An aquifer is a body of sand, gravel, fractured bedrock, or other earth material that can provide groundwater from a well or spring in useful amounts, such as to supply a household. Likewise, a karst aquifer is an aquifer that occurs within limestone geology, where the limestone (or other easily dissolved rock) has been partially dissolved so that some fractures are enlarged into passages that carry the groundwater flow.
The location and size of these passages is unpredictable. In the Loudoun area, the limestone geology is made up of a limestone conglomerate or collection of irregularly sized and shaped limestone chunks cemented in a carbonate-rich clay matrix.
Groundwater flow in karst aquifers is significantly different from that of other aquifers because of the solutionally enlarged conduits. In sand or fractured bedrock aquifers, surface water slowly infiltrates downward through the soil and groundwater moves laterally very slowly, sometimes only a few feet per year. But in karst aquifers, surface water may quickly enter the groundwater system through sinkholes and swallets and move quickly through the dissolved conduits in the rock.
The nature of the groundwater flow system and its more direct connection to the surface causes karst areas to be extremely vulnerable to groundwater contamination from surface sources. Another serious problem occurs when the dissolved areas of limestone rock underground become so large that they can no longer support the overlying soil and collapse forming a sinkhole.