Hurricane Harvey’s floodwaters triggered a spill of almost a half-million gallons of gasoline from two storage tanks along the Houston Ship Channel, marking the largest spill reported to date from a storm that slammed into the heart of Texas’ huge petrochemical industry.

The spill measured 10,988 barrels, or more than 461,000 gallons, and occurred at a petroleum tank farm in Galena Park operated by Magellan Midstream Partners, according to the Oklahoma-based company and accident reports submitted to federal officials.

Some of the spilled fuel flowed into a waterway adjacent to the ship channel, a heavily-industrialized area that’s lined with dozens of petrochemical facilities, the reports said.

Gasoline is more volatile than oil, meaning it evaporates more quickly after it’s spilled. But it’s also more likely to catch fire and can more rapidly penetrate the soil and potentially contaminate groundwater supplies.

Magellan spokesman Bruce Heine said the gasoline that reached the small, unnamed waterway had been contained. The spilled fuel was sprayed with foam to prevent it from releasing harmful vapors, he said.

“Federal and state regulators have been on-site during the recovery and clean-up procedures,” Heine said. “Clean-up activities at the facility are continuing and we are currently removing and replacing affected soil.”

The U.S. Environmental Protection Agency said in an emailed statement that it was not aware of any environmental damage from the spill outside of Magellan’s Galena Park facility. The agency said there was a chance the gasoline would enter the ship channel but agency personnel were not aware of that happening.

The Texas Commission on Environmental Quality planned to investigate the accident to determine if Magellan had complied with its operating permit and state rules that require the company to disclose any air pollution emitted by the spilled gasoline, CEQ spokeswoman Andrea Morrow said.

Environmentalists criticized officials for not being proactive in publicizing the spill and warning Houston-area residents that it had occurred.

Luke Metzger, director of Environment Texas, said the group had received anecdotal reports of people in the Houston area suffering headaches, respiratory problems and other symptoms associated with exposure to chemicals in contaminated floodwaters.

“They ought to know exactly how much gasoline was spilled, where it is now, how the state is containing it, and whether they should worry about any ongoing public health threats,” said Kara Cook-Schultz with the advocacy group TexPIRG.

The spill was first reported to state and federal officials on Aug. 31, but no volume was given at that time. The company later that day reported the spill was 1,000 barrels. In a report filed last week and released Monday, Magellan put the spill more than 10 times higher at 10,988 barrels. That’s equivalent to the combined volume of about 51 tanker trucks used to deliver gasoline to service stations.

Heine said state and federal regulators had been notified promptly of the company’s best assessments of the volume.

The Associated Press has identified more than two dozen spills from fuel and chemical tanks that failed during Harvey. At least 14 tanks failed when their roofs sank under the weight of Harvey’s unprecedented rainfall. Others were swept away by floodwaters.

Including the Magellan spill, the accidents have released more than 600,000 gallons of fuel.

Heine said what happened with Magellan’s tanks was “related to flooding associated with the hurricane,” but would not elaborate and said the cause was still under investigation.

Government regulations do not require tank owners to take specific steps to make the tanks flood resistant, although researchers have warned for years that they are prone to break open during severe hurricanes. In 2005 during Hurricane Katrina storage tank failures spilled millions of gallons of fuel into floodwaters.