Hydrogeology of the observation well site at the United States Geological Survey National Center, Reston, Virginia

Abstract

The U. S. Geological Survey's National Center is on a 105-acre tract straddling rocks of two distinct types. These are pelitic schists of late Precambrian and early Paleozoic age; and sandstone, shale, siltstone, and conglomerate of Triassic age. Within the Triassic part is a diabase dike and an associated contact metamorphic zone. Two observation wells and two core holes were drilled on the part of the property underlain by Triassic rocks. The wells are 10-inch and 8-inch diameter and were drilled to depths of 165 feet and 205 feet. The wells were drilled to monitor water levels, for equipment testing, and to determine the hydraulic properties of the Triassic rocks. Geophysical logs were run, and good correlations were found between the electric and caliper logs and the lithologic logs prepared from drill cuttings and cores. An aquifer test was made and was analyzed by the Theis method which considers nonleaky artesian conditions and the Streltsova method which considers flow in fractured rocks. The test results suggest that transmissivity of the sandstone aquifer is about 40 ft2/day. Flowmeter surveys showed that the water comes from two thin zones parallel to the bedding. The flowmeter surveys, core samples, and geophysical logs suggest that the Triassic sandstone is a fractured-rock aquifer. Water is probably derived from partings and joints parallel to bedding which in turn are fed by vertical joints and fractures. A water sample taken at the close of the aquifer test indicates that the water has a low dissolved solids content, is soft, and is generally of excellent quality.