BAYOU CORNE — Explosive concentrations of gas were found this week in soil underneath two homes and a shed built on concrete slabs in the Bayou Corne community near a large swampland sinkhole, officials said Wednesday.

Workers with Texas Brine Co. contractor Tetra Tech drilled small holes through the slabs and collected the gas Tuesday through a quarter-inch tube from the top 1 to 2 inches of soil underneath, officials said.

The gas appears to be of a sufficiently low pressure so as not to migrate through the slabs to pose an explosive risk inside structures, parish officials said. Air monitoring also has not detected above-normal gas levels inside the homes, officials said.

But Louisiana Office of Conservation officials said Tuesday that the slab findings coupled with prior observations of gas making its way to shallow soils and strata at low pressures — such as bubbling in rain puddles near utility line guy wires — verify the need for safety measures being recommended by authorities.

The measures include installation of in-home air monitors and ventilation systems, as well as testing of residential crawl spaces and slabs.

“The Office of Conservation and Assumption Parish Incident Command continue to advise property owners who have not allowed installation of monitoring equipment or under-slab ventilation systems to do so,” officials said in the statement.

Scientists have said for months that methane gas is in the aquifer and even shallower layers underneath the community. They have expressed fears that the failure of a Texas Brine salt dome cavern last year had released to surface areas oil and gas from deep formations alongside the Napoleonville Dome.

An evacuation order applying to the area has remained in place for eight months, in part due to concerns over the colorless and odorless gas accumulating in enclosed spaces in homes, undetected, to explosive levels.

The cavern failure also is suspected of causing the sinkhole, which now has a surface area of more than 13 acres and a depth of 160 feet, according to latest estimates.

Samples of the gas under the slabs have been collected and sent off for further testing, said John Boudreaux, director of the Assumption Parish Office of Homeland Security and Emergency Preparedness.

He said it is not clear if the gas is so-called “swamp gas,” which naturally results from the breakdown of biological matter, or from the deeper formations.

Sonny Cranch, a spokesman for Texas Brine, said early indications are that the samples are, in fact, swamp gas.

“We expect that when the analysis comes back in a few days, it will be swamp gas,” he said.

Texas Brine officials added in a statement that Weatherford Labs recently analyzed gas samples taken from separate shallow probe wells in the immediate Bayou Corne neighborhood.

“All of those samples were determined to be biogenic gas, which is otherwise known as swamp gas,” officials said in the statement.

Boudreaux said scientists working for the Office of Conservation think the formation gas released by the cavern failure may be pushing up existing swamp gas while the deeper, released formation gas works its way toward the surface.

Boudreaux said the slab testing, which began Tuesday, was ordered by the Office of Conservation as part of the response to the sinkhole east of the Bayou Corne community. The Office of Conservation also has ordered Texas Brine to collect and burn off the gas with vent wells.

Boudreaux said the gas in the soil is being found at 100 percent of the lower explosive limit of the gas.

Lower explosive limit works like a marker, showing how closely gas concentrations are approaching a critical point in flammability: when they can produce a flash fire with an ignition source.

Methane, for example, reaches 100 percent of its lower explosive limit when it reaches a 5 percent concentration in the atmosphere, Boudreaux said.

But in an indication the gas is not getting through the slabs, Boudreaux said, the gas had to be vacuumed from the subsurface.

“It does not seem to be under any pressure,” he said.

Cranch also said inside air monitors for flammable gas concentrations and hydrogen sulfide gas have not detected either of the gases inside the structures where the slab testing occurred.

Cranch added that none of the 80 pairs of detectors in 35 homes have yet to find above-normal accumulations of flammable gas or hydrogen sulfide, though there have been false alarms.

Boudreaux said that the explosive gas concentrations found in the soil Tuesday may reflect the small of amount of air underneath the slabs, where little air movement occurs.

As a result, he said, the resulting small amount of gas in the soil would dissipate very quickly if it did reach a larger volume of ambient air.

Cranch said that in all, 13 property owners have agreed to the slab monitoring. Nineteen other property owners have refused.

He said a third home also was tested this week, but no gas was found under the slab. Only one of the three tested residences is occupied, Cranch said.

Carol Vincent, 69, said she lives in one of the houses on Sauce Piquante Lane where gas was detected. She said workers tested her slab Tuesday and told her Wednesday about the flammability.

She said she is remaining in the house and has been told by the workers it was safe to do so. She said she has been living in the home with her husband, despite the parish-issued evacuation order, but not for much longer.

“We’re fixing to buy a house,” Vincent said. “We’re already starting that because we’re not staying here.”

She said her in-home monitors did go off Tuesday, but she was told it was a false alarm.

Texas Brine officials said the gas under the slabs will be tested again in a week.