Metal-contaminated sediments in a semi-closed basin: Implications for recovery

Sediment cores, collected from a contaminated zone in the Ria de Aveiro (Portugal), were sectioned, under nitrogen, and centrifuged to remove the pore waters. The sediment characteristics, including acid volatile sulphide (AVS) concentrations, were determined, together with total and available metals (Fe, Mn, Cd, Cu, Pb, Zn) and the total dissolved metals in the pore waters. Peak concentrations in total metals of the sediments were observed at various depths in the core as a result of time-dependent, industrial discharges. The fraction of total metal released by a mixture of hydroxylamine hydrochloride and acetic acid (HAA) ranged from 24% for Cu to 74% for Zn and enzymatic digestion by proteinase K released <10% of total metal. The pore waters had elevated dissolved metal concentrations concomitant with the peaks in total and available metal and with a maximum in AVS concentration. Equilibrium calculations indicated that the major dissolved phase species was MHS2−, with minor quantities of M(HS)2. The diffusive fluxes for sediment–water exchange of the metals were insignificant, the mobility of the metals being hindered by sulphide formation. Thus, the metals are likely to remain trapped in these sediments, thereby delaying recovery from contamination.