Groundwater residence times in Shenandoah National Park, Blue Ridge Mountains, Virginia, USA: a multi-tracer approach
Groundwater residence times in Shenandoah National Park, Blue Ridge Mountains, Virginia, USA: a multi-tracer approach
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DOI：
10.1016/s0009-2541(01)00317-5
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发表时间：
2001-09
期刊：
影响因子：
3.9
通讯作者：
L. Plummer;E. Busenberg;J. Böhlke;D. L. Nelms;R. Michel;P. Schlosser
中科院分区：
文献类型：
--
作者：
L. Plummer;E. Busenberg;J. Böhlke;D. L. Nelms;R. Michel;P. Schlosser
对美国弗吉尼亚州蓝岭山脉峰顶附近的谢南多厄国家公园（SNP）泉水和威尔斯井水的化学和同位素性质进行了监测，以获得地下水停留时间的信息。调查的时间尺度包括季节（雨季，1996年4月，旱季，1997年8月至9月），每月（1999年3月至9月）和每小时（30分钟的时间间隔记录的特定电导率和温度，1999年3月至2000年2月）。采用氚/氦-3（3 H/3 He）、氯氟烃（CFCs）、六氟化硫（SF6）、硫-35（35 S）和水的稳定同位素（δ 18 O和δ 2 H）等多种环境示踪剂，估算了34个泉水和15口威尔斯井的浅层地下水排放停留时间。泉水最可靠的年龄似乎是基于SF6和3 H/3 He，大多数年龄在0-3年之间。这一范围与根据氟氯化碳浓度估算的表观年龄相一致;然而，由于1995年之后氟氯化碳大气增长曲线趋于平稳，因此基于氟氯化碳的年龄具有很大的不确定性。泉水的35 S（>1.5年）和δ 18 O（平均停留时间为5年）的季节变化表明，泉水的表观年龄偏高。35 S和δ 18 O数据所显示的较高年龄反映了通过非饱和带的时间，在35 S的情况下，S可能与土壤或生物质发生吸附和交换。在1996年4月采集的泉水样品中，由3 H/3 He数据得到的表观年龄（中位年龄为0.2年）低于由SF6数据得到的表观年龄（中位年龄为4.3年），与1997年夏末旱季期间由3 H/3 He数据得到的中位年龄（0.3年）和SF6数据得到的中位年龄（0.7年）形成对比。SNP地区1999年4个春季的月样SF6表观年龄仅为1.2 ~ 2.5±0.8年，与1997年的SF6表观年龄一致。泉水中的空气过剩量低（0-1 cm 3 kg −1），N2-Ar温度随季节变化。He和Ne的浓度超过溶解度平衡表明，溶解的气体没有分馏。在N2-Ar温度的季节性变化表明浅，季节性补给，过量的He和Ne数据表明沃茨主要局限于气体交换在浅，山坡，地下水泉系统。来自破裂岩石中的威尔斯井的水含有高达8 cm 3 kg − 1的过量空气，年龄在0-25年之间。在特定的电导率和温度的瞬态响应，观察在1999年9月的大降水事件的几个小时内，在春季放电，这两个参数最初增加，然后下降到低于风暴前基流值的值。地下水的停留时间表明，冲洗率的移动的大气成分通过地下水流排水SNP的高海拔平均不到3年的基流条件。
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 SF6and3H/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 by35S (>1.5 years) and seasonal variation of δ18O (mean residence time of 5 years) for spring discharge. The higher ages indicated by the35S and δ18O data reflect travel times through the unsaturated zone and, in the case of35S, possible sorption and exchange of S with soils or biomass. In springs sampled in April, 1996, apparent ages derived from the3H/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 from3H/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 SF6apparent ages of only 1.2 to 2.5±0.8 years, and were consistent with the 1997 SF6data. Water from springs has low excess air (0–1 cm3kg−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 cm3kg−1of 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.