But parish officials said none of the in-home air monitors installed in the community have had hits for dangerous levels of gas accumulating in residences, indicating the gas is not getting through the slabs.
John Boudreaux, director of the Assumption Parish Office of Homeland Security and Emergency Preparedness, said the gas was detected by drilling through the slabs Tuesday and collecting the gas that rises from the underlying earth through the new holes.
Samples of the gas have been sent off for further testing, Boudreaux said, but the gas collected was found at 100 percent of its lower explosive limit.
Though expressed as a percentage, the lower explosive limit does not reflect the percentage of gas in the air.
The limit works more like a marker, showing how close gas concentrations are approaching a critical point in flammability: when they can produce a flash fire with an ignition source.
Methane reaches its full lower explosive limit when it has a 5 percent concentration in the air.
Boudreaux pointed out that the 100 percent readings for the lower explosive limit 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 actually dissipate very quickly if it did reach ambient air.
Scientists have known for months that methane gas is in the aquifer and even shallower layers underneath the community. The methane is one suspected consequence of the failure of a Texas Brine salt dome cavern nearby. Scientists also think the cavern’s failure last year caused a sinkhole with a surface area that has now expanded to more than 13 acres in size.
An evacuation order for the area has remained in place for eight months, in part due to the risk of the colorless and odorless methane accumulating in homes, undetected, to explosive levels.
The Louisiana Office of Conservation ordered Texas Brine to collect and burn off the gas with vent wells and also to install in-home monitors to guard against explosive accumulations in closets and other enclosed spaces.
Those orders also called for the slab testing now under way, Boudreaux said.