Ground-water supplies in shale and sandstone in Fairfax, Loudoun, and Prince William Counties, Virginia

The Triassic rocks of northern Virginia may be a potential source of moderately large supplies of ground water for municipal end industrial use if the performance of two deep wells drilled at the site of the new Dulles International Airport is a criterion. These two wells produced 327 and 600 gpm (gallons per minute) from depths of 860-and 955 feet in sedimentary rocks in an immediate area where the previous maximum depth reported was 180 feet and the maximum yield 12 gpm.

Chemical analyses of the water indicates that it is extremely hard--533 and 500 ppm (parts per million) in the two wells--and would require treatment to be satisfactory for domestic and some industrial uses. However, water of better quality may be present at greater depths, and it may be possible to case off the more highly mineralized water. Further exploration and sampling of water from various depths will be necessary for efficient development of the Triassic groundwater reservoir.