Contaminant Flux through Capped and Uncapped Sediments

2012-12-16T00:00:00Z (GMT) by Priscilla Z. Viana
Cd, Cr, Pb, Ag, As, Ba, Hg, CH3Hg and CN transport through sand (25 cm), granular activated carbon (GAC, 2 cm), organoclay (2 cm), shredded tires (10 cm) and apatite (2 cm) caps was modeled by deterministic and Monte Carlo methods. Effective caps prevented above-cap concentrations from exceeding USEPA acute criteria at 100 yr assuming below-cap concentrations at solubility. Sand caps performed best under diffusion due to the greater diffusive path length. Apatite had the best advective performance for Cd, Cr and Pb. Organoclay performed best for Ag, As, Ba, CH3Hg and CN. Organoclay and apatite were equally effective for Hg. Monte Carlo analysis was used to determine output sensitivity. Sand was effective under diffusion for Cr within the 50% confidence interval (CI), for Cd and Pb (75% CI) and for As, Hg and CH3Hg (95% CI). Under diffusion and advection, apatite was effective for Cd, Pb and Hg (75% CI) and organoclay for Hg and CH3Hg (50% CI). GAC and shredded tires performed relatively poorly. Although no single cap is a panacea, apatite and organoclay have the broadest range of effectiveness. Cap performance is most sensitive to the partitioning coefficient and hydraulic conductivity, indicating the importance of accurate site-specific measurement for these parameters. This study also quantified the magnitude of organic and metal contaminant facilitated transport from the sediment to the water column due to gas ebullition at 14 urban waterway locations. The magnitude of the ebullition-facilitated measured fluxes indicates that gas ebullition is an important pathway for release of both polycyclic aromatic hydrocarbons (PAHs) and heavy metals from buried sediments in urban freshwater systems. Comparison of direct benthic release rates to ebullition facilitated rates suggests that total PAHs are released at significantly greater rates by biogenic gas production. Although the increase in release rate is not as great for metals, ebullition facilitated release rates are frequently greater than benthic release. Mechanistic and empirical models developed in this study may be used to predict in situ gas ebullition flux and ebullition-facilitated contaminant flux.