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In this manuscript, we present data from a demonstration in situ capping site (100 m × 100 m) in Hamilton Harbour, Lake Ontario, Canada. A layer of clean medium to coarse sand with the average thickness of 35 cm was placed at the site in the summer of 1995. Concentration of Zn, Cr, and Cd in the original sediments reached values over 6000, 300 and 15 μg/g, respectively. The predicted consolidation of the uppermost one meter of sediment was about 14 cm, which was in good agreement with values obtained from comparisons of moisture content values of pre-capping and post-capping cores. A thin layer of fresh moderately contaminated sediments has started to develop on the top of the cap. In general, the concentrations of elements were greater in porewater than in the overlying water, e.g., the concentration of Fe and soluble reactive phosphorus were 1000 times, and those of Mn 100 times greater. There was a significant reduction in the vertical fluxes of all the trace elements after the capping of the contaminated sediments.