TY - JOUR
T1 - Groundwater residence times in Shenandoah National Park, Blue Ridge Mountains, Virginia, USA
T2 - A multi-tracer approach
AU - Plummer, L. N.
AU - Busenberg, E.
AU - Böhlke, J. K.
AU - Nelms, D. L.
AU - Michel, R. L.
AU - Schlosser, P.
N1 - Funding Information:
This study was conducted in cooperation with the National Park Service. The work was supported by funds from the US Geological Survey National Research Program, and the Office of Ground Water, US Geological Survey. Precipitation and air samples were collected by Shane Spitzer, Shenandoah National Park at the Big Meadows Air Monitoring Station. Meteorological data were provided by the National Park Service Air Resources Division. We thank Tyler Coplen, US Geological Survey, Reston, VA for stable isotope data. Field assistance from Julian Wayland, Gerolamo Casile, Michael Doughten, Peggy Widman, Wandee Kirkland, and Anne Burton (USGS, Reston, VA), Steve Richards (NPS, SNP), Vanessa Trompetter (Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand), and John Weaver (CSIR, Stellenbosch, South Africa) is gratefully acknowledged. We thank Gerolamo Casile and Julian Wayland for chlorofluorocarbon analyses, Peggy Widman for dissolved N 2 and Ar analyses, Wandee Kirkland for SF 6 analyses, and Michael Doughten for water chemistry analyses. Janet Hannon assisted with the precipitation isotope study. Computer and graphical assistance from Sarah Parnes and Brian Norton is gratefully acknowledged. The manuscript was improved by reviews from Karen C. Rice (USGS, Charlottesville, VA), Stephanie D. Shapiro (USGS, Reston, VA), Martin Stute (Lamont-Doherty Earth Observatory, Palisades, NY), Peter Cook (CSIRO, Australia), D. Kip Solomon (University of Utah, Salt Lake City, UT), and an anonymous reviewer. Lamont-Doherty Earth Observatory of Columbia University contribution No. 6208.
PY - 2001
Y1 - 2001
N2 - Chemical and isotopic properties of water discharging from springs and wells in Shenandoah National Park (SNP), near the crest of the Blue Ridge Mountains, VA, USA were monitored to obtain information on groundwater residence times. Investigated time scales included seasonal (wet season, April, 1996; dry season, August-September, 1997), monthly (March through September, 1999) and hourly (30-min interval recording of specific conductance and temperature, March, 1999 through February, 2000). Multiple environmental tracers, including tritium/helium-3 (3H/3He), chlorofluorocarbons (CFCs), sulfur hexafluoride (SF6), sulfur-35 (35S), and stable isotopes (δ18O and δ2H) of water, were used to estimate the residence times of shallow groundwater discharging from 34 springs and 15 wells. The most reliable ages of water from springs appear to be based on SF6 and 3H/3He, with most ages in the range of 0-3 years. This range is consistent with apparent ages estimated from concentrations of CFCs; however, CFC-based ages have large uncertainties owing to the post-1995 leveling-off of the CFC atmospheric growth curves. Somewhat higher apparent ages are indicated by 35S (> 1.5 years) and seasonal variation of δ18O (mean residence time of 5 years) for spring discharge. The higher ages indicated by the 35S and δ18O data reflect travel times through the unsaturated zone and, in the case of 35S, possible sorption and exchange of S with soils or biomass. In springs sampled in April, 1996, apparent ages derived from the 3H/3He data (median age of 0.2 years) are lower than those obtained from SF6 (median age of 4.3 years), and in contrast to median ages from 3H/3He (0.3 years) and SF6 (0.7 years) obtained during the late summer dry season of 1997. Monthly samples from 1999 at four springs in SNP had SF6 apparent ages of only 1.2 to 2.5 ± 0.8 years, and were consistent with the 1997 SF6 data. Water from springs has low excess air (0-1 cm3 kg-1) and N2-Ar temperatures that vary seasonally. Concentrations of He and Ne in excess of solubility equilibrium indicate that the dissolved gases are not fractionated. The seasonal variations in N2-Ar temperatures suggest shallow, seasonal recharge, and the excess He and Ne data suggest waters mostly confined to gas exchange in the shallow, mountain-slope, water-table spring systems. Water from wells in the fractured rock contains up to 8 cm3 kg-1 of excess air with ages in the range of 0-25 years. Transient responses in specific conductance and temperature were observed in spring discharge within several hours of large precipitation events in September, 1999; both parameters increased initially, then decreased to values below pre-storm base-flow values. The groundwater residence times indicate that flushing rates of mobile atmospheric constituents through groundwater to streams draining the higher elevations in SNP average less than 3 years in base-flow conditions. Published by Elsevier Science B.V.
