An agglomerate for use in economical bulk treatment of contaminated sediments with minimal environmental impact is formed from a sorbent, bentonite clay and sand. The agglomerate has sufficient density so as to sink through a water column into sediment below the water column and is still sufficiently light as to be capable of mixing with the sediment when subjected only to bioturbation. The agglomerate can be formed into pellets and applied to a water column over contaminated sediment by broadcast methods, so as to permit economical remediation of contaminated sediment with negligible environmental impact.