Abstract

 A critical step in evaluating and selecting contaminated sediment remediation options is the production of a forecast of the long-term trend of exposure concentrations that will occur under natural conditions without active in situ remediation. This Natural Attenuation forecast provides a baseline reference against which to compare alternative sediment management options and to evaluate the risk reduction they can achieve over time. Model development and application at a number of contaminated sediment sites has indicated that one of the most important parameters for accurately forecasting Natural Attenuation is the depth of the upper mixed (i.e., "active") sediment layer. The upper mixed sediment layer is defined by the depth of surficial sediments where biological activity and physical processes promote sediment mixing and resulting active exchange of the contaminant of concern between the sediments and the water column. The upper mixed sediment layer essentially acts as the biologically available reservoir of historical loadings, determining risks to ecological and human health. Model sensitivity analysis at one of our sediment management sites, the Lower Fox River, has demonstrated that the rate at which PCB exposure decreases with time under natural processes is inversely proportional and very sensitive to the depth of mixing in the surficial sediments of the system. In this presentation, it will be shown that increasing assumed mixed layer depth from 10 cm (determined by radio-isotope depth profiling) to 30 cm smears the existing PCB profile and thus, artificially increases the simulated recovery half-time - as measured by the rate of change of the upper sediment PCB concentration - from ~12 years to >50 years. For this reason we strongly recommend that part of developing a site conceptual model should be sediment contaminant and radioisotope sampling to determine and characterize the spatial extent and depth of the upper mixed sediment layer.