The odor is coming from an area inside the former Concord Naval Weapons Station, near the Concord Pavilion.
Some Concord residents say the strong smell of crude oil has been unbearable.
"I could smell it in the air. So as soon as they take the earth away from what it was covering up, it really comes out," said Quintin Grady, a Concord resident.
A representative from Conoco-Philips -- the owner of the pipeline -- called neighbors explaining why they detected an odor coming from behind their homes, located on the former Concord Naval Weapons Station.
The message left on a resident's phone said, "...and that odor was of the nature, so they started excavating to see if there's a possible pin-hole leak or something."
The odor this time was noticeable, but on Nov. 8, 2011 neighbors say the fumes were so overwhelming many of them had to stay in hotels.
"I'm still dealing with the cough and the sinus infection and headaches from the initial one," said Brenda Grady, a Concord resident.
The Bay Area Air Quality Management District issued a public nuisance violation to Conoco-Phillips for the November leak. By phone, Conoco-Philips simply said it was doing maintenance work and it would get back to us with details. They never did.
"I've emailed them. They don't do nothing [sic]. They really don't. It's sad to say," said Nick Alderson, a Concord resident.
Contra Costa County Hazmat says it's actually been in contact with Conoco Philips and they've been told the oil has actually migrated under private property.
Lisa Hoctor lives directly behind the site and her backyard is a habitat for the federally threatened red-legged frog.
"My fear is the oil has penetrated down to where the frogs are hibernating and it might have killed the frogs," said Hoctor.
Crews worked throughout Tuesday night to fix the problem and by Wednesday night the fumes were gone, but the neighbors still have a lot of unanswered questions.