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 sediment precipitation/dissolution, sorption/desorption of metals, aquatic and surface-chemical transformations, pore water flow, and diverse biological processes including bioturbation and bioirrigation. This complexity often confounds assessment of the potential adverse effects of metals in contaminated sediments, and hinders development of effective remediation schemes. To support management and remediation of contaminated sites, the objectives this project were to improve understanding of key processes that regulate the behavior and effects of metals in contaminated sediments, and to develop approaches for improved characterization of sediments obtained from contaminated sites to foster the development of better site conceptual models. The project specifically sought to determine how the interplay of physical, chemical, and biological processes controls the transformation, mobility, bioavailability, and toxicity of metals in contaminated sediments.
A series of laboratory experiments were executed to study the coupled effects of overlying hydrodynamics, bioturbation, and biogeochemical processes on mobility, bioavailability, and toxicity of metals in contaminated sediments. Gust chamber and laboratory flume experiments with the capability to provide precisely controlled hydrodynamic conditions were performed with contaminated sediments collected from two sites: Lake DePue (LDP), IL and Portsmouth Naval Shipyard (PNS), ME. Experiments were performed both with and without bioturbating organisms (burrowing worms). Metals concentrations were characterized in overlying water, porewaters, and sediments over the course of experiments to assess the effects of hydrodynamic and biological processes on metals distribution, speciation, mobility, and efflux. Resulting metals bioavailability and toxicity also were evaluated using multiple test organisms in targeted experiments.
Oxidation of surficial sediments promoted the formation of more mobile metal species that represented a source of metals to both the porewater and overlying water. Hydrodynamics of the overlying water column clearly influenced the efflux of metals from the sediments. In coarser grained PNS sediments, hydrodynamic forcing substantially promoted the release of copper (Cu) to the overlying water. These effects were damped in finer-grained sediments. Sediment resuspension also transitorily mobilized particulate metals. Metals concentrations in the water column increased substantially during resuspension and decreased back to the pre-resuspension levels following cessation of sediment transport. However, little mobilization of dissolved metals was observed from the resuspended sediments, and resuspension did not substantially increase metals bioavailability or acute toxicity to multiple test organisms (based on observed survival of Hyalella azteca and Daphnia magna; bioluminescence of Pyrocystis lunula; and survival, growth, and tissue metal concentration of Neanthes arenaceodentata during and following resuspension events). Redeposited sediments exhibited increased metal bioavailability and acute toxicity to H. azteca, indicating potential for adverse ecological impacts due to changes in metal speciation associated with flow perturbations and sediment resuspension. Bioturbation greatly altered sediment structure, mixed sediments leading to enhanced exposure of buried sediments to oxic conditions, and increased oxygen delivery into the sediments by pumping oxygenated water into burrow structures. As a result, bioturbation substantially increased the flux of metals from the sediments to both the overlying water and porewater.
The results of this project improve the assessment of risks at Department of Defense (DoD) contaminated sediment sites by enhancing the knowledge base of fundamental processes that govern the behavior and effects of metal contaminants in sediments and providing strategies for measuring key effects including metals efflux, bioavailability, and toxicity under complex site conditions. The experimental approaches developed in this project to measure the effects of flow forcing, sediment resuspension, bioturbation, and bioirrigation can be directly employed in site assessments using sediment cores or homogenized sediments obtained from DoD sites.