Geochemical influences on metal partitioning in contaminated estuarine sediments

Abstract

Stormwater runoff has resulted in heavy metal contamination throughout much of the Port Jackson estuary, Sydney, Australia. Metal partitioning was investigated in the benthic estuarine sediments of Iron Cove, an off-channel embayment of Port Jackson. Contamination was greatest near the stormwater canal, where sediments were anoxic and contained high concentrations of sulfide in the porewater. Away from the canal a layer of non-cohesive, sub-oxic surficial sediment containing high dissolved iron was found overlaying a more cohesive substratum. At all sites, porewater Cd, Cu, Ni, Pb and Zn were <2.5 g L^–1, and negligible metal release was observed upon sediment resuspension. According to water quality guidelines, the ecological risk posed by dissolved metals from the Iron Cove sediments is low. Estimated fluxes of Cd, Cu, Ni, Pb and Zn from the sediments were calculated to be <0.2 mol m^–2 day^–1. The rapid oxidation then hydrolysis of iron(II) in porewaters caused a drop in pH and the formation of iron hydroxide precipitate. These processes may affect dissolved metal concentrations; hence, oxidation of samples must be avoided during sampling and extraction procedures. Sediment-bound zinc was the metal most easily mobilized.