Laboratory simulator cells were employed in a preliminary assessment of the effectiveness of a sand cap for containing highly mercury-contaminated sediments, and the resultant impacts on the fate of mercury beneath the cap. Experimental observations in simulator cells with a 10 mm sand cap showed no significant mercury migration into the cap layer over a period of approximately eight months. The total mercury flux through a cap and into the overlying water was undetectable, compared to ~10–2 to10–3 ng m–2 s–1 from exposed uncapped sediment. These results indicate that the sand cap can effectively delay and reduce the migration of mercury into the overlying water. Experimental observations also showed that the presence of a cap decreased the ratio of methylmercury (MeHg) to total mercury. Redox potential and oxygen concentration measurements showed aerobic conditions extended only a few millimetres into the sand cap, causing reducing conditions throughout the underlying contaminated sediment. It is postulated that the sediment beneath the cap was less conducive to mercury methylation, which tends to occur only where sulphate reduction is first initiated, and not in fully reduced sulphide rich environments.

Key words: capping remediation, mercury contaminated sediment, mercury transport, methylation