* Plummer, L N nplummer@usgs.gov
, U.S. Geological Survey, MS 432, Reston, VA 20192
United States
Busenberg, E , U.S. Geological Survey, MS 432, Reston, VA 20192
United States
Bohlke, J K, U.S. Geological Survey, MS 432, Reston, VA 20192
United States
Nelms, D L, dlnelms@usgs.gov
U.S. Geological Survey, 1730 East Parham Rd., Richmond, VA 23228
United States
Michel, R L, U.S. Geological Survey, MS 434, Menlo Park, CA 94025
United States
Schlosser, P, Lamont-Doherty Earth Observatory of Columbia University, Route 9W, Palisades, NY 10964
United States
Chemical and isotopic properties of water discharging from springs in Shenandoah National Park (SNP) were monitored to obtain new information on ground-water residence times, seasonal variation of ground-water chemistry, and vulnerability of water supplies to anthropogenic inputs. Investigated time scales included seasonal (wet season, April, 1996; dry season, August-September, 1997), monthly (March, 1999 through September, 1999) and detailed (30-minute intervals for specific conductance, temperature, dissolved oxygen and pH, from March, 1999 through February, 2000). The residence times of shallow ground water discharging from thirty-five springs were estimated using (3H/3He), chlorofluorocarbons (CFCs), sulfur hexafluoride (SF6), sulfur-35 (35S), and stable isotope data (d18O and dD of water). A median residence time of 5 years (range 0 to 10 years) was estimated from the magnitude of seasonal variations in stable isotope composition of spring discharge and precipitation, which was consistent with the ages determined from concentrations of SF6, CFC-113, and CFC-12. 35S (half-life: 87 days) was not detected in most spring discharge, even after major precipitation events, which indicated residence times greater than 1.5 years or mixtures containing relatively small fractions of recent recharge. The 3H/3He data indicated shorter residence times (0-2 years) than those based on stable isotopes, CFCs and SF6, possibly as a consequence of gas exchange in the subsurface workings of constructed springs. Any gas exchange would affect all gas tracers with the highest impact on helium. Twenty-nine of the thirty-five springs sampled during high base flow in April, 1996 were sampled again during a prolonged drought (August-September, 1997). Spring discharge during the dry season was about 10% of that observed in the wet season. The decline in flow was accompanied by small but significant changes in some chemical parameters. The median specific conductance increased by 8.5%, from 36.2 µS/cm in April to 40.5 µS/cm during the dry season, while the median concentrations of Ca, Cl, HCO3, and SiO2 increased by 10, 17, 15, and 26%, respectively. In contrast, concentrations of Mg, SO4, and B decreased by 3, 32, and 14%, respectively, between April and late summer. Increases in Ca, HCO3, and specific conductance were correlated with warming of the water discharging from springs of about 1 degree C between winter and summer. Transient response in specific conductance and temperature was observed in spring discharge within 30 minutes of large precipitation events in September, 1999. Initially the specific conductance and temperature increased, and was followed by a decrease in temperature and specific conductance to values that were below those of pre-storm base-flow. The d15N and d34S values of the nitrate and sulfate in ground water at SNP are similar to those of local atmospheric deposition. The ground-water residence times indicate that flushing rates of mobile atmospheric constituents through ground water to streams draining SNP could be in the order of several years in base-flow conditions. CFC-11 concentrations were as much as 10 times that of modern air-equilibrated water in some samples from wells near Park facilities, indicating that some water supplies in SNP have been affected by human activities in the Park.
Plummer, L.N., Busenberg, Eurybiades, Bohlke, J.K., Nelms, D.L., Michel, R.L., and Schlosser, Peter, 2000, Ground-water residence times and water-quality trends in spring discharge from Shenandoah National Park, Virginia [abs.]: Eos, May 9, 2000, American Geophysical Union, p. S260.
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