Cey Bradley D, Hudson G Bryant, Moran Jean E, Scanlon Bridget R
Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA.
Ground Water. 2009 Sep-Oct;47(5):646-59. doi: 10.1111/j.1745-6584.2009.00562.x.
Water table temperatures inferred from dissolved noble gas concentrations (noble gas temperatures, NGT) are useful as a quantitative proxy for air temperature change since the last glacial maximum. Despite their importance in paleoclimate research, few studies have investigated the relationship between NGT and actual recharge temperatures in field settings. This study presents dissolved noble gas data from a shallow unconfined aquifer heavily impacted by agriculture. Considering samples unaffected by degassing, NGT calculated from common physically based interpretive gas dissolution models that correct measured noble gas concentrations for "excess air" agreed with measured water table temperatures (WTT). The ability to fit data to multiple interpretive models indicates that model goodness-of-fit does not necessarily mean that the model reflects actual gas dissolution processes. Although NGT are useful in that they reflect WTT, caution is recommended when using these interpretive models. There was no measurable difference in excess air characteristics (amount and degree of fractionation) between two recharge regimes studied (higher flux recharge primarily during spring and summer vs. continuous, low flux recharge). Approximately 20% of samples had dