Hazardous Substance Research Centers/South & Southwest

In-Situ Containment and Treatment: Engineering Cap Integrity and Reactivity

Principal Investigators

Joseph B. Hughes and Mark Wiesner, Rice University
Billy Edge, Texas A&M University
K.T. Valsaraj, Louisiana State University

Objectives: Contaminant transport and fate in sediment caps is dependent on the structure of the sediment cap, which in turn depends upon the methods used to construct the cap. Current approaches to constructing sediment caps largely focus on stabilizing or isolating underlying contaminated materials. However, variations in current construction techniques may lead to variations in contaminant transport across the cap, as well as differences in cap aging, stability and permeability. The construction of second-generation "reactive" caps will likely require a more sophisticated understanding and control of the depositional processes involved in constructing a cap. This research will address fundamental processes controlling sediment deposition as they affect the engineering of sediment caps as currently practiced, and as applied to the development of advanced capping technologies. We will test the hypothesis that the surface chemistry of conventional capping materials can be altered to achieve a desired cap structure and/or reactivity.

Approach: A lab-scale flume will be constructed to study the effect of release methods, materials selection, and ambient conditions such as cross-flow and salinity on cap structure. Cap structure will be quantified in macroscopic terms using parameters such as permeability, porosity, and grain-size distribution. The micro-scale structure will be characterized in terms of the fractal dimension of the deposited mass. Experiments will be conducted under conditions of stagnant deposition (column studies) using suspensions of particles ranging in size from approximately 1 µm to 1 mm. Mixtures of particle sizes will be investigated including deposition of fine particles onto a gravel substrate. Various surface treatments for the capping material will be investigated through the addition of polymeric materials. Changes in capping material surface chemistry (adhesion probability and adsorptive capcity) will be quantified.

In parallel with this work, simulations of particle deposition will be performed using Lagrangian methods in which the trajectories of individual particles are calculated from second-law principles. This involves the integration of the individual stochastic equations for particle motion and the calculation of individual particle trajectories from randomly selected release points over the surface. Computer models based on Monte Carlo techniques will be developed to investigate particle deposition in 3-dimensional Euclidean space on a plane. Characteristics of the simulated caps will be compared with experimentally determined characteristics to better understand limitations in the numerical model. It is hoped that such a comparison will lead to reasonable predictions of deposit morphology as function of fundamental physico-chemical parameters for a given cap-forming technique.

Expected Results: This work will allow us to form caps with a desired set of characteristics. Better control over cap formation and methods for constructing caps where the micro- and nano-scale properties of the cap can be designed will improve our ability to control transport and targeted reactivity of contaminants within the cap. Such improvements will allow for better long-term containment and remediation of hazardous materials in sediments.