The leak was detected around 11:10 a.m., when a hissing noise was heard and a white haze seen in the back of the 2400 Monument Blvd. store, said Contra Costa County Fire Inspector George Laing.
The store was evacuated immediately, and hazardous materials teams were called to the scene.
"This was a huge concern in an occupied store on a Sunday afternoon with an unknown gaseous release," Laing said. "Everybody was very concerned about what the release might be."
And with justification, as it turned out.
The leaking gas was determined to be a dangerous refrigerant from the store's mechanical refrigeration system. The gas in question is an asphyxiant, meaning it sinks to the ground and displaces oxygen, and it is odorless, "so you could walk into a cloud of it and not know," Laing said.
While the store's refrigeration rooms are equipped with sensors to detect such leaks, this leak from a pipe outside the refrigeration areas. Fortunately, the gas release was visible, Laing said.
The store was aired out and monitored until the gas dropped below hazardous levels, and a store repair technician was called to the scene to fix the refrigeration system.
As of 3:30 p.m., the store had still not been reopened, as county environmental health officials were checking for remaining signs of contamination.
Laing said the incident, which resulted in no injuries or exposures to the , was an example of excellent coordination among public safety agencies in responding to a serious situation.
Responding agencies included the Contra Costa County and Concord departments, hazardous materials teams from San Ramon Valley and Contra Costa County fire departments, as well as Contra Costa County Environmental Health, Concord police and AMR ambulances.