Hillsborough releases plan to deal with landfill sinkhole

The Hillsborough County Solid Waste Department presented a plan today to fix a massive sinkhole at its Southeast County Landfill in Lithia. The plan includes regular monitoring of dozens of wells both belonging to the county and on private property and a way to fill the crater in without contaminating groundwater drinking supplies.

The expansive plan was sent to the Florida Department of Environmental Protection this afternoon. The goal: have all the work done and the hole filled in by June, before the rainy season.

The four-phase remediation plan includes injecting concrete grout into the crater to stabilize the tumbling walls and collapsing bottom. That will allow county workers to excavate earth from the hole to create a safe environment for workers to permanently plug the hole and for scientists to figure out why it collapsed in the first place.

The report said the landfill was placed at the site in the early 1980s because experts had said that sinkholes in that area wouldn't be a problem. What caused this sinkhole, the report said, has yet to be determined, but hydrologists have noticed a drop in groundwater levels over the weeks leading up to the earthen collapse.

The clay liner that lies beneath the landfill and protects groundwater reserves from contaminates in the landfill, was damaged by the sinkhole, the report said, and a priority of the county is to get that repaired.

Additionally, the county will punch two new monitoring wells into the ground and take regular samples from existing wells from as far as a half-mile away to see what, if any, impact the massive sinkhole is having on groundwater quality.

The sinkhole opened up last week, raising concerns that it could contaminate the wells of nearby residents. Since then, the crater has stabilized and isn't growing larger or deeper, but it is perched precariously close to underground methane gas collection systems and pipes containing water - called leachate - that comes into contact with garbage in the dump.

A plan of action includes hydrological charts, geological graphs and topographical maps of the area and is chock full of scientific data.

Scientists from county and state agencies have been busy over the past 11 days trying to come with a way to fix the hole without contaminating water supplies.

On Tuesday, the county presented a detailed plan that lays out how it hopes to determine the effect on the aquifer as well as nearby streams and tributaries.

The plan is in three phases and has already been implemented, the report said. That involves taking regular samples from monitoring wells around the landfill; some as close as 140 feet from the crater and one that is 438 feet away.

The county also plans to take samples from wells of six nearby residents to see if their water quality has changed.

Additionally, there are four currently unused irrigation wells that had provided water to an orange grove a half-mile away. The county plans to install pumps in those four wells that reach deep into the aquifer and begin drawing water for testing.

The crater opened up last week. The depression formed on Dec. 13 and a day later, it caved in. At its widest, the hole measured 80 feet wide, 108 feet from where the earth begins to crack around the crater, and had fallen to a depth of about 60 feet.

The sinkhole has seemingly stabilized and is not as deep as it was in recent days, officials say. They attribute that to the sides of the crater falling in to the bottom and filling it up.