Sinkholes, Karst Geology, and Your Central Florida Well: What the Polk County Events Tell Us

In January 2024, a sinkhole approximately 70 feet deep and 15 feet wide opened on Royal Crest Drive in Highland City, Polk County. In November of the same year, a second sinkhole, roughly 8 feet wide and 20 feet deep, formed at the same property. Two sinkholes in 10 months at the same address is unusual even for Central Florida, but the underlying geology that caused them is universal across the Orlando metro. Understanding karst behavior matters for anyone on a private well, anyone considering well-drilled property, and anyone whose drinking water comes from the Floridan Aquifer, which is essentially everyone in Osceola, Orange, Seminole, and Polk Counties.

What Karst Actually Is

Central Florida's geology is defined by limestone and dolomite, carbonate rocks that slowly dissolve when contacted by mildly acidic water. Rainwater picks up CO₂ from the atmosphere and soil, forming carbonic acid. Over geologic time, that acid has dissolved the underlying carbonate rock into an extensive network of voids, caves, fissures, and conduits. This dissolved-rock landscape is called karst.

The Floridan Aquifer, the drinking water source for approximately 10 million people across Florida, Georgia, Alabama, South Carolina, and Mississippi, covering roughly 100,000 square miles, is hosted in these karst limestone formations. The same dissolution processes that create water-storage capacity also create structural instability.

How Sinkholes Form

A typical Central Florida sinkhole forms when:

1. A void in the underlying limestone grows gradually over decades or centuries through continuing dissolution.
2. A confining clay layer above the limestone bridges the void at the surface.
3. Changes in groundwater levels (drought lowering water tables; heavy rains saturating the clay) destabilize the bridge.
4. The clay bridge collapses into the void below, and surface soils cascade downward to fill the new space.
5. At the surface, a hole appears, sometimes slowly over hours (a "cover-subsidence" sinkhole), sometimes in seconds (a "cover-collapse" sinkhole).

Cover-collapse sinkholes are the dramatic ones, Highland City's 70-foot-deep January 2024 event is a classic example. Cover-subsidence is slower and easier to miss until significant damage has occurred.

Sinkholes and Wells, The Direct Connection

When a sinkhole opens, it can directly breach the confining clay layer that normally separates surface water (and surface contamination) from the Floridan Aquifer below. In ordinary conditions, the confining layer provides natural filtration, surface runoff, septic effluent, fertilizer, pesticides, and pathogens all take months to years to migrate through clay and soil before reaching the aquifer. A sinkhole creates a shortcut.

For private well owners in the area of a new sinkhole:

• Immediate contamination risk: Surface water can reach the well's water-bearing zone within hours or days, carrying with it anything that was on the surface or in the shallow subsurface.
• Elevated turbidity: Collapse events often mobilize sediment into the aquifer, causing temporary turbidity spikes.
• Structural damage to the well itself: Wells within 100–200 feet of a sinkhole can experience casing damage, grout seal failure, or loss of well yield.

Even if your well is not directly in the sinkhole footprint, the regional hydraulic connectivity of karst means contamination entering at one point can propagate to nearby wells faster than in non-karst geology.

FDEP's Delineated Areas Program

Florida Department of Environmental Protection regulates groundwater protection in areas where contamination has been identified or where geology creates elevated risk. The Delineated Areas Program, authorized under Chapter 373 F.S. Part III and codified in Chapter 62-524 F.A.C., designates specific zones with enhanced requirements.

Within a Delineated Area:

• New wells must meet stricter construction standards (deeper casing, improved grout, approved annular seal).
• Mandatory water quality testing by the County Health Department before the well can be cleared for potable use.
• Certain land uses may be restricted.
• Existing wells may be scheduled for periodic monitoring.

If you're considering buying property with a private well in Central Florida, checking whether the parcel is inside a Delineated Area should be a standard due-diligence item.

Florida DOH Well Surveillance

Florida Department of Health maintains a Well Surveillance Program that monitors threats to private wells from industrial contamination sites, agricultural operations, and hydrogeologic risks. DOH coordinates with county health departments to test wells in documented risk areas; if your well is in a surveillance zone, you may be eligible for free testing on a periodic basis.

What This Means for Homeowners

Existing well owners:

1. Know your well construction. Ask your well driller or the previous owner for the well log (total depth, casing depth, casing material, grout information). Shallower wells in karst are higher-risk for sinkhole-related contamination.
2. Test annually at minimum. Coliform, nitrate, hardness, pH, and iron as a baseline panel. More frequent testing if you're near agricultural operations, septic-dense neighborhoods, or known contamination sites.
3. Watch for signs of sinkhole activity: new cracks in foundations, doors that stop fitting their frames, slumping ground near trees or structures, sudden changes in well water clarity or pressure.
4. If a sinkhole opens anywhere within 500 feet of your well: do not drink from the well until a certified laboratory confirms no coliform contamination.

Prospective property buyers:

• Check FDEP's Delineated Areas database for the parcel.
• Request the well log before closing.
• Get an independent water quality test (not just the seller's most recent test).
• Check for sinkhole activity on adjacent parcels within a mile radius, county records and Florida Geological Survey databases both track documented events.

Whole-House and Point-of-Use Treatment for Karst Wells

Even a properly constructed well in karst terrain benefits from multi-barrier treatment:

• Sediment pre-filter: Captures turbidity spikes from nearby surface disturbances.
• UV disinfection: Inactivates coliform bacteria that may enter through hydraulic connections. Requires low turbidity upstream.
• Whole-house carbon: Removes agricultural herbicides, pesticides, and volatile organic compounds that may reach the aquifer through sinkhole-mediated pathways.
• Under-sink RO: Provides final polishing for drinking water, removes nitrates, any residual bacterial contaminants, and dissolved inorganic concerns.

Free Well Water Testing

Pure Agua tests private wells throughout Osceola, Orange, Seminole, and Polk Counties. We test coliform, nitrate, hardness, iron, pH, and sulfur, and we know the karst geology across Central Florida. 5.0★ rated with 200+ Google reviews.

Schedule your free well water test or call (407) 512-8342.