The over-arching objective of this project was to determine the environmental significance of resuspended metal contaminated sediments, considering spatial and temporal issues as they relate to exposure, fate, and real-time vs. ecological effects. The focus was on strong resuspension events, such as those caused by propeller wash, which prevail in harbors and navigation systems. The metals that were studied were cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn). A physical-chemical model was developed that predicts metal contaminant speciation, partitioning and transport, and the resulting exposures linked to biological effects in these dynamic ecosystems.

A wide range of sediment types were used as their physicochemical characteristics greatly influence metal release dynamics. Sediments known to have metals related issues were obtained from five locations and analyzed. Three sites were freshwater:  a reference sediment from Duck Lake (Muskegon, MI), Idaho river bank (Blackbird Mine, IN), and Lake DePue (DePue, IL). Freshwater sediments were used initially due to difficulty and delays in obtaining marine sediments. Two sediments were eventually collected at marine harbors:  San Diego Bay (San Diego, CA) and Portsmouth Naval Shipyard (Portsmouth, NH).  

Initial stages of this project involved developing the Sediment Flux Exposure Chamber (SeFEC), which was modified from a previous design. These chambers are useful in laboratory experiments, as they are inexpensive to construct and easily allowed for multiple replications. In this research, the SeFEC was used to assess sediment-water exchange of metal bioavailability and toxicity.

Benthic flux chambers were also constructed based on the Gust erosion chamber design involving the combination of a plate stirrer with a center port for outflow. The flux chambers were modified so that pore water flow could be superimposed in addition to the turbulent shear applied to the sediment surface. This also allowed testing of various combinations of pore water motion induced by boundary shear, direct inflow and outflow of pore water (which is induced in the field by larger-scale boundary exchange processes), and sediment resuspension.

Resuspension events were simulated in the Gust chamber by increasing the rate of disc rotation in the Gust erosion head and by increase water flow through the chamber. For those studies simulating resuspension, full bed erosion was generated and maintained for four hours. After the resuspension event, samples for dissolved and particulate metal concentration were collected approximately every hour to capture temporal dynamics of metal flux. Various sediment characteristics were analyzed including total metals, acid volatile sulfide (AVS), simultaneously extracted metals (SEM), and total organic carbon (TOC).

Propeller wash resuspension events in San Diego Bay and Pearl Harbor were simulated in April and August 2012, respectively. The objective of the field work component was the collection of discrete seawater samples, representative of background and conditions within the resuspended sediment plume generated by a tug boat propeller. These samples were manipulated in the laboratory for quantification of the load of metal and organic contaminants of concern associated with a suite of sediment-size fractions, information that will be used for parameterization of predictive hydrodynamic models.

The Tableau Input Coupled Kinetic Equilibrium Transport (TICKET) chemical model was subsequently developed for incorporation into the Particle Tracking Model (PTM) to assess metal release and bioavailability in field setting.  The combined TICKET-PTM effectively simulated the movement of a large number of dissolved and particulate parcels through complex open water flow fields using a Lagrangian framework. Along with transport, a simplified version of the TICKET chemical model was used to track the oxidation of metal sulfide [MS(s)] to “labile” metal concentrations, and to determine the partitioning of “labile” metal between dissolved and particulate phases.  The TICKET-PTM was applied to a pilot field test of a propeller-wash event in San Diego Bay as a proof of concept evaluation of the model capabilities.  Model results demonstrated the importance of transport, oxidation kinetics and metal partitioning behavior in assessing the effects of propeller-wash events on copper bioavailability in the overlying water.

