Limestone Geology and Sinkholes

Limestone geology, also known as Karst, is quite prevalent in the eastern United States. Limestone often dissolves in water, creating voids where groundwater flows like an underground river –gnawing away rock over the eons.

The natural wearing away of limestone over the millenia creates fantastic caves, unique ecosystems and touristic wonders (think Luray Caverns). However, in developed areas, human activities and new development can accelerate the natural pace of erosion–causing sinkholes, threatening buildings and roads, and contaminating groundwater.

PEC co-sponsored an information session entitled “Living on Limestone–Protecting Your Investment” to provide more information on Karst Topography. Below are three presentations from the session.

Karst Protection Efforts in Clarke County by Alison Teetor, Clarke County Natural Resource Planner

Karst Geophysics by Andy Forrest, P.G., Forrest Environmental Services, Inc.

Karst in Loudoun County by Dr. Malcolm S. Field, Hydrogeologist, National Center for Environmental Assessment, USEPA

Loudoun’s Limestone Dilemma

In Loudoun, limestone is located primarily from the southern edges of Leesburg north to the Potomac– and generally bounded by the Catoctin Mountains to the west. In order to limit the potential for new sinkholes or groundwater contamination, the Loudoun Board of Supervisors is considering a Limestone Overlay District.

This new district would help protect the health, safety and welfare of residents by reducing new sources of pollution, and help to limit pollution pathways to groundwater sources through sinkholes and other breaks in the earth’s surface. The Limestone Overlay District would apply to new development and would:

  • Create setbacks to known sinkholes and underground drainage ways
  • Provide additional guidance for development practices to ensure the safety of new structures
  • Limit development grading practices which could change surface water drainage patterns and increase the instability of subsurface geology

In order to respond to the concerns of residents, the Board of Supervisor’s Transportation and Land Use Committee has been carefully reviewing the proposed ordinance and making some adjustments to the language to increase the minimum structure or disturbance size needing county review, deleting language restricting garden size and debating the notice requirements. The Committee will continue its work through December and may be ready to refer it back to the full Board in January for review and a vote.

UPDATE: The Limestone Overlay District was approved by the Loudoun County Board of Supervisors in the winter of 2010!