Firefighters have been spraying the outside of the tank with water hoses to cool the chemicals heating up inside and prevent an explosion.
Lee Zeldin, head of the US Environmental Protection Agency, said Sunday the "most likely scenario" is a "low-volume release," where officials will be able to "monitor, neutralise, and contain the threat".
"The Orange County Fire Authority is working to keep the temperature of the tank down. That is very important," he said on CNN.
He said keeping the temperature under 85 degrees F (29.4 degrees C) is key.
The pressurised tank overheated on Thursday and began venting vapours at a company site in Garden Grove 60 km south of downtown Los Angeles, according to the Orange County Fire Authority.
No injuries have been reported. Air monitoring tests have so far found that air pollution around the evacuation zone is within normal limits, and specialised equipment has been deployed to ensure no gas is released from the compromised tank, state and federal environmental officials said Saturday.
Some Garden Grove residents filed a class-action lawsuit on Saturday against GKN Aerospace Transparency Systems, the company that operates the facility where the tank is located.
Lawyers for residents living in the evacuation zone argued in their federal court lawsuit that regardless of what happens next, property values in the surrounding community are sure to be impacted.
"There is no good outcome here for the people who live nearby," the lawyers wrote in a statement.
"In the best-case scenario, a slow, controlled leak still forces residents out of their homes for an indefinite period, disrupting families, businesses, and daily life. In the worst case, a catastrophic explosion could send a plume and debris across a far wider area, damaging thousands of properties and exposing residents to serious health risks."
Spokespeople for the company didn't immediately respond to an email seeking comment Sunday.
Officials said the valves on the tank are broken, which prevented crews from removing the chemical or relieving the pressure on the tank, said Craig Covey, Orange County Fire Authority division chief.
Firefighters' first hope is to find a way to cool off the chemical inside the tank so it won't leak or explode.
If that is not possible, Purdue University engineering professor Andrew Whelton said it would be best if the tank sprang a leak so the chemical could be mostly contained.
If the temperature inside the tank continues to increase, the pressure will continue to build as the methyl methacrylate converts from a liquid to a gas, because officials said the pressure relief valves on the tank were no longer working.
Whelton said it's unlikely that firefighters would consider creating a hole in the tank because of fears that could create a spark that might ignite the volatile and flammable gas.
Drones were monitoring temperatures at 10-minute intervals to watch for any spikes and planning was underway to ensure a possible leak could quickly be prevented from spreading into waterways or the ocean, Covey said.
