Metal equilibration and bioavailability in laboratory-contaminated (spiked) sediments used for the development of whole-sediment toxicity tests

The equilibration and bioavailability of metals in laboratory-contaminated sediments has been investigated in order to provide better guidance on acceptable procedures for spiking sediments with metals for use in the development whole-sediment toxicity tests. The equilibration rates of Cd, Cu, Ni and Zn spiked into three estuarine surface sediments with varying properties were investigated. Changes to sediment pH, redox potential, porewater and acid soluble metals, acid-volatile sulfide and bacterial activity during equilibration, effects of temperature and disturbances following equilibration are reported. The addition of metals to sediments caused major decreases in pH and increases in redox potential as metals displaced iron(II) into the porewaters and both added metals and iron (following oxidation) were hydrolyzed. Equilibration rates of porewater metals varied considerably and were dependent on sediment and metal properties. For the oxic/ sub-oxic sediments studied, metal spikes of Cd, Cu, Ni and Zn appeared near equilibrium after 25-45, 10-15, 30-70 and 20-40 days respectively. Acid-soluble metal concentrations decreased during the equilibration period indicating the metals become more strongly associated with the sediments with time (less bioavailable). Both the addition of metals to sediments and incubation (conditioning) of sediments caused changes to bacterial activity. Spiked sediments were shown to equilibrate more slowly at lower temperatures resulting in high porewater metal concentrations. Disturbances to equilibrated sediments caused by sample manipulation caused significant iron(II) oxidation and losses of metals from porewaters. The importance of documenting spiking and equilibration procedures and carefully measuring and reporting sediment parameters is highlighted so that contaminant bioavailability and exposure pathways can be interpreted and organism sensitivity accurately determined.