RIVIERA BEACH — A phosphorus grenade and a 55-gallon drum of hydrogen fluoride washed ashore on Singer Island Wednesday, Riviera Beach Fire Rescue reports. A beachgoer found the material along the shore in Singer Island at about 11 a.m. The area has since been cleared and blocked off by city police and the Palm Beach County Sheriff’s Office’s hazardous material unit, said fire rescue spokesman Dwayne Watson. The drum of hydrogen fluoride was open, but investigators are unsure if it leaked into the water, Watson said.