PAH Transport in Amended Sediment Caps

By isolating contaminated sediments, capping can effectively reduce exposure to contaminants and the potential for contaminant transport into the food chain. However, typical sand caps have little sorption capacity to retard the transport of hydrophobic contaminants such as PAHs that can be mobilized by groundwater flow. The overall objective of this research was to develop and improve engineering tools for more efficient cap designs by enhancing the scientific understanding of organic contaminant transport through sediment caps and the role of sorbent amendments in enhancing cap performance. Laboratory column experiments were performed using contaminated sediments and capping materials obtained from a creosote contaminated EPA Superfund site. The study examined activated carbon and peat amendments to the cap as ways to enhance contaminant retardation and evaluated the effect of biological activity in the cap on PAH transport. A major contribution of this research was the measurement of very low concentrations of a range of PAHs in cap pore water using a recently developed solid phase microextraction analytical technique. Azoic laboratory column experiments demonstrated rapid breakthrough of lower molecular weight PAHs when groundwater seepage was simulated through a column packed with coarse sand capping material. After eight pore volumes of flow, most PAHs measured showed at least 50% of initial source pore water concentrations at the surface of 65 cm capping material. PAH concentration in the cap solids was low and comparable to background levels typically seen in urban depositional sediment, but the pore water concentrations were high. Thus, performance monitoring of sand caps need to include measurement of PAHs in cap porewater. Column experiments with a peat amendment delayed PAH breakthrough. The most dramatic result was observed for caps amended with activated carbon at a dose of 2% by dry weight. PAH concentrations in the pore water of the activated carbon amended caps were 3-4 orders of magnitude lower (0.04�0.02 �g/L for pyrene) than concentrations in the pore water of the source sediments (26.2�5.6 �g/L for pyrene) even after several hundred pore volumes of flow. Enhancing the sorption capacity of caps with activated carbon amendment even at a lower dose of 0.2% demonstrated a significant impact on contaminant retardation suggesting consideration of active capping for field sites prone to groundwater upwelling or where thin caps are desired to minimize change in bathymetry and impacts to aquatic habitats. The major trends of PAH transport through sorbent amended caps were reasonably predicted using standard approaches used in groundwater transport modeling Azoic caps were also compared with biologically amended caps. The biodegradation potential of PAHs in sand caps used for sediment remediation was high in the presence of oxygen and nutrients. An initial lag phase in the biological amended cap allowed the early breakthrough of some low molecular weight PAHs; however, over time PAH degradation occurred in the column reducing the concentrations through the cap. When pore water concentrations of 10 parent PAHs and 14 alkyl clusters were measured, the surface of the biological cap remained toxic (based on the US EPA's narcosis model). Results from this study indicate sorbent amended caps combined with biological activity may lead to long-term sustainable cap designs even in areas prone to groundwater upwelling