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

Abstract

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 ( 3 H/ 3 He), chlorofluorocarbons (CFCs), sulfur hexafluoride (SF 6 ), sulfur-35 ( 35 S), and stable isotopes ( δ 18 O and δ 2 H) 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 SF 6 and 3 H/ 3 He, 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 35 S (>1.5 years) and seasonal variation of δ 18 O (mean residence time of 5 years) for spring discharge. The higher ages indicated by the 35 S and δ 18 O data reflect travel times through the unsaturated zone and, in the case of 35 S, possible sorption and exchange of S with soils or biomass. In springs sampled in April, 1996, apparent ages derived from the 3 H/ 3 He data (median age of 0.2 years) are lower than those obtained from SF 6 (median age of 4.3 years), and in contrast to median ages from 3 H/ 3 He (0.3 years) and SF 6 (0.7 years) obtained during the late summer dry season of 1997. Monthly samples from 1999 at four springs in SNP had SF 6 apparent ages of only 1.2 to 2.5±0.8 years, and were consistent with the 1997 SF 6 data. Water from springs has low excess air (0–1 cm 3 kg −1 ) and N 2 –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 N 2 –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 cm 3 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.