| | Limestone Geology refers to areas that are underlain by carbonate rocks such as limestone, dolomite and marble. Carbonate rocks will dissolve when exposed to acid and since normal rainwater is slightly acidic, it will dissolve these rocks over time. This slow process will eventually produce a landscape characterized by fluted and pitted rock surfaces, vertical shafts, sinkholes, sinking streams, springs, subsurface drainage systems and caves. These features are called “karst features” and the resulting landscape is called "karst terrain." Karst terrain is also characterized by abundant rock outcrops, closed depressions, and limited surface drainage.
| Karst Aquifers 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. |
Click on the image above for a larger view of Loudoun County's limestone geology and karst features. You will need the Adobe Reader which may be downloaded here for free.
| 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. Are You Located Within the Limestone Geology Region? | The region of Loudoun County that has karst geology is located in an area roughly one mile either side of State Route 15 from just south of Leesburg, north to the Potomac River bridge. (Click on the map at right for a detailed view). The region is bounded sharply to the west by the Bull Run Fault which runs at the base of Catoctin Mountain through Loudoun County.
Information is also available through the county's Online Mapping System. |  | Issues Specific to Water Wells Located in Limestone Geology Because surface water can rapidly enter into the groundwater system and move quickly through channels created in the limestone rock, private drinking water wells located in karst aquifers are more susceptible to contamination than those in other types of aquifers. These contaminants can include bacteria (from animal waste or failing drainfields), pesticides and fertilizers in runoff, and hazardous products that were not properly disposed (such as motor oil and other household chemicals). To minimize the risk of contamination, please refer to our Best Practices for Private Water Wells document.
Additionally, heavy pumping at rates that significantly lower groundwater levels may affect the subsurface "plumbing" and result in sinkhole collapses, ground subsidence, and de-watering of neighboring wells and ponds. These are some of the potential problems that can occur in karst areas. As long as people in the community understand the unique geology and take reasonable precautions, wells can be safely used in these areas. Types of Pollution that can Influence Limestone Geology Water Wells Because of the rapid transport of water from the ground surface in karst terrain, there is a high risk of pollution. Streams and surface runoff that enter sinkholes or caves bypass the natural filtration through the soil and provide a direct conduit for contaminates to enter the groundwater. Because groundwater in karst aquifers travels quickly, contaminates can be transmitted quickly to wells and other drinking water sources in the areas connected to that aquifer. There are two main types of pollution that can influence karst geology water wells. There is nonpoint-source pollution and point-source pollution. Nonpoint-source Pollution Nonpoint-source pollution comes from a widely distributed source, and is not easily identified. It's also called "runoff pollution." Types of nonpoint-source pollution can include: - Runoff from golf courses, roads, and parking lots
- Soil washed from farm fields & construction sites
- Animal waste from feedlots
- Waste from failing drainfields
- Pesticides from lawns
- Trash dumped in sinkholes, and
- Fertilizers
Because it is difficult to detect the source of nonpoint-source pollution, homeowners with wells in the limestone area should be diligent about handling and disposing of potential pollutants and periodic testing of their water supply. Point-Source Pollution Point-source pollution enters into the groundwater supply or aquifer through an easily identified distinct location through a distinct route. Most of the pollution that enters karst aquifers will be from nonpoint-source pollution; however, there are some instances where pollution may occur from point-source pollution. Potential types of point-source pollution that may occur are: - Wastewater
- Leaking fuel storage tanks
While the source of these types of pollution is much easier to identify, citizens should still be diligent in testing their water supply of private drilled wells to ensure that there is no contamination. Best Practices for Private Water Wells Whether Loudoun residents live in karst geology regions or not, the Loudoun County Health Department recommends diligence in taking care of private water wells. Homeowners assure their safety, the safety of their family, and the safety of their community when they continue to upkeep and maintain their private water wells. Please click on the link below for the Loudoun County Health Department's "Best Practices for Private Water Wells" document. Limestone Geology Private Water Wells Frequently Asked Questions The Loudoun County Health Department has compiled a list of frequently asked questions for homeowners living in karst geology. These FAQs can be accessed by clicking the link below. Links to More Information on Limestone Geology and Private Water Wells |