Assessing quality and beneficial uses of Sargassum compost

•• Sargassum composts successfully, but should be used in low human-contact areas.•• Arsenic concentrations met U.S. EPA standards but not SCTLs for residential use.•• Compost met fecal coliform U.S. EPA standards for large-scale outdoor piles only.•• Radish plants will grow in Sargassum compost, but arsenic detected in vegetable.•• Beneficial uses recommended for fill, ornamental plants, and mangrove restoration.