The Reston Groundwater Dating Laboratory


Dating of Young Ground Water Presentation

Dating of young ground water with CFCs, SF6, 3H, and 3H/3He: Examples from the Blue Ridge and Valley and Ridge of parts of Virginia and Pennsylvania

L. Niel Plummer, US Geological Survey, 432 National Center, Reston, VA 20192


Chlorofluorocarbon (CFC), sulfur hexafluoride (SF6), tritium (3H), and tritium/helium-3 (3h/3he) concentrations in discharge from wells and springs in karst or fractured rock of the Blue Ridge and Valley and Ridge of parts of Virginia and Pennsylvania indicate that ages and mixing fractions can be interpreted using a binary mixing model in which a young component (apparent age of 0-25 yrs) is diluted to varying extents with old, pre-tracer water. In contrast, SF6 and 3h/3he data for water from shallow springs that discharge from residuum overlying fractured crystalline rock of the Blue Ridge can be interpreted using either exponential or piston-flow models (mean ages 0-3 yrs). Use of multiple tracers and "tracer plots" can eliminate some mixing models and refine estimates of mean tracer age. About half of the 3h/3he samples (39 total) from the Valley and Ridge karst have initial tritium concentrations consistent with piston flow (0-15 yrs, unmixed). The rest of the samples are shown to be mixtures of young and old (pre-bomb) water. The CFC data also demonstrate cases of piston flow and binary mixing in the Valley and Ridge, but can be affected by contamination. Other samples demonstrate apparent removal of CFC-11 and CFC-113, and excesses of SF6 and 4He from terrigenic sources. The tracer data show that most ground water from fractured and/or karstic aquifers is highly susceptible to contamination. Use of Halon 1211 in air rotary drilling shows that the effects of drilling can leave a long-lasting imprint on "natural" environmental tracer concentrations.