The Los Angeles Times reported an Orange County bomb squad was called in after people working in a drainage ditch found the deadly cache shortly before 9 a.m. The FBI joined the investigation, the newspaper said.
Orange County Sheriff's Office spokesman Jim Amormino said bomb technicians detonated the six grenades and other materials after the area was cordoned off.
"It was a dangerous situation," Amormino said. "It could have caused major structural damage to the bridge."
No arrests had been made but he said the person responsible appeared to have a good understanding of explosives.
"Possibly in the military," he said. "Possibly somewhere else. We don't know yet."