DEBARY — Mary Sue Scott couldn’t believe her eyes when she visited Gemini Springs Park one day last week.
A sinkhole is growing in a meadow dozens of feet from the spring run, forcing Volusia County officials to cordon off three large areas with temporary fencing while they try to figure out what’s happening below ground.
When Scott, a DeBary resident and advocate for the Volusia County park, heard about the sinkhole, someone told her a third spring had opened. “Are we going to have to change the name to Triplet Springs?” she quipped.
The sinkhole, which began forming last December, isn't a new spring, but it is connected to the pair of springs that give the park its name. When county officials brought in a sand-like filler material to pour into the hole to try to fill it up, they discovered the material flowed out into the popular park's twin springs.
Now, the county may turn the sinkhole into an informational display.
When the hole first appeared in the ground near a large oak tree last December, it was a small area about 18 inches wide, said Tim Baylie, the county's parks director. Now the hole continues to widen. It is several feet long, several feet wide, and more than 10 feet deep. Of greatest concern: A cavern beneath the hole is much wider than the hole at the surface.
The collapsing ground is thought to be related to the heavy rains and high water table after Hurricane Irma, and is one of at least 50 sinkholes reported to the state since the hurricane came through last September, said Jon Arthur, director of the Florida Geological Survey, a division of the Florida Department of Environmental Protection. The county brought in the Geological Survey to help investigate the sinkhole and help the county determine how to manage it.
At first county officials hoped it would be a quick fix. But as it began to get more complicated, Bailey said that's when the first bright yellow temporary fencing went up. The first fencing closed off an area more than 9,000 square feet in size. The county brought in geotechnical engineers and construction engineers to look at the sinkhole.
Rain flowing over the ground and into the hole at Gemini Springs has caused the original opening to get bigger and more of the ground has fallen away, said Baylie. Rainfall draining into the ground at the spot may have helped influence formation of the sinkhole.
The sinkhole is known as a cover collapse, the kind of sinkhole that “makes headlines,” said Jon Arthur, state geologist and director of the Geological Survey. “It’s the exception, not the rule. It’s the slower occurring sinkholes that are more common.”
A cover collapse sinkhole forms when the roof of an underground cavern gradually moves upward through layers of limestone and soil underground until it collapses in on itself. The hole suddenly visible at the ground surface may be much smaller than the cavern underneath.
In early April, the Geological Survey brought in ground-penetrating radar and determined the cavity below ground is much larger than the several-feet wide hole at the surface.
The cavity below ground is narrower at the top and wider below, a sort of hourglass or half-hourglass shape, said Arthur, who hasn't personally visited the site but was briefed by staff who did.
“There was concern about how far out that collapse might work its way out and if there were any other possible areas that could be troublesome,” he said. The research with the ground-penetrating radar "did reveal there might be a couple of areas of some concern.”
This week, two more areas were fenced off in the meadow, each more than 3,000 square feet.
It's possible the Gemini Springs sinkhole could grow larger, Arthur said. "Sinkholes have been known to reactivate with changing conditions and this one was reported to have flowing water at the base."
The Survey's radar could not see well beneath the water in the hole and could not determine how deep the cavity is, said Arthur, who graduated from Seabreeze High School in Daytona Beach in 1978. The water table was visible about 10 feet below the land surface.
Rainfall, underground water flow and soil types play a role in determining when an underground cavern will begin to give way in the layers of limestone, caverns and soil below ground. After a major rain event, water flowing downward through the layers of rock and soil, especially after a period of drought, can create the erosion that causes sinkholes to form. The flow of water can pull down the sediment from above, causing the collapse. “If there’s a place for it to go, if there’s a hole in the subsurface," said Arthur, a collapse occurs when the "right conditions exist at the right depth."
The county will contract with a geological consulting firm to more closely examine the sinkhole and surrounding area, Baylie said. The firm will look for any trouble areas that could collapse in the future.
A meadow slopes downward toward the area where the sinkhole has developed and the springs that are just beyond it. Because all the water flows that direction, Baylie said they’re going to have to work to control the erosion.
Baylie thinks the county and its contractor will try to open up the sinkhole, punching through the smaller hole at the surface to the larger cavern below. Then he hopes to make it “a feature of the park." He plans permanent, attractive fencing with informative signs explaining Florida’s karst geology, how sinkholes form, and how water flows through aquifers below ground to become springs.
He hopes it will help people understand what is happening beneath their feet, he said.
The county's tentative plan to open up the sinkhole is "innovative," said Arthur.
“For a unique instance like this, in an area that’s a natural setting and they want to keep it that way, that’s an innovative approach that I hadn’t heard about,” said Arthur. “If you induce the sinkhole to form, you’re sort of taking control of the situation, and it’s not going to surprise you later. When you punch through it you actually know what you’re going to get." Arthur said he's not aware of anyone who has tried a similar approach in a public park setting.
Mary Sue Scott and others visiting the park are surprised to find the large roped off areas. "It's huge," Scott said. But it's also interesting, she said, "nature working its course."
Check out the state's sinkhole database:
Florida Department of Environmental Protection subsidence incidents