Investigating the Effect of Bioirrigation on In Situ Porewater Concentrations and Fluxes of Polychlorinated Biphenyls Using Passive Samplers.

Polychlorinated biphenyl (PCB) fluxes from contaminated sediments can be caused by mechanisms including diffusion, bioirrigation, and resuspension, but it is often unclear which mechanisms are important. In the Lower Duwamish Waterway (Seattle, Washington), the presence of abundant benthic macrofauna suggests that porewater bioirrigation may be an important mechanism for PCB transport from the bed into the overlying water column. In this field study, the fluxes of PCBs due to bioirrigation were quantified by using (a) polyethylene (PE) samplers to quantify in situ and ex situ (i.e., equilibrium) PCB porewater concentration profiles and (b) measurements of the geochemical tracer 222Rn to quantify the rate of porewater exchange with overlying water. The results showed that bioirrigation caused sorptive disequilibrium with the surrounding sediment, which led to lower in situ porewater concentrations than expected from sediment concentrations. The combined fluxes of seven PCB congeners (Σ7PCBs) were 1.6-26 ng/m2/day for the three field sites, similar in magnitude to the upper limit estimates of diffusive fluxes calculated assuming water-side boundary layer control (Σ7PCBs = 1.3-47 ng/m2/day). Moreover, the depleted in situ porewater concentrations imply lower diffusive flux estimates than if the ex situ porewater concentrations had been used to estimate fluxes (Σ7PCBs = 89-670 ng/m2/day). These results suggest that nondiffusive PCB fluxes from the sediment bed are occurring and that quantifying in situ porewater concentrations is crucial for accurately quantifying both diffusive and nondiffusive PCB fluxes.

Effect of bioirrigation on sediment-water exchange of methylmercury in Boston Harbor, Massachusetts.

Coastal marine sediments are important sites of methylmercury (MMHg) production, and dissolved efflux provides an important source of MMHg to near-shore, and possibly offshore, water columns and food webs. We measured the flux of MMHg across the sediment-water interface at four stations in Boston Harbor that span a range of infaunal population densities and bioirrigation intensities. At each station we carried out total MMHg flux measurements using core incubations and collected near-surface pore waters to establish MMHg gradients for diffusive flux calculations. The flux cores were also imaged by CT scanning to determine the distribution of infaunal burrows, and pore-water sulfide and 222Rn profiles were measured. Total MMHg fluxes, measured using core incubations, ranged from -4 to 191 pmol m(-2) d(-1), and total MMHg fluxes were strongly correlated with burrow densities at the stations. Estimated diffusive fluxes, calculated based on MMHg concentration gradients below the sediment-water interface, were much lower than total fluxes at three of the stations, ranging from 2-19 pmol m(-2) d(-1). These results indicate that MMHg exchange may be significantly enhanced over molecular diffusion in bioturbated sediments. Furthermore, burrow density provides a strong predictor of total MMHg flux. Pore-water exchange of both dissolved MMHg and 222Rn, a naturally occurring pore-watertracer, increased across the range of observed burrow densities, suggesting that the presence of burrows enhances both MMHg production and flux.