BAYOU CORNE —Bulldozers and men have been at work building the next phase of a 1.5-mile-long berm and upgraded levee system aimed at containing the briney and oily contents of the Assumption Parish sinkhole.
Black geotextile fabric and white geosynthetic liner — about 7,800 linear feet of each — are being laid on top of a sand base that was finished in February, Texas Brine Co. officials said in a written response to questions. The white liner contains a special clay and is used to contain landfills and ponds.
Bulldozers are then spreading and compacting clay on top of the liner and the fabric, an estimated 7,300 cubic yards in all, company officials said.
Conservation Commissioner Jim Welsh ordered construction of the containment system last year as the primary means of keeping the oil and briney water feeding into the sinkhole from below from infiltrating surrounding cypress forests and scenic bayous.
Once completed by July 1 with limestone and drainage structures for heavy rain, the berm will contain 71 acres encompassing the growing 13-acre sinkhole, rise to 5.5 feet above ground level and have a designed lifespan of at least 20 years, Texas Brine officials said in their written statement.
Office of Conservation officials said in a written response to questions that the sinkhole is likely to widen and become more shallow over time even after periodic tremors end but said the hole itself isn’t likely to go anywhere.
“The sinkhole itself is likely to be a permanent feature even after the subsurface has fully stabilized,” officials said.
Conservation said they remain in emergency response mode and have set “no end point” for the berm.
The sinkhole formed last year after the failure of a Texas Brine salt dome cavern deep underground. About 350 people in the Bayou Corne and Grand Bayou communities have been evacuated for more than eight months.
Tom Killeen, state Department of Environmental Inspection Division administrator, said it is likely the briney water in the sinkhole will freshen over time from inflowing groundwater.
Monitoring around the berm containment area will continue at least until the sinkhole water becomes homogeneous with the surrounding waterways, but there is no way to know how long that will take, he said.
“I foresee a longterm monitoring plan staying in place,” Killeen said.
Texas Brine officials, who will oversee the berm, said in a statement that filling the sinkhole would not be viable currently due to the sheer volume of the hole, the massive truck traffic required to fill it and the large hole that would be left where fill would be removed.
Conservation officials said they have discussed filling the hole, which has a volume of 1.2 million cubic yards, but are waiting on new seismic data to see what impact the dirt’s added weight would have on the rumbling fractured rock zone lying underneath the sinkhole.
Texas Brine officials said removing the brine is not viable now due to sinkhole stability now but said they will continue to evaluate the idea.
Rodney Mallett, a spokesman for DEQ, said that continued water quality testing since the sinkhole emerged in early August have shown that salt and other contaminants have not escaped into surrounding waterways and affected swamps and fisheries.
While virtually fresh at the surface, the sinkhole’s deepest waters have a projected salinity that would be about 25 percent salt by volume. The ocean has a salinity of 3.5 percent. Salinity in some parts of the Great Salt Lake in Utah reaches 28 percent, according to the U.S. Geological Survey Utah Water Science Center.
Ralph Portier, a professor in the LSU Department of Environmental Sciences, said the salt water poses a risk to fish, invertebrates and the freshwater vegetation, pointing the effect years of salt water intrusion has had on the state’s coastal wetlands, as an example.
While oil and gas condensates coming up from the sinkhole can biodegrade over the long term, he said, salt and other inorganic compounds will not.
He said using a berm to control the site’s hydrology is not any different than what would be done at an industrial site.
“Salt water is denser than fresh water,” Portier said.
“If you hydrologically control the water, you are not going to have that much off-site movement of water,” Portier said.
The heavier, denser brine, Conservation officials say, will tend to stay below fresher water on the top of the sinkhole.
Conservation officials added that any brine moving up the underground fracture zone beneath the sinkhole and its surrounding berm may rise to an aquifer in the area that is only used by industry.
But, for similar reasons related to brine’s density, the brine wouldn’t reach the root zone of the swamp vegetation without first displacing all the fresher aquifer water above the brine.
Conservation officials said that over the very long term — they were asked about a span over hundreds to thousands of years — the sinkhole is likely to fill in or become indistinguishable from the land around it, given Louisiana’s soft soils and frequent floods.