Individuals

• Timothy E. Ford, Ph.D. Research Interests: transport and transformations of contaminants in aquatic systems, survival of pathogens in the environment, biofilms in water distribution systems.

• James P. Shine, Ph.D. Assistant Professor of Environmental Chemistry, Harvard School of Public Health. Research Interests: Fate of contaminants in aquatic environments, metal speciation and bioavailability.

• Raveendra V. Ika, Research Specialist, Harvard School of Public Health.

• Chunhua Liu, Ph.D., Harvard School of Public Health.

Objective

Theoretical simulations suggest that capping is very efficient in preventing contaminant transport to the overlying water. For example, published calculations based on molecular diffusion alone suggest that it would take more than 900 years for trichlorophenol to break through a 45 cm cap. However, there have been few detailed investigations of capping efficiency under conditions of advective flux, with past research focusing on diffusive fluxes. This study is designed to investigate the efficiency of capping under conditions of both advective and diffusive fluxes. Advective fluxes are a function of submarine groundwater discharge (SGD), tidal effects and changing chemistries from interactions of groundwater, pore water and seawater.

Approach and Preliminary Findings

In laboratory studies, we have shown that over a six month period no metal (Ag, Cd, Cr, Cu, Ni, Mo, Pb, Zn) transport occurs into capping material in simulator cells under undisturbed conditions. Hence, it is reasonable to assume that sedimentation rates in depositional areas may be sufficient to prevent contaminant breakthrough by molecular diffusion. In order to evaluate advective transport of metals through capping material, we have established a series of capping simulator cells with different groundwater chemistries and flow rates. Our data suggested that advective transport of sediment pore water may potentially lead to a very high flux of metals to the overlying water after capping. The time to reach this breakthrough depended on the depth of capping, property of capping material and flow rate of SGD. When coming to the steady-state stage, advective transport of metals is highly significant under conditions of SGD compared with diffusion process. Results also suggested that sorption capacity of metals by capping material sand may be very small. In general, results from first set of lab experiment suggest that SGD may significantly effect capping efficiency and, therefore, greater emphasis on groundwater hydrology is critical before selection of near-shore capping sites for disposal of contaminated sediment. The second set of lab experiment was conducted to study mechanism of metal transport under conditions of SGD and influence of metal transport by different factors such as depth of sediment, SGD flow rate, acidity of SGD, and Dissolved Oxygen (DO) in SGD. A mathematical model was set up to model metal transport through capping material under conditions of SGD considering physi-chemistry, hydrology factors.