Summary of PNNL Component of Project No.86729 Mechanisms of CCl4 Retention and Slow Release in Model Porous Solids and Sediments

A migration-resistant fraction (MRF) is a portion of polluted sediment's contaminant inventory that exhibits slow release. MRF's are formed as a result pollutant aging. Slow release is a key process that controls organic contaminant transport and fate in a plume long after the major portion of the contaminant inventory of a source term has been depleted or removed. Slow release rates are not well understood nor are they commonly accounted for in subsurface numerical transport models. In this project, we propose to study the accumulation and slow-release behavior of carbon tetrachloride (CCl4) MRF as a function of time, contaminant concentration and different physicochemical properties of sediments. Experiments will be conducted at macro- and microscopic scales under saturated conditions. The results will be used to (1) develop a mechanistic description of slow release of CCl4 in the subsurface environment and (2) lay the groundwork for improving the robustness of numerical models that predict organic contaminant transport and fate under natural conditions. The outcomes of this study are expected to improve the conceptual model of CCl4 subsurface transport at different physical scales and have an impact on remediation and site closure decision-making at DOE sites, especially in situations involving the potential application of natural attenuation