While one relief well near the Assumption Parish sinkhole was “shut-in and locked” after hydrogen sulfide emissions were detected, no similar problems have been found in recent testing of four other relief wells, a scientist for Shaw Environmental and Infrastructure Inc. said Thursday.

Shaw scientist Gary Hecox said in a blog post Thursday that researchers found no detectable hydrogen sulfide concentrations in gas being flared from two relief wells. The other two relief wells had “low concentrations of hydrogen sulfide consistent with a swamp environment,” he wrote.

Hydrogen sulfide is a colorless gas that is flammable and poisonous at high concentrations. The naturally occurring gas has a foul rotten-egg odor, can occur in natural gas deposits and is a known risk with oil and gas exploration.

Hecox said decaying trees and vegetation in swamps produce hydrogen sulfide.

“While low concentrations of hydrogen sulfide have been detected at some of the bubble sites, the concentrations are well below those that would pose a human health risk,” Hecox said in the blog. Bubble sites are where natural gas travels through the water to the surface.

He also said no hydrogen sulfide was found in the 18 shallow Geoprobe wells that check for the presence of gas.

About 150 homes in the Bayou Corne and Grand Bayou areas have remained evacuated since Aug. 3, when the large sinkhole was found in the swamps between the two communities.

Scientists believe a Texas Brine salt cavern inside the Napoleonville Dome failed and initiated a chain of events that created the sinkhole and released methane and crude oil.

Louisiana Department of Natural Resources officials, private industry scientists and contractors have been setting up vent wells to burn off the methane in an effort to get evacuees back in their homes and camps.