By isolating contaminated sediments, capping can effectively reduce exposure to contaminants and the possibility of contaminant transport into the food chain. However, because contaminants are left in place, caps generally require long-term monitoring, and the risks of contaminant breakthrough or sediment resuspension persist. Many contaminated marine sediment sites reside in shallow, coastal areas that are often impacted by advective processes (i.e., groundwater flow, tidal pumping, and wave pumping), sorption-controlled diffusive processes, and bioturbation. These forces may contribute to the advective flux of contaminants through sediments and, ultimately, through a sediment cap.

The objective of this project was to enhance the scientific understanding of contaminant migration through sediment caps in areas with significant groundwater potential or tidal fluctuations. Through a combination of laboratory and field studies, this project specifically sought to (1) examine contaminant mobility over time through an existing sediment cap at the Wyckoff/Eagle Harbor Superfund site in Washington; (2) measure the influence of porewater flux via groundwater advection and tidal pumping; (3) quantify aqueous contaminant mobility in the laboratory; and (4) evaluate the fundamental mechanisms that contribute to polycyclic aromatic hydrocarbon (PAH) sorption and retention in the laboratory.

Building on the current knowledge of contaminant transport phenomena, this project combined field and laboratory studies to examine the fate and transport of PAH contaminants through a sediment cap installed at Eagle Harbor. Field studies consisted of electrical resistivity measurements that were collected at varying depths in the sediment cap and then used to identify areas where fresh groundwater may be entering the marine environment. Thirteen continuous cores were collected in both the cap and native sediment using a vibracoring technique in areas of the harbor where freshening was measured and where no freshening occurred. The core samples were then sectioned in centimeter-thick segments, and a PAH profile was conducted by depth using an in-field rapid screening technique called enzyme-linked immunosorbent assay (ELISA). Results of the ELISA tests were used to profile the distribution of the total PAH analyses (t-PAH) in the sediment cores and to identify segments for further GC/MS analyses off-site. Laboratory studies were conducted at the University of Maryland Baltimore County (UMBC) using sediment collected from the Eagle Harbor site. At UMBC, laboratory-scale columns were constructed to simulate the vertical flux of contaminants. Solid phase microextraction (SPME) was used to measure low PAH concentrations in milliliter-size samples.

During field studies, the rapid sediment characterization (RSC) method proved to be a successful approach for quantifying the sediment t-PAH profile in real-time, making it an effective decision tool for locating additional core samples. The results from the RSC method paralleled the results produced from the more detailed GC/MS method and may provide a cost-effective alternative at this and future sites. In selected cores, experiments indicated that there appeared to be some migration pattern or mixing of PAH-contaminated sediments within the cap profile and some indication that there may be other anthropogenic sources contributing to the t-PAH concentration on the cap surface; however, additional laboratory analyses is needed to provide insight into the mechanisms by which PAH translocation in the cap profile is taking place.

Results from this project build on the current knowledge of contaminant transport phenomena in caps and provide insight into the hydraulic and chemical mechanisms affecting migration of contaminants at capped sites. This will aid the Department of Defense and other federal agencies in developing more cost-effective sediment cap design and construction by identifying sites where capping is appropriate and predicting long-term cap performance and potential risks to human health and the environment. In addition, the results will have value for understanding the behavior of other hydrophobic contaminants (e.g., polychlorinated biphenyls) and for contaminant behavior in natural and engineered systems.