Developing models for the assessment and the design of the in situ remediation of contaminated sediments

Abstract

The sediments in natural environment serve as sinks for contaminants from historical release, particularly hydrophobic organic compounds (HOC) and heavy metals. In-situ remediation, including monitored natural recovery (MNR), in-situ treatment (e.g. sorbing amendment) and in-situ capping, is one of the few alternative economically viable options with a proven record of success for sediment remediation. Modeling is often used to compare in-situ remedial approaches and design a system of meeting long term remedial goals. The fate and transport of contaminants in a remediation system is commonly modeled using a generalized advection-dispersion-reaction equation with potentially different physical and chemical properties in each layer. An analytical solution was developed with computational efficiency and unconditional stability for the multi-layered transport problem with linear processes and was shown to be more convenient for sensitivity analyses and parameter estimation and implement. A numerical model, CapSim, has been developed to model the transport and fate under more general conditions. Several important processes in sediment environments, such as nonlinear and kinetically limited sorption, steady and periodic advection, bioturbation, consolidation and deposition, are incorporated in the model. The current model also allows description of multiplied coupled chemical reactions. It builds on a simpler numerical model of Lampert (2009). It allows assessment of the transport and fate of chemicals under the most important dynamic sediment processes. Performance reference compounds (PRC) are often used to support passive sampling as a means of monitoring sediment processes and in situ remedial processes. An analytical solution was developed for modeling the release of PRC and uptake of target compounds in cylindrical passive sampling system. In the presence of nonlinear sorbents such as activated carbon, the interpretation and application of PRCs is more difficult. The fate and transport model CapSim was used to simulate the behavior of PRCs and target compounds in a passive sampling system with activated carbon. The impacts from the non-linear sorption of the compounds in activated carbon as well as the competitive sorption between an isotope-labeled PRC and the non-labeled compound are discussed.