The spill was reported at about 7:25 a.m. at the lagoon, located near Violetti Road, Battalion Chief Andy Pforsich said.
The acid is part of a chlorination system used to manage the lagoon's water, Pforsich said.
The spill was reported by a park employee who came to work and noticed the odor of the acid before seeing the leak. Three park employees experienced respiratory discomfort due to the spill. They were treated and released at the scene, Pforsich said.
About 580 gallons were initially reported to have spilled, but after crews went out to inspect the spill, it was determined that only 40 gallons were unaccounted for, according to Pforsich.
"It probably just got into the soil immediately around the processing area," he said. "We don't believe it reached the lagoon."
The lagoon was closed as a precaution and Sonoma County public health officials did testing at the lagoon this afternoon to confirm that there was no threat from the acid, Pforsich said.
Jim Leddy, Sonoma County's community and governmental affairs manager, said the lagoon was scheduled to reopen at 2:30 p.m., and that the lagoon got a "clean bill of health" from investigators.
The cause of the spill is under investigation, but "it looked like one of the gaskets gave out" in the chlorination system, Leddy said.