TAMPA — A huge sinkhole that sucked in garbage and ash will be shored up with a concrete mixture while groundwater around it is monitored for pollution, a Hillsborough County plan submitted Wednesday shows.
The hole started to open Dec. 14 at the Southeastern County Landfill in Lithia. It grew the next day, but has stayed roughly the same size since — now 108 feet wide and 60 feet deep.
About 70 percent of the material that disappeared into the hole was landfill waste, and half of that was ash from incinerated garbage, the Hillsborough County Public Utilities report says. The rest was soil.
Still, the county says there is little reason to believe the sinkhole has caused additional contamination. But it will increase its monitoring, the report to state environmental officials says.
After the hole opened, crews lowered a home videocamera deep into the cavity to check for signs of contaminated water, called leachate, leaking inside. The video clips didn't show signs of it, the report says.
The utilities department has said it doesn't yet know the cause of the sinkhole. The report says the location of the landfill was selected in part because of a historical lack of sinkholes there.
In January, dozens of sinkholes had formed in eastern Hillsborough after farmers drained aquifers to protect plants during a streak of below-freezing days.
The sinkhole that opened at the landfill also came during a cold spell when farmers sprayed their fields. But the landfill, on County Road 672, lies about 20 miles from that farming area around Dover and Plant City.
With a review to determine the cause still to come, the Public Utilities Department said it plans to dig new monitoring wells, fill the sinkhole and repair part of the landfill's clay liner system.
Two new monitoring wells will be installed west and downgrade of the sinkhole.
Officials said the clay liner slopes east on the eastern side of the sinkhole, making it unlikely that stormwater could run into the hole from that side.
Berms and piping were also installed to divert stormwater away from sinkhole.
Nine groundwater wells, three surface water points and some private wells will be monitored monthly unless the testing reveals a need for more, the county plan says.
Filling the hole has been broken into three phases:
First, workers will start injecting concrete grout beneath the waste to stabilize the sinkhole. Next, the waste and soils around the hole will be excavated. Then scientists will study the extent of the sinkhole before recommending how to permanently seal it with concrete grout.
Once the hole has been sealed, the clay liner in the area of the hole will be repaired.
The plan says the Public Utilities Department wants to finish the work before June — the start of the rainy season.
The county hopes to use that section of the landfill again after the hole has been sealed.