Approximately 20% of Loudoun County's residents, primarily those living in the western and northern portions of the county, rely on groundwater from a well. Community water supply wells serve residents in the towns, villages, and some of the subdivisions, but many residents in the rural areas of the county rely on a private well located on their property. As the county's population continues to increase, protection of groundwater resources is increasingly important.
In addition to dozens of community water supply wells, there are an estimated 15,000 domestic groundwater wells that supply water to individual households in Loudoun. There are additional wells that are used for other purposes such as agriculture, industry, business, schools, recreation, and monitoring.
The occurrence and availability of groundwater varies within the county, due partly to the complex geology below the surface. Bedrock occurs throughout the county at various depths beneath the overlying layers of soil and unconsolidated sediments. Fractures and cracks occur in the bedrock and below the water table they are filled with groundwater. Groundwater pumped out of wells comes from these fractures but the location and number of fractures is highly variable due to rock type and other factors. Consequently, wells that are located near each other may yield very different amounts of water.