On 3 August 2012 an area of woodland 100 m across in Assumption Parish, Louisiana, unexpectedly sunk 130 m into the ground, the tops of the Cyprus trees just visible through the mire that filled the hole. Local authorities have issued a mandatory evacuation order on about 150 homes in the area; this is short of a full compulsory evacuation order, and does allow people the option of remaining in their homes for the time being. The nearby Highway 70 was briefly close, while damage to a natural gas pipeline was investigated, but has now reopened.
Areal photograph of the Assumption Parish sinkhole. The tops of trees can be sen in the water that has flooded the hole. ABC News.
When the sinkhole opened up bubbling was observed at the surface and an oily sheen observed, causing concern that the hole could be linked to previous oil and gas exploration in the area, with the possibility of radioactive contamination (crude oil in some areas contains radioactive material). But investigations by the Louisiana Department of Environmental Quality revealed the oily material to be mostly diesel oil and found no trace of radioactive material.
The hole is now being linked to a brine extraction operation run by the Texas Brine Company in the area until last year. Such operations extract salt from subterranean deposits by injecting water into the saltbeds then removing the water with the dissolved salt, which can then be recovered and sold. Such operations can cause subsidence by removing layers of salt from underneath overlying rock beds. The Texas Brine Company, which still retains responsibility for the site, is drilling an exploratory borehole to try to determine the state of the salt caverns beneath the area, and is said to have made a 'substantial contribution' to a fund to assist residents of the evacuation zone, though how this fund will be administered is still not clear.
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