KNOXVILLE, Iowa — A sinkhole that baffled Marion County officials and residents may be the result of a limestone mine, according to a final site investigation report by the Iowa Department of Agriculture and Land Stewardship (IDALS).
The sinkhole is located in the east right of way of 135th Place, just south of a limestone quarry operated by Bruening Rock Products, and extends onto a private landowner's field.
IDALS concluded the sinkhole formed from a collapse of a limestone mine based on the available evidence.
The department claims roof problems in the area are known to exist, based on a sinkhole repair due to a limestone mine collapse that occurred in 2009.
Furthermore, IDALS referenced conversations between Marion County and Bruening Products that indicated other roof collapses in the portion of the mine.
However, Bruening Rock Products told officials that "this portion of the mine is abandoned and that the extent of their mining did not go south of the house", according to the site investigation report.
When a Mine Safety and Health Administration inspector entered the mine to investigate, they informed Marion County that the southeast portion of the mine is currently underwater.
"Therefore, no underground limestone mine observations were possible near the sinkhole for verification," the site investigation report reads.
Still, there is little evidence to suggest the sinkhole could have been caused by anything else, IDALS said.
"Even though the limestone quarry indicated they did not mine this far south, their maps are not very detailed or georeferenced to the surface," the report reads.
Originally, officials believed the sinkhole could have been formed from an abandoned coal mine. The area has reported the existence of abandoned coal mines in the past.
However, this theory was disproven when IDALS bore into the ground and found only one coal seam, "which is unlikely to have been mined because of its small thickness", the report said.
Since it cannot be definitively determined if the sinkhole is related to an underground coal mine, IDALS is not authorized to spend grant money to mitigate the sinkhole.
"We recommend this report be provided to Bruening Rock Products for their use," the report states.
After receiving the report on May 4, Marion County met with "interested parties" on Thursday to discuss next steps, according to a press release.