Use of Chlorofluorocarbons to Delineate Ground-Water-Flow Systems, Naval Weapons Station Yorktown, Yorktown, Virginia

Nelms, David L., U.S. Geological Survey, 1730 E. Parham Rd., Richmond, VA 23228, dlnelms@usgs.gov


Dating of ground water with chlorofluorocarbons (CFC's) at the Naval Weapons Station Yorktown, Va. indicates the presence of two different recharge age profiles and flow systems in the shallow ground-water system. Older recharge age profiles and evidence of degradation of CFC's occur under the topographically elevated areas of the station, where the Cornwallis Cave aquifer is confined. Ground water in these elevated areas typically has low dissolved oxygen concentration and recharge ages that range from 17 to 35 years. Steep, downward vertical gradients in the elevated areas reflect long flowpaths associated with subregional flow. Recharge age profiles in the deeply dissected valleys, where the Cornwallis Cave aquifer is unconfined, are typically younger than those in the elevated areas. Water from springs and wells in the valleys (discharge areas) have high dissolved oxygen concentration and recharge ages that range from less than 2 to 10 years. The dissected areas of the station have vertical gradients less than those in the elevated areas and have short flowpaths associated with local flow systems. Excess air concentration from dissolved gas analyses also indicates that differences in flowpaths exist between the elevated and dissected areas. The combination of CFC's dating with geohydrologic mapping, water-level monitoring, and field water-quality measurements indicate that both local and subregional flow systems are present at the station. These findings can aid in the definition of potential contaminant transport pathways for ongoing remedial investigations on and near the station.


Nelms, D.L., 1997, Use of chlorofluorocarbons to delineate ground-water-flow systems, Naval Weapons Station Yorktown, Yorktown, Va. [abs.]:  Geological Society of America Abstracts with Programs, v. 29, no. 6, p. 76.


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