A geologist would describe a sinkhole as a large depression formed by subsiding (sinking) ground. In prehistoric times, natural forces caused most sinkholes. Cave-ins, quicksand, and landslides are related phenomena.
Here's the common source of natural sinkholes. Rainwater picks up carbon dioxide from the atmosphere, which makes it slightly acidic. The water then passes through soil, encounters soluble rocks, and dissolves pockets that weaken them. The resulting porous bedrock is called "karst."
Karst occurs in both evaporite (salt, gypsum) and carbonate (limestone) rock strata. More than one quarter of the world's population either lives on karst or drinks groundwater from it. In the U.S., 20% of the land surface is karst. Karst aquifers supply almost half (40%) of American drinking water. Karst topography creates the fantastic landscapes of places like Mammoth Cave, Carlsbad Caverns, the Shenandoah Valley, and Yosemite.
From the first water wells and outhouses to contemporary hydraulic-fractured natural gas wells, human digging has also induced sinkholes. Particularly vulnerable: poorly graded land after excavation, rerouted surface water, water impoundment, roadways that have settled unevenly, underground piping, collapsed septic and underground fuel tanks, water main breaks, dredging, blasting areas, unstable dams, underground mines (especially coal), overpumped water wells, brine wells, and fossil fuel wells, both producing and abandoned.
As well as increasingly acid rain, anthropogenic climate changes associated with earth vibration, rising sea levels, and permafrost melting can also sink land. The risk most imminently threatens low-lying islands and flat coastal plains.
In the United States, formerly glacial areas like New Hampshire, Minnesota, and Oregon have few karst areas. However, other regions are particularly vulnerable. Texas, Alabama, Missouri, Kentucky, Tennessee, and Pennsylvania all have numerous sinkholes. Louisiana is pockmarked with them.
Florida, site of last week's fatal residential collapse, is often characterized as having more of them than any other state: 15,000 verified at last count. One of the most productive aquifers in the world lies below the state. An area of about 100,000 square miles (reaching into southern Alabama, southeastern Georgia, and southern South Carolina as well as containing all of Florida) holds this vast supply of groundwater. It serves the large cities of Savannah and Brunswick, Georgia and Jacksonville, Tallahassee, Orlando, and St. Petersburg, as well as smaller communities in between.
Land development leads to more and more sinkholes and groundwater hazards in an area like this. Unfortunately, both ignorance and wilful cost-cutting limit our ability to plan around these hazards. Many new irrigation, building construction, infrastructure, and even mining projects do not adequately consider the problems sinkholes can cause.
And some of them simply defy an affordable fix.
“Sufficiently stable” is a criterion geologists often use to describe an area's potential for development. However, it applies only to existing conditions and is limited to current technology. Future technology and land uses cannot rely on measurements from the past. Environmental geologists at a U.S. Geological Survey conference in 2008 gave potent advice to excavators and builders: "Avoiding the most hazardous areas by preventive planning is the safest strategy for development in sinkhole-prone areas."
Florida's pressured government admits that "We do not have sufficient staff to visit all new sinkholes." Insurance claims from sinkholes in the state tripled between 2006 and 2010. Two years ago, the rapidly rising claims forced the Florida legislature to revise statutes related to residential insurance and property loss. The changes made it harder for property owners to claim damages from sinkholes, despite their increasing numbers.
Although we know that sinkholes can affect water supplies, groundwater effects and other hazards are still rarely quantified and poorly understood. And the Sunshine State is not the only one reaching new lows because of sinkholes.
Award-winning science writer Sandy Dechert covers developments and environmental issues in conventional, solar, wind, biomass, large and small hydroelectric, and geothermal energy. She detailed events and policy at last fall's 18th UN climate change summit meeting in Doha, Qatar. Sandy has also reported on extreme weather disasters, including superstorm Sandy, winter storm Nemo, and the massive summer wildfires of the past decade.
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