In the hours following a flaring incident on July 6, the company initially reported that a fire in a compressor unit at its Martinez facility led to the release of more than 100 pounds of hydrogen sulfide.
That day Kristin Marczak, a company spokeswoman, told reporters the episode was similar to a "car running and you have an unplanned shutdown of that car. We have a number of units within the plant that operate, and one of those units had kind of an unplanned shutdown."
It is now clear there were five refinery problems at the site over four days, leading to the release of more than 8,500 pounds of gas, according to documents Shell recently filed with Contra Costa County officials and a notification it made to the Governor's Office of Emergency Services.
Environmentalists say this revelation follows a pattern of local oil refineries initially underestimating the severity of malfunctions at their facilities.
"The potential impact from the incident was more serious than we initially realized," said Randy Sawyer, the county's chief environmental health and hazardous materials officer.
The problems began just after 1 a.m. when a loss of pressure inside a refinery unit set off unplanned flaring. During that operation all three of the refinery's flare pilot lights were accidentally extinguished by water, which had accumulated due to a faulty drainage system.
The pilot lights are supposed to remain on to ensure that gas released to the flaring system is burned off, which is considered safer than releasing the chemicals unburned.
Two hours later, while crews were investigating that problem, a small lubrication oil fire triggered one of Shell's hydrocracker units (which help convert lower-quality fuels into higher-quality ones) to go offline, causing another flaring operation, resulting in the release of unburned gas again.
Due to the failures, close to 1,500 pounds of methane, 860 pounds of hydrogen gas and 260 pounds of hydrogen sulfide were released into the air from the refinery that morning.
The release of hydrogen sulfide accounts for five weeks' worth of the typical emissions of that chemical from the refinery, according Bay Area Air Quality Management District spokesman Tom Flannigan.
Perhaps more concerning, the malfunctions also led to the release of close to 5,700 pounds of unspecified "non-methane hydrocarbons." This could include chemicals like propane and benzene.
This amount may have been dangerous, according to Anthony Wexler, director of the Air Quality Research Center at UC Davis.
"This is where the concerns may be," Wexler said in an email, calling the release of those gases "an unspecified mix of potential nasties and lots of it."
Two days later the refinery's catalytic cracking unit "unexpected shutdown," according to documents Shell filed with the Governor's Office of Emergency Services. That caused another round of flaring that lasted close to six hours, leading to the release of 45 pounds of sulfur dioxide, Shell said.
The following day, July 9, in the process of re-starting that unit, the refinery released close to 400 pounds of sulfur dioxide in yet another round of flaring.
The malfunctions have alarmed environmentalists.
"It's quite concerning to see the quantities of dangerous gases," said Hollin Kretzmann, a staff attorney at the Center for Biological Diversity. "As is often the case, what was reported initially drastically underestimated the true extent of the release."
"The fact that there were multiple flaring events in rapid succession over the course of a few days shows that whatever safety measures and protocols the refinery has in place are not adequate to prevent these accidents from happening," Kretzmann said in an email.
Local air regulators issued a notice of violation against Shell for the July 7 incident for failure to maintain continuous operation of its flare pilots.
The Bay Area Air Quality Management District is investigating the malfunctions, Flannigan said in an email.
The district sent staff to the scene several hours after the first refinery malfunction, according to an agency incident report filed that day.
The district said then its investigators did not observe any odors downwind of the refinery and it did not receive any complaints from residents near the facility.
Shell told the district it found no readings on its downwind ground-level monitors in excess of regulatory standards for hydrogen sulfide and sulfur dioxide.
Representatives for the company have yet to respond to requests for comment.
But in its report to the county, Shell says it's in the process of making a series of changes to its Martinez facility to prevent a similar incident.
This story includes updated information from the Bay Area Air Quality Management District and Governor's Office of Emergency Services.