In SeFEC exposures, three sediment types (Idaho, San Diego, and Lake DePue) had a total SEM > AVS, indicating that the amount of metals present exceeded the sulfide binding capacity in the sediment, indicating these sediments may cause toxicity in organisms. Across all sediment types an average decrease in pH (0.14) and dissolved oxygen (DO) (0.24 mg/L) was observed. The drops in pH and DO indicate potential oxidation of metal sulfides, resulting in dissolved metals released into the water. Increased dissolved metal concentrations were slight, and increased metals could be a product of easily exchangeable sediment particles, rather than metal sulfide release, or potentially a combination of both. Survival of H. azteca varied across sediment types in SeFEC exposures. The most striking result was the zero survival in the Lake DePue bedded sediments. The toxicity was not observed immediately following the resuspension event, but in later tests of the bedded sediments.  This was the only occurrence in which bedded sediments were more toxic than resuspended and suggested that redeposited sediment at this site had greater toxicity than sediments not resuspended.

Gust chamber exposures utilized Lake DePue sediment which yielded decreased zinc concentrations during bioturbation studies. Cu, nickel (Ni), and Zn levels all exceeded probable effect levels (PELs). Additionally, bioturbation increased abundance of particulate Manganese (Mn) binding ligands.Resuspension of metals also increased both particulate Fe and Mn ligands in Gust chambers. Based on Gust chamber exposure data, there was variable H. azteca survival outcomes depending on treatment.High and low density L. variegatus exposures showed acute toxicity only during the combined bioturbation and resuspension events. The most acute toxic effects were observed when no worms were present, which is counter-intuitive. 

It appeared that most resuspension events, whether conducted in the laboratory or field, were non-toxic events due to the short duration of the exposures. Metals released from sediments were quickly scavenged by Fe and Mn oxyhydroxides becoming non-bioavailable and then settled out onto the surficial sediments. If there were metal sensitive epibenthic species, such as Hyalella azteca, in the area where these resuspension events occur then they may be chronically exposed feeding subsequent feeding on the metal elevated particles. The laboratory studies with the SEFEC and Gust chambers confirmed a lack of effects and the chemical and particle exposure dynamics supported those findings.

Model results showed that during short-term resuspension events, metal bioavailability was limited due to the slow oxidation of metal sulfides, and the binding of released metal to natural organic matter (NOM) and hydrous iron oxide (HFO). This latter effect was enhanced by: (i) the production of additional HFO binding sites during the oxidation of reduced iron phases (e.g., FeS(s), siderite (FeCO₃(s))); and (ii) minimal decreases in pH due to pH buffering and negligible acid production during FeS(s) and FeCO₃(s) oxidation and precipitation to HFO and elemental sulfur (an intermediate sulfur oxidation state). The TICKET-PTM was applied to a pilot field test of a propeller-wash event in San Diego Bay as a proof of concept evaluation of the model capabilities. Model results demonstrated the importance of transport, oxidation kinetics and metal partitioning behavior in assessing the effects of propeller-wash events on copper bioavailability in the overlying water.

The laboratory and modeling work that was performed as part of this project has provided a much clearer understanding of the critical processes that control metal bioavailability and ultimately toxicity effects associated with propeller-wash events. Additional research would be beneficial in assessing differences in metal sulfide oxidation rates in freshwater and marine systems. The development of protocols is also needed for characterizing sediment chemistry prior to resuspension. Further model development should be considered for cases where subsequent oxidation of elemental sulfur to sulfate (and an associated release of acid) may occur. Finally, additional testing of the TICKET-PTM should be performed be a variety of field conditions that are likely to be encountered at DoD sites.   

This research helped improve fundamental understanding of the behavior and ecological impacts of resuspended sediments that contain elevated levels of priority metals (Zn, Cu, and Pb) and oxy-anions (arsenic). This advances DoD’s ability to manage these priority contaminants by contributing detailed, fundamental, and general knowledge of the processes that control their stability, mobility, and bioavailability in sediments. Additionally, this project was performed in cooperation with the USACE who were primarily responsible for national guidance on dredging operations and are actively involved in sediment and contaminant management in harbors and waterways. Results will be transitioned through publication of peer-reviewed journal articles, technical reports, technical symposia, and short course training. It is anticipated that this research will lay the foundation for technical guidance for use in dredging, site risk assessments, and feasibility studies for possible remediation efforts.