Environmental impact of ongoing sources of metal contamination on remediated sediments

A challenge to all remedial approaches for contaminated sediments is the continued influx of contaminants from uncontrolled sources following remediation. We investigated the effects of ongoing contamination in mesocosms employing sediments remediated by different types of active and passive caps and in-situ treatment. Our hypothesis was that the sequestering agents used in active caps and in situ treatment will bind elements (arsenic, chromium, cadmium, cobalt, copper, nickel, lead, selenium, and zinc) from ongoing sources thereby reducing their bioavailability and protecting underlying remediated sediments from recontamination. Most element concentrations in surface water remained significantly lower in mesocosms with apatite and mixed amendment caps than in mesocosms with passive caps (sand), uncapped sediment, and spike solution throughout the 2520h experiment. Element concentrations were significantly higher in Lumbriculus variegatus from untreated sediment than in Lumbriculus from most active caps. Pearson correlations between element concentrations in Lumbriculus and metal concentrations in the top 2.5cm of sediment or cap measured by diffusive gradient in thin films (DGT) sediment probes were generally strong (as high as 0.98) and significant (p<0.05) for almost all tested elements. Metal concentrations in both Lumbriculus and sediment/cap were lowest in apatite, mixed amendment, and activated carbon treatments. These findings show that some active caps can protect remediated sediments by reducing the bioavailable pool of metals/metalloids in ongoing sources of contamination.

Removal of low levels of Cu from ongoing sources in the presence of other elements - Implications for remediated contaminated sediments

Mesocosms were used to investigate the effects of Cu influx, alone and in the presence of other elements, on sediments remediated by active caps, passive caps, and in situ treatment. Competitive interactions between Cu and other elements were investigated because contaminants often co-occur. Elements in surface water remained at significantly lower concentrations in mesocosms with apatite and mixed amendment caps than in mesocosms with passive sand caps or uncapped sediment. Element concentrations in Lumbriculus variegatus were significantly higher in untreated sediment than in active caps and significantly related to element concentrations in sediment measured by DGT probes. The cumulative toxicity of Cu mixed with other elements was greater than the toxicity of Cu alone in treatments without active caps, but the ability of active caps to control Cu was not affected by the presence of other elements. Active caps can protect remediated sediments by reducing bioavailable elements in ongoing contamination.

Effect of bioturbation on contaminated sediment deposited over remediated sediment

A challenge to all sediment remediation technologies is the continued influx of contaminants from uncontrolled sources following remediation. However, contaminants deposited on sediments remediated with chemically active sequestering agents may be affected by the sequestering agents resulting in reduced impacts. We deposited sediment contaminated with As, Cd, Cu, Ni, Pb, and Zn over clean sediment capped with the sequestering agent, apatite, and clean uncapped sediment in laboratory mesocosms to simulate the recontamination of remediated sediment by influxes of particle-bound contaminants. Cap effectiveness was assessed in the presence and absence of the bioturbating organism Corbicula fluminea based on metal fluxes to sediment pore water and surface water, the distribution of mobile contaminants in sediment and surface water measured by Diffusive Gradients in Thin Films, and contaminant bioaccumulation by Lumbriculus variegatus. The metal sequestration capacity of apatite caps was unaffected or improved by bioturbation for all elements except As. Effects with uncapped sediment were metal specific including reductions in the bioavailable pool for Ni, Cd, and to a lesser extent, Pb, increases in the bioavailable pool for As and Cu, and little effect for Zn. It is likely that the reductions observed for some metals in uncapped, clean sediment were the result of burial and dilution of contaminated sediment combined with chemical processes such as sequestration by minerals and other compounds. These results indicate that apatite caps can control recontamination by metals regardless of bioturbation but point to the complexity of sediment recontamination and the need for further study of this problem.