AB - Chemical and isotopic properties of water discharging from springs and wells in Shenandoah National Park (SNP), near the crest of the Blue Ridge Mountains, VA, USA were monitored to obtain information on groundwater residence times. Investigated time scales included seasonal (wet season, April, 1996; dry season, August-September, 1997), monthly (March through September, 1999) and hourly (30-min interval recording of specific conductance and temperature, March, 1999 through February, 2000). Multiple environmental tracers, including tritium/helium-3 (3H/3He), chlorofluorocarbons (CFCs), sulfur hexafluoride (SF6), sulfur-35 (35S), and stable isotopes (δ18O and δ2H) of water, were used to estimate the residence times of shallow groundwater discharging from 34 springs and 15 wells. The most reliable ages of water from springs appear to be based on SF6 and 3H/3He, with most ages in the range of 0-3 years. This range is consistent with apparent ages estimated from concentrations of CFCs; however, CFC-based ages have large uncertainties owing to the post-1995 leveling-off of the CFC atmospheric growth curves. Somewhat higher apparent ages are indicated by 35S (> 1.5 years) and seasonal variation of δ18O (mean residence time of 5 years) for spring discharge. The higher ages indicated by the 35S and δ18O data reflect travel times through the unsaturated zone and, in the case of 35S, possible sorption and exchange of S with soils or biomass. In springs sampled in April, 1996, apparent ages derived from the 3H/3He data (median age of 0.2 years) are lower than those obtained from SF6 (median age of 4.3 years), and in contrast to median ages from 3H/3He (0.3 years) and SF6 (0.7 years) obtained during the late summer dry season of 1997. Monthly samples from 1999 at four springs in SNP had SF6 apparent ages of only 1.2 to 2.5 ± 0.8 years, and were consistent with the 1997 SF6 data. Water from springs has low excess air (0-1 cm3 kg-1) and N2-Ar temperatures that vary seasonally. Concentrations of He and Ne in excess of solubility equilibrium indicate that the dissolved gases are not fractionated. The seasonal variations in N2-Ar temperatures suggest shallow, seasonal recharge, and the excess He and Ne data suggest waters mostly confined to gas exchange in the shallow, mountain-slope, water-table spring systems. Water from wells in the fractured rock contains up to 8 cm3 kg-1 of excess air with ages in the range of 0-25 years. Transient responses in specific conductance and temperature were observed in spring discharge within several hours of large precipitation events in September, 1999; both parameters increased initially, then decreased to values below pre-storm base-flow values. The groundwater residence times indicate that flushing rates of mobile atmospheric constituents through groundwater to streams draining the higher elevations in SNP average less than 3 years in base-flow conditions. Published by Elsevier Science B.V.
KW - Environmental tracers
KW - Groundwater age
KW - Residence time
KW - Shenandoah National Park
KW - Springs
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U2 - 10.1016/S0009-2541(01)00317-5
DO - 10.1016/S0009-2541(01)00317-5
M3 - Article
AN - SCOPUS:0034887975
SN - 0009-2541
VL - 179
SP - 93
EP - 111
JO - Chemical Geology
JF - Chemical Geology
IS - 1-4
ER -