The objective of the project was to assess the potential for placing a cap at the experimental Mud Dump site and to determine reductions in environmental impacts related to capping. Contaminated sediment from dredging projects in the Hudson Estuary, Newark Bay, and contiguous waters were capped first with fine sediments from the Bronx River and Westchester Creek, then with sand from the Ambrose Channel. The capping resulted in a layer of sand about 1m thick lying atop the contaminated sediment. Physical, chemical and biological studies were carried out to determine if the capping effort yielded an intact cap showing resistance to erosion, and if the effort reduced loss of organic and inorganic toxicants from the contaminated material to the water column. It was determined that a cap was successfully placed at the experimental dump site. The cap was still intact and in place after 16 months. Cap erosion was minor; predictions of cap life were in excess of 20 years under normal meteorological conditions. Major storm events, however, are capable of eroding the cap and exposing the contaminated material. During the 16 months of study, the contaminated material decreased in volume by about 4%. Part of the decrease was due to compaction and part was due to loss of solids during dumping and deposition.