Coupled Effects of Flow, Biogeochemistry, and Bioturbation on Mobility and Efflux of Metals from Low-Permeability Sediments

Sediments, with their resident organisms, represent a highly complex system. Contaminant behavior and effects are highly dependent on an interrelated suite of sedimentary processes, including flow hydrodynamics, biogeochemical processes, sediment resuspension, and biological processes. While many of these processes have been extensively studied, interactions between them are not. In particular, the coupled effects of these processes on the mobility of metals remain poorly defined. Accordingly, this research aims to improve understanding how the interplay of physical, chemical and biological processes controls the speciation, mobility, and efflux of metals in low-permeability sediments.;Gust chamber experiments were conducted with in situ collected sediment cores to understand the mechanisms that drive the net efflux of Zn from surface sediments to the overlying water and porewater. Short term resuspensions (4-hour) were also imposed over these sediment cores to assess the effects of resuspension on speciation and efflux of Zn. X-ray absorption spectroscopy (XAS) results, in conjunction with bulk chemical measurements of porewater, overlying water, and sediment, revealed that the oxidative dissolution of Zn sulfide minerals near the sediment water interface (SWI) mobilized Zn to the porewater and overlying water. Results from this experiment also showed that both steady slow porewater transport and rapid episodic resuspension events are important to the release of metal from fine-grained, low permeability sediments.;Overlying hydrodynamics play critical roles in controlling surface water-porewater exchange in permeable sediments, but these effects have rarely been considered in low permeability sediments. Two sets of Gust chamber experiments were performed with two sediments had similar permeability (~10-15 m 2) but different particle size distribution. Contaminated sediments were subject to different levels of flow shear stresses with and without resuspension. Results revealed that hydrodynamic forces enhanced the mobilization and efflux of Cu from coarser-grained sediments, but not the finer-grained sediments. These results showed that the mobility and efflux of metals are likely to be influenced by overlying hydrodynamics even in low permeability sediments, and these effects are mediated by sediment heterogeneity.;To understand how long-term sediment resuspension influences the speciation and release of metals, two-week batch experiments were conducted. The combination of XAS results and bulk chemical results revealed that long-term resuspension substantially oxidized ZnS, but only a small proportion (15%) of the Zn was released to the aqueous phase. This suggests that sorption onto reactive surfaces and precipitation of hydrozincite strongly limited Zn solubility and thus retained the major proportion of Zn in suspended particles.;Effects of hydrodynamic forcing, sediment resuspension, and bioturbation by marine polychaete Nereis virens were evaluated both independently and together in a six-month flume experiment. Bulk chemical measurements showed that hydrodynamic forcing without resuspension or bioturbation slightly enhanced the efflux of dissolved Cu. On the other hand, sediment resuspension caused a significant release of dissolved Cu, and interactions between hydrodynamics and bioturbation further enhanced Cu efflux. Overall, the results indicated that the release of metals from low-permeability sediments is greatly enhanced by interaction between flow and bioturbation.