Sediment amendments are materials that can be mixed into the surface layer of sediment in rivers, streams, bays and lakes. The amendments help reduce the exposure of organisms living in sediment and surface water to contaminants found in porewater, the water between sediment grains. The technology occurs “in situ” (in place) which avoids removing sediment for treatment or disposal. In situ sediment amendments are used to treat a range of contaminants, including PCBs, petroleum products, solvents, pesticides and metals.