Hazardous Substance Research Centers/South & Southwest

Contaminant Release During Removal and Resuspension

Principal Investigators

Danny D. Reible, J.W. Fleeger, and John Pardue, Louisiana State University
Mason Tomson, Rice University

Mason Tomson and Amy T. Kan, Rice University
Louis J. Thibodeaux, Louisiana State University

Objectives: In resuspension of contaminated sediments during dredging heavy metals, such as Pb, Cd, Cu, and As, represent several special challenges and will be the focus of this HSRC research. During resuspension generally the largest physical-chemical effect, with respect to heavy metal sorption, is the change in redox of the freshly disturbed sediments. At the point of dredging the sediments are suspended in the river bottom and there is an immediate increase in solid surface area and corresponding immediate change in the physical chemical parameters that characterize the water. Following these immediate changes there will be several time scales that are applicable: 1) the slower redox processes; 2) the desorption kinetics; and 3) the relative rates of redeposition of the sediment particles. The objective of this study is to understand the dynamics and kinetics of heavy metal release processes.

Approach: Uncontaminated and contaminated sediments will be obtained from rivers or bayous in LA coordinated by Louis Thibodeaux. Sediment samples and redox conditions will be preserved as undisturbed as possible. Several different sediments, characterized as having different representative sediment properties (see proposal for details) will be used. Associated or overburden water will either be used directly or simulated in the laboratory. Next, experimental methods similar to the US Army Corps WES DRET method will be performed in the laboratory. These initial experiments will yield solution phase concentrations of metals versus time. Results of these preliminary batch experiments with field sediments will be used as the baseline metal release rates. The impact of known changes in sediment/solution conditions during resuspension will be simulated with these field sediments, including redox and dissolved oxygen, pH, ionic strength, and temperature. The effect of individual parameters on heavy metal sorption and desorption will be studied in batch experiments based upon the results of these tests with actual contaminated sediments. With laboratory contaminated sediments, multistep sorption experiments will be performed to systematically saturate specific binding sites on the solid. Once the range of interest for a particular contaminant-solid combination has been identified the method of "constant composition" desorption will be used for a few combinations to obtain precise stoichiometry, kinetics, and equilibrium information at fixed chemical potential driving force. Key samples will be used for extensive characterization by modern surface methods, such as atomic force microscopy (AFM) and extended range XAFS (EXAFS).

Expected Results: The relative interplay between immediate physical-chemical changes, redox, heavy metal desorption, and redeposition for real sediments will be modeled by changing one parameter at a time. Change in solution and solid surface redox is expected to be the most important parameter controlling heavy metal release during dredging. How this redox varies and thereby alters the kinetics of heavy metal release is not known, but is probably related to sediment properties such as sulfide/oxide content and to sediment organic matter. Once key descriptors have been identified, simplified assays and predictors will be developed for routine use. The final hypothesis to be tested is that sorption and desorption of heavy metals can be modeled using readily available or measurable properties of sediments and dredged materials along with properties of potentially impacted surface water bodies. Understanding the key physical and chemical parameters that affect heavy metal desorption during dredging and resuspension will enable regulators and field practitioners to use only a few key sediment/water parameters and reliably predict the environmental risk in specific dredging operations.