Sinkholes and Karst Terrain

Sinkholes are bowl-shaped, funnel-shaped, or vertical-sided depressions in the land surface that form over underground voids. These depressions, which can range in size from a few feet to several hundred feet in diameter and depth, usually result from the natural collapse of the roofs of caves eroded in soluble bedrock, but they can also result from man-made activity such as mining, groundwater pumping, or the failure of sewer and storm water drains.  Subsidence of the ground is usually gradual, but on occasions it can be sudden and dramatic.

In regions of carbonate bedrock such as limestone or dolomite, rainwater percolating though organic soil becomes slightly acidic and as it comes into contact with the bedrock, it slowly dissolves the carbonate minerals.  Over time, this persistent process can create extensive systems of underground fissures and caves.  The surface of such a region is often pocked with sinkholes and closed depressions.  This type of topography is called karst terrain.  In well-developed karst terrain, chains of sinkholes form what are known as solution valleys and streams frequently disappear underground.

A poor understanding of Karst terrain has led to land-use practices that pose significant economic and environmental impacts to households and communities.  Sinkhole collapse, either slow or dramatic, regularly causes considerable damage to buildings, highways, rails, bridges, pipelines, storm drains, and sewers.  In addition, sinkholes provide a pathway for surface water to directly enter groundwater aquifers, so the potential for pollution is high because of the minimal filtering of surface water.

Sinkhole formation is closely related to local hydrologic conditions, and human-induced changes to the local hydrology commonly accelerate the process.  Diverting surface water, pumping groundwater, and constructing reservoirs all contribute to sinkhole collapse.  An extreme example occurred in Florida on February 25, 1998, when, during the flushing of a newly drilled irrigation well, hundreds of sinkholes up to a hundred and fifty feet across formed over a twenty-acre area within a few hours.  Runaway urbanization and development dramatically increases water usage, alters drainage pathways, and overloads the ground surface.  According to the Federal Emergency Management Agency, the number of human-induced sinkholes has doubled since 1930, while insurance claims for related damages has increased 1,200 % from 1987 to 1991, costing nearly $100 million.  Subsidence is not covered by standard homeowners insurance.

Virginia counties containing karst terrain. Modified from Virginia Natural Heritage Karst Program.

In Virginia, the principal area affected by sinkholes is the Valley and Ridge province, an extensive karst terrain underlain by limestone and dolomite, but the narrow marble belts in the Piedmont and some shelly beds in the Coastal Plain are also pocked with sinkholes. Dramatic collapses that swallow homes or persons have happened in Virginia, but generally are rare.  The most notable incidents occurred in the City of Staunton: on August 11, 1911, parts of several homes and the firehouse were lost in a series of sinkholes on Baldwin Street and Central Avenue, and on October 28, 2001, a 45-feet deep chasm opened up on Lewis Street.  In April of 2000, thirty-two sinkholes were reported in the upper Shenandoah Valley after seven inches of rain fell after a long dry spell. 

Sinkholes regularly cause problems along roads and highways in the Commonwealth.  During the past thirty years, VDOT’s Staunton District has recorded 350 sinkholes that have damaged roads throughout the district.  In March 2001 a nine-mile stretch of Interstate 81 in Augusta County was closed after the sudden appearance of three sinkholes, the largest measuring 20 feet long, 11 feet wide and 22 feet deep.  Repairs cost over $100,000.  On October 5, 2004, the right southbound lane of I-81 just north of the Exit 118C ramp in Montgomery County collapsed.

Groundwater contamination is a universal problem in populated areas overlying karst terrain.  Karst aquifer contaminants in Virginia have included petroleum products, herbicides, solvents, fertilizers, sheep and cattle dip, sewage, dead livestock, and household garbage, along with septic tanks falling into underground caves.  In the late 1800s, a Shenandoah County community was subjected to a cholera outbreak due to the pollution of the local karst aquifer.  A significant concern is the vulnerability of karst aquifers to contamination along the I-81 corridor, where hazardous materials are regularly transported and accidents are increasingly routine.  For some chemicals that do not readily mix with water, contamination can be widespread and remain in the groundwater for many years.  Most of Virginia’s karst region follows Interstate 81, and twenty-seven of Virginia’s counties lie in this zone, where hundreds of thousands of people get their drinking water from springs and wells.

