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South Carolina State Climatology Office
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South Carolina Drought News Release

South Carolina Department of Natural Resources
Land, Water and Conservation Division
South Carolina Drought Response Program
Department of Natural Resources News (803) 734-4133

NEWS RELEASE #02 - 43 September 6, 2002


DORCHESTER SINKHOLES CAUSED BY DROUGHT, THEN HEAVY RAINS

Recent sinkholes that occurred in Dorchester County were likely caused by the combination of extended drought and then heavy rains, according to a geologist with the S.C. Geological Survey.

Ralph Willoughby, geologist with the S.C. Geological Survey, said sinkholes are one kind of "subsidence" feature - where the land surface drops down. "Sinkholes form where limestone or lime sediment occurs near the surface, the limestone is eroded by the small concentration of acid in ground water, and where ground water fluctuates at some level below the surface," Willoughby said. "Sinkholes most commonly occur where the limestone is relatively near the surface.

"In northern Dorchester County near Rosinville, between Indian Field Creek and Four Hole Swamp, geologically young sediments at the surface are composed of quartz sand and clays, are 18-20 feet thick and overlie thicker deposits of limestone. Limestone is composed largely of calcium carbonate. Rainwater in South Carolina is slightly acidic. When rainwater falls and enters the groundwater system, the small amount of acid reacts to dissolve some of the calcium carbonate in limestone. In time, dissolution forms channels or networks of channels in the rock or lowers the top of the limestone, or both.

"During drought conditions ground water is withdrawn, the groundwater level is lowered, and the limestone or the overlying sediments may collapse," Willoughby said. "During wetter conditions ground water is added and the groundwater level is elevated. The sudden raising and subsequent lowering of ground water that follows a period of concentrated rainfall can trigger a sinkhole."

The recent and continuing period of extended drought, alone, may be sufficient immediate cause of the Rosinville sinkhole, according to Willoughby. Recent heavy rains in Dorchester County, coupled with the extended drought, are a probable contributing factor.

The geological configuration of South Carolina determines that sinkholes are localized in the state. Sedimentary limestone is absent in the Piedmont area. In general, only parts of the Coastal Plain are "favorable" for the development of sinkholes, Willoughby said. Limestone of appreciable thickness is relatively near the surface (less than 30-40 feet) mainly in two regions: in central South Carolina (eastern Orangeburg County, western Berkeley County and northern Dorchester County) and in eastern South Carolina (inland parts of Horry County and adjoining areas).

The S.C. Geological Survey is part of the S.C. Department of Natural Resources Land, Water and Conservation Division.

For more information: Contact Ralph Willoughby at (803) 896-7716

Find out more about the State Climatology Office at http://www.dnr.sc.gov/climate/sco/ or by calling (803) 734-9100.

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