Groundwater residence times in Shenandoah National Park, Blue Ridge Mountains, Virginia, USA: a multi-tracer approach

Chemical and isotopic properties of water discharging from springs and well s in Shenandoah National Park (SNP), near the crest of the Blue Ridge Mount ains, 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, 19 99) and hourly (30-min interval recording of specific conductance and tempe rature, March, 1999 through February, 2000). Multiple environmental tracers , including tritium/helium-3 (H-3/He-3), chlorofluorocarbons (CFCs), sulfur hexafluoride (SF6), sulfur-35 (S-35), and stable isotopes (delta O-18 and delta H-2) of water, were used to estimate the residence times of shallow g roundwater discharging from 34 springs and 15 wells. The most reliable ages of water from springs appear to be based on SF6 and H-3/He-3, with most ag es in the range of 0-3 years. This range is consistent with apparent ages e stimated from concentrations of CFCs; however, CFC-based ages have large un certainties owing to the post-1995 leveling-off of the CFC atmospheric grow th curves. Somewhat higher apparent ages are indicated by S-35 (> 1.5 years ) and seasonal variation of delta O-18 (mean residence time of 5 years) for spring discharge. The higher ages indicated by the S-35 and delta O-18 dat a reflect travel times through the unsaturated zone and, in the case of S-3 5, possible sorption and exchange of S with soils or biomass. In springs sa mpled in April, 1996, apparent ages derived from the H-3/He-3 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 H-3/He-3 (0.3 years) and SF6 ( 0.7 years) obtained during the late summer dry season of 1997. Monthly samp les from 1999 at four springs in SNP had SF, apparent ages of only 1.2 to 2 .5 +/- 0.8 years, and were consistent with the 1997 SF6 data. Water from sp rings has low excess air (0-1 cm(3) kg(-1)) and N-2-Ar temperatures that va ry seasonally. Concentrations of He and Ne in excess of solubility equilibr ium indicate that the dissolved gases are not fractionated. The seasonal va riations in N-2-Ar temperatures suggest shallow, seasonal recharge, and the excess He and Ne data suggest waters mostly confined to gas exchange in th e shallow, mountain-slope, water-table spring systems. Water from wells in the fractured rock contains up to 8 cm(3) kg(-1) of excess air with ages in the range of 0-25 years. Transient responses in specific conductance and t emperature were observed in spring discharge within several hours of large precipitation events in September, 1999; both parameters increased initiall y, then decreased to values below pre-storm base-flow values. The groundwat er residence times indicate that flushing rates of mobile atmospheric const ituents through groundwater to streams draining the higher elevations in SN P average less than 3 years in base-flow conditions. Published by Elsevier Science B.V.