Posted: Sep 5, 2012 4:45 PM by AP
BATON ROUGE, La. (AP) - Hurricane Isaac seems to have had little effect on a sinkhole in Assumption Parish that transformed hundreds of feet of swampland into slurry, but after the storm trudged past, three new bubbling gas sites were discovered and officials were trying to find the source.
There were reports of bubbling gas in the area before the sinkhole appeared, though officials aren't quite sure how they are connected. The three new sites were discovered Tuesday in Bayou Corne, Grand Bayou and Triche Canal.
Representatives from the state Department of Environmental Quality say preliminary tests around the three new sites indicate no abnormal readings. The department is also monitoring the areas around the sinkhole and no harmful readings have been found, DEQ Secretary Peggy Hatch said in a statement.
Kim Torres, a spokeswoman for the Assumption Parish Police Jury, said the sinkhole did not increase in size despite Isaac lashing the parish with heavy rains and wind, but the "magic question" - the cause behind the new bubble sites - is still unanswered.
Three natural gas pipelines in the area have been flared off and depressurized and Crosstex Energy LP of Dallas has said they will reroute its 36-inch natural gas pipeline away from the sinkhole.
Experts believe a brine cavern encased within the Napoleonville salt dome might be the cause behind the 476-foot sinkhole that toppled an acre of bald cypress trees when it first appeared over a month ago.
Greg Hancock, a professor of geology at the College of William and Mary, has been following the saga of the sinkhole. He said the additional rain Isaac dumped on the area likely didn't contribute to the growth of the slurry area because there was already so much water around. The sinkhole is filled with brackish water and officials have said it might be converted to a lake once they discover its source.
He said natural sinkholes are usually formed in rock types that are relatively soluble, like limestone or salt domes. Water infiltrating the ground and dissolving the rock can create a natural cavern that then renders the ground above less stable. Ultimately, a sinkhole is born when the rock collapses in on itself.
However, since the sinkhole near Bayou Corne already existed before the storm arrived he speculated that the additional rainfall would do little to the sinkhole's size or the salt cavern underneath.
"In some cases, having material saturated with water reduces stability. In this case, there's plenty of water already around, so that probably isn't playing much of a role here in generating an additional collapse," he said.
Hancock also said the new bubbling sites were likely not related to the storm.
Patrick Courreges, a spokesman for the state Department of Natural Resources, said an aerial survey from the U.S. Environmental Protection Agency detected no harmful levels of natural gas or radiation in the area. He said crews will drill a new water well halfway between the salt dome and nearby Bayou Corne to test the aquifer for natural gas.
"We wanted to drill something that would be in the path of where you'd expect natural gas to migrate," he said.
All 23,000 residents in the parish were told to evacuate their homes as the storm approached Louisiana on Aug. 28, and drilling on an exploratory well was halted while operators waited for the storm to pass. A mandatory evacuation was already in place for the residents of Bayou Corne after Gov. Bobby Jindal declared a state of emergency when the sinkhole materialized Aug. 3.
The brine cavern was plugged and abandoned in 2011 by Houston-based Texas Brine Co., and company spokesman Sonny Cranch said that while the storm delayed drilling of the exploratory well, operations are expected to begin again late Wednesday.
"We will move at a steady pace from now on. They're waiting for the cement to cure, or harden properly, before they commence drilling," Cranch said.
For several weeks before the sinkhole appeared residents had been reporting mysterious gas bubbles. Parish officials also reported tremors had been felt more than two months and seismic readings from the U.S. Geological Survey were able to narrow down the concentration of the earthquakes to the western edge of the dome, which is where the Texas Brine salt cavern lies.
Cranch said the company is also back on track after a short delay in providing weekly housing checks to evacuated residents. He said they wrote around 150 checks of $875 each to residents.