State law prohibits the dumping of waste into sinkholes, and some Virginia counties have implemented ordinances about sinkhole dumping and outfalls.  Meanwhile, the Virginia Health Department discourages the use of karst springs as public water supplies and requires periodic testing of those karst springs that are used.  The Virginia Department of Conservation and Recreation’s Natural Heritage Karst Program is responsible for groundwater and habitat protection in karst areas, supported by EPA Section 319 Clean Water Act Program.  Presently, the USGS is working with various state geologic surveys and to develop a National Karst Map, but this is not detailed enough for planning and zoning purposes. 

Areas over underground mine workings are also susceptible to subsidence.  Mine collapses have resulted in losses of homes, roadways, utilities and other infrastructure.  Subsidence is often exacerbated by the extensive pumping of groundwater associated with underground mining.  Abandoned coal mines occur in Buchanan, Dickenson, Lee, Scott, Russell, Tazewell, Wise, Montgomery, and Pulaski counties in southwest Virginia; and Henrico, Chesterfield and Goochland counties in the Richmond coal basin.  Abandoned underground gypsum mines occur in Smyth and Washington counties.  Information of past mining activity can be obtained from the Virginia Division of Geology and Mineral Resources.

Signs of sinkhole formation:

Although a sinkhole can form without warning, specific signs can signal potential development:

• Slumping or falling fence posts
• Wilting vegetation                  
• Discolored well water
• Structural cracks in walls, floors, or foundations.

References:

Fosse, R. M., 1968, Surface subsidence and collapse caused by groundwater withdrawal in carbonate rock areas: Proceedings of the 23rd International Geological Congress, Prague, v. 12, p. 155-166.

Hubbard, D. A., Jr., 1983, Selected karst features of the northern Valley and Ridge province, Virginia: Virginia Division of Mineral Resources, Publication 44, one sheet.

Hubbard, D. A., Jr., 1988, Selected karst features of the central Valley and Ridge province, Virginia: Virginia Division of Mineral Resources, Publication 83, one sheet.

Hubbard, D. A., Jr., 1999, Highways in Virginia karst: Resource and hazard considerations: 50th Annual Highway Geology Symposium & TRB Karst Meeting, Proc. & Field Trip Guide, 13-22.

Hubbard, D. A., Jr., 2001, Selected karst features of the southern Valley and Ridge province, Virginia: Virginia Division of Mineral Resources, Publication 167, one sheet.

Hubbard, D. A., Jr., 2003, Use of regional sinkhole mapping for sinkhole susceptibility maps, in Beck, B. F., ed., ASCE Geotechnical Special Publication No. 122, p. 61-71.

Hubbard, D. A., Jr. and Sterrett, R. M., 1994, Groundwater contamination in Virginia karst: Case studies (abs.): Program and Abstracts, 37th Annual Meeting of the Association of Engineering Geologists, Williamsburg, VA, p. 49.

Sowers, G. F., 1976, Mechanisms of subsidence due to underground opening, in Subsidence over mines and caverns, moisture and frost actions, and classification: Washington D.C., Transportation Research Board Record 612, National Academy of Science, p. 2-8

Weary, D. J., 2005, An Appalachian Regional Karst Map and Progress Towards a New National Karst Map: USGS  http://pubs.usgs.gov/sir/2005/5160/PDF/sir2005-5160part3A.pdf

Web sites:

The Virginia Department of Conservation and Natural Resources web site concerning the Virginia Natural Heritage Karst Program.
http://www.dcr.virginia.gov/natural_heritage/karsthome.shtml

The National Karst Program
http://www.nature.nps.gov/nckri/map/maps/index.html

Karst: The stealthy hazard: Geotimes article by J. Cobb, and J. Currens, 2001
http://www.geotimes.org/may01/feature2.html

*** Please note you will need Adobe's Acrobat Reader, Microsoft Word Viewer in order to view any documents on this page.