Ground-Water Dating to Predict Aquifer Susceptibility in Crystalline Terranes of Virginia

Nelms, David L., U.S. Geological Survey, 1730 E. Parham Rd., Richmond, VA 23228, Plummer, L. Niel, and Busenberg, Eurybiades, U.S. Geological Survey, 12201 Sunrise Valley Dr., MS 431, Reston, VA 20192


Various dating techniques can provide insight into the movement of ground water in crystalline terranes. These techniques have been applied in two crystalline terranes of Virginia and are currently being used in a statewide aquifer susceptibility study. Young ground water (less than 13 years old) from public supply wells was identified in a typical Piedmont setting, where the thickness of saprolite ranges from 60 to 110 feet and water zones occur between 110 and 360 feet below land surface. Evidently, macropores or remnant rock fabric in the saprolite coupled with pumpage allows for rapid movement of ground water through the saprolite part of the flow system. Ground water tended to be younger in the igneous rocks than in the metamorphic rocks; this difference, however, may be related to factors other than rock type, such as pumpage. Findings were similar in a mountainous setting of the Blue Ridge, except that young water was identified at greater depths than in the Piedmont. In both settings, the lithology and thickness of the saprolite were not limiting factors on ground-water movement, and no relation between well depth and apparent age of ground water is evident.

Knowledge gained from these previous studies became the fundamental premise of the Virginia Aquifer Susceptibility study, where age determinations will be used as a guide for the classification of areas in terms of the susceptibility of ground water to near-surface contamination. This is a cooperative study with the Virginia Department of Health (as part of the State Source Water Assessment program) to determine the intrinsic susceptibility of regional aquifers in Virginia that serve as public water supplies. The information will be used to screen public ground-water supplies and identify those that require detailed source-water assessments.


Nelms, D.L., Plummer, L. Niel, and Busenberg, Eurybiades, 1999, Ground-water dating to predict aquifer susceptibility in crystalline terranes of Virginia [abs.]: Geological Society of America Abstracts with Programs, v. 31, no. 3, p. 78.


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