GROUNDWATER EXPLORATION AND DEVELOPMENT IN SEDIMENTARY AND IGNEOUS ROCKS OF THE MESOZOIC CULPEPER BASIN, FAUQUIER COUNTY, VIRGINIA - A CASE STUDY

Groundwater resources were developed within fractured bedrock aquifers in Northern Virginia to serve as commercial and potable water supplies. The development of these groundwater sources was complicated by active groundwater withdrawals from local public and domestic water supply wells. Detailed exploration, drilling, and bedrock aquifer testing programs led to the development of four water supply production wells.

The project site is located within the central portion of the Mesozoic Culpeper Basin and is underlain by diabase intrusives and shallow to moderately dipping, sedimentary rocks and basalt flows. Aquifers within all rocks types are comprised of interconnected structural discontinuities such as fractures, fracture zones, fault zones, and partings along bedding planes.

A continuous 5-day pumping test (PT) was conducted on five potential production wells. Pumping of all five wells at a cumulative rate of 825 gallons per minute (1,180,000 gallons per day) caused water level impacts on 23 of the 29 observation wells monitored. The results of the test showed that: 1) The basalt flows largely acted as aquitards, creating two distinct aquifers within the sedimentary rocks; 2) Impacts on local groundwater levels were greatest within the sedimentary rocks parallel to bedding; these impacts were observed as far as 5000+ feet from the pumping wells; 3) Discrete, laterally extensive, structural discontinuities did exist locally which influenced groundwater levels during the groundwater withdrawal testing period.