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Previous studies conducted in our laboratories have shown that submarine groundwater discharge (SGD) can significantly increase metal fluxes from capped contaminated sediment to the overlying water. Five columns were set up in the laboratory to evaluate the effects of environmental factors such as groundwater pH, sediment depth, and groundwater flow rate on metal transport from capped contaminated sediment under conditions of SGD. Acidified groundwater discharge was shown to enhance the mobility of all tested metals except Mo. Although much of the released metal was adsorbed by the capping material, significant increases of initial or steady-state fluxes to the overlying water were observed for Ni, Cu, Zn, Cd, Pb, and Mn. Additional sediment depth enhanced steady-state fluxes for all tested metals except Mo, Cd, and Pb. Increased SGD rates did not significantly change the average metal concentration in the outflow to the overlying water for most metals; however, all metal releases were higher due to the greater flow at increased SGD rates. The residence time and the redox conditions may be important in evaluating environmental effects on capping efficiency.

Theoretical estimations and laboratory studies suggest that capping can effectively retard contaminant transport from sediments under undisturbed conditions. However, contaminated near-shore areas, commonly selected as capping sites, are frequently subjected to submarine groundwater discharge (SGD). Column experiments were set up in the laboratory to simulate metal transport through sediment and capping material in the presence and absence of SGD. In the absence of SGD, capping enhanced Mo flux and initial Mn flux while having no effect in retarding Fe flux, presumably due to altered redox conditions. This effect was more pronounced in the presence of SGD (4.7 x 10(-4) m/h specific discharge). Capping enhanced Cd flux and initial fluxes of Ni, Cu, and Zn under conditions of simulated SGD, which may be caused by co-transport with Mn and Fe and oxidation of sulfide. Capping retarded Cr and Pb fluxes and steady-state Ni, Cu, Zn, and Fe fluxes in the presence of simulated SGD. However, capping efficiency decreased relative to that with no SGD. Elevated Mn concentration was detected at the capping surface with simulated SGD. Results indicate that advective flow may lead to significantly higher metal fluxes than those under undisturbed conditions.