, product of the oxidation of original sulphides. The mobility of metals varied significantly with shifts in pH. Downstream, available and soluble metals concentrations decreased mainly due to precipitation by increments in pH. As a general pattern, no metal was bioaccumulated by any tested plant. Thus, native vegetation has adopted physiological mechanisms not to accumulate metals. This information allows the understanding of the effect of mining activities on stream contamination, enforcing the immediate intervention to reduce risks related to metals' mobility.