Considerations for capping metal-contaminated sediments in dynamic estuarine environments

The effects of tides, bioturbating organisms and periods of anoxia on metal fluxes from contaminated harbour sediments in a shallow tidal estuarine bay were studied, together with capping technology options for the containment of metal contaminants. Zinc fluxes from the sediments were high, ranging from 10 to 89 mg Zn /m2/day. In the absence of capping, experiments in corer reactors showed that simulated tidal processes increased zinc fluxes 5- fold. Fluxes were also greater in the presence of sediment-dwelling organisms. If organisms were removed, and recolonising organisms later added, their bioturbation activities initially lowered zinc fluxes, but fluxes gradually reached steady state at the higher levels seen previously. Capping materials physically isolate contaminated sediments, provide a binding substrate for metals released from the sediment, and importantly create an anoxic environment below the cap, that stimulates the formation of insoluble metal sulfides. Clean sediment (5 mm) was the most effective capping material in reducing zinc fluxes. Zeolite/sand mixtures (10 mm) also greatly reduced these fluxes, but significant breakthrough of zinc occurred after two weeks. Sand (20 mm) was not effective. The presence of organisms disturbed capping materials and increased zinc fluxes. Installed capping materials should have depths of \u3e30 cm to minimise organisms burrowing to contaminated sediments beneath