Porewater dynamics of silver, lead and copper in coastal sediments and implications for benthic metal fluxes.

To determine the conditions that lead to a diffusive release of dissolved metals from coastal sediments, porewater profiles of Ag, Cu, and Pb have been collected over seven years at two contrasting coastal sites in Massachusetts, USA. The Hingham Bay (HB) site is a contaminated location in Boston Harbor, while the Massachusetts Bay (MB) site is 11 km offshore and less impacted. At both sites, the biogeochemical cycles include scavenging by Fe-oxyhydroxides and release of dissolved metals when Fe-oxyhydroxides are reduced. Important differences in the metal cycles at the two sites, however, result from different redox conditions. Porewater sulfide and seasonal variation in redox zone depth is observed at HB, but not at MB. In summer, as the conditions become more reducing at HB, trace metals are precipitated as sulfides and are no longer associated with Fe-oxyhydroxides. Sulfide precipitation close to the sediment-water interface limits the trace metal flux in summer and autumn at HB, while in winter, oxidation of the sulfide phases drives high benthic fluxes of Cu and Ag, as oxic conditions return. The annual diffusive flux of Cu at HB is found to be significant and contributes to the higher than expected water column Cu concentrations observed in Boston Harbor. At MB, due to the lower sulfide concentrations, the association of trace metals with Fe-oxyhydroxides occurs throughout the year, leading to more stable fluxes. A surface enrichment of solid phase trace metals was found at MB and is attributed to the persistent scavenging by Fe-oxyhydroxides. This process is important, particularly at sites that are less reducing, because it maintains elevated metal concentrations at the surface despite the effects of bioturbation and sediment accumulation, and because it may increase the persistence of metal contamination in surface sediments.