Modeling Organic Contaminant Transport through Reactive Media

We modeled the transport of 12 organic compounds through five different reactive media under diffusion and advection using deterministic and Monte Carlo methods to evaluate their effectiveness for sediment capping and/or liners. A broad range of media contaminant site scenarios are provided to aid in the selection of reactive media to address a broad range of organic sediment contaminants under diffusion or advection scenarios. Organic compounds evaluated include monoaromatics, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and alkanes covering a wide range of organic pollutants commonly found in contaminated sites. We investigated the performance of the reactive media granular activated carbon (GAC), organoclay (OC), shredded tires (ST), and apatite (AP) in comparison with the performance of conventional sand media. Based on the modeling results, a 2-cm GAC layer is predicted to be as effective to reduce organic contaminant transport as a 1- to 3-m-thick sand layer. Performances of GAC and OC were similar under both diffusion and advection due to their low observed diffusivity coefficients. As expected, low molecular weight compounds (e.g., monoaromatics and naphthalene) present a higher potential release risk. In contrast, pyrene, hexachlorobiphenyl, and tetradecane do not present toxicity concerns at the 95% confidence level after 100 years under diffusion and/or advection (except in AP/sand configurations). Lower acute toxicity contaminants such as benzene and dichlorobiphenyl are predicted to behave similarly for most media under advection, with partial breakthrough within 100 years. Performance uncertainty is substantially lower for organic contaminant transport simulations compared with those in a previous metal contaminant transport study. The relative GAC and OC effectiveness increases with increasing contaminant hydrophobicity. This behavior may be a key decision factor to select GAC and/or OC media at sites with highly hydrophobic and low toxicity/solubility ratio contaminants such as dichlorobiphenyls.