The release also set in motion Contra Costa County's automatic telephone dialing system, county and refinery officials said.
But residents of Tormey, a 17-home enclave that is the nearest community downwind from the refinery, said they did not hear any siren, and some complained that the warning phone call was not timely enough.
Officials of the refinery, the county hazardous materials division and the Rodeo-Hercules Fire Protection District said the release, which began around 4 a.m., did not endanger human health.
Refinery spokesman Bill Tanner said crews working on a gas line accidentally vented some excess natural gas that was mixed with hydrogen sulfide, a gas that gives off a telltale rotten-egg smell.
County hazardous materials specialist Paul Andrews said the first call to the county came in at 4:08 a.m. Sirens sounded about 4:20 a.m. and officials activated the telephone warning system about 4:30 a.m., Andrews said. The system dialed residents in Rodeo, Tormey and Crockett. Andrews could not immediately say how many people were reached.
The pipeline that leaked Sunday was part of a loop that normally directs excess gases to a flare, Andrews said. Workers capped the line at 6:15 a.m., and the all-clear went out shortly afterward, Tanner said.
Crockett-Carquinez Fire Chief Jerry Littleton said his department got the first call from a resident at 4:14 a.m. The shelter-in-place order was issued at 4:37 a.m. and was lifted at 7:01 a.m., and the all-clear was issued at 7:47 a.m., Littleton said.
The odoriferous gases that triggered the shelter-in-place order included hydrogen sulfide and other reduced-sulfur gases known as mercaptans, Andrews said.
The odors of the gases are strong enough to cause headaches or nausea, Andrews said.
An air inversion at the time of the leak prevented the gases from dispersing, Andrews said.
"Warm air close to the ground and cold air aloft pushed everything down, and there was very little wind," Andrews said. The smell primarily affected Tormey and Crockett, Andrews said.
Tanner said the incident is under investigation.
"We're very sorry to have created this situation," said Tanner, who praised fire district, county hazardous materials and Bay Area Air Quality Management District crews for the quickness and efficiency of their responses.
"We work very hard to ensure that we don't have an impact on the community, and we didn't do a good job this time," Tanner said. "We'll investigate and put procedures in place to make sure it doesn't happen in the future."
Residents of Rodeo and Crockett interviewed by the Times confirmed they heard a siren. Most of those interviewed by the Times who have local numbers said they got phone calls; others who have cell phones said they did not. Some residents said they may have missed calls that went instead to voice mails and were not recorded. One Rodeo woman said she hung up the phone, thinking the recorded message was from a telemarketer.
But four Tormey residents said they had not heard a siren. Dale Barber said no siren even exists in Tormey and that the only siren that is ever heard in the hamlet, and only on certain days, is one in Crockett. Others said they hear the Tormey siren regularly when it is tested the first Wednesday morning of each month but insisted it did not sound Sunday.
"There was no siren," said Kimberly Norris, adding she knows she did not sleep through it either.
"About 3:30, 4 (a.m.) I started smelling something," Norris said, adding she reached someone at the refinery gate who said he would call back. "About an hour after, I got the shelter-in-place."
Her neighbor, Mary Arca, said she also did not hear a siren and received a phone call at 5:18 a.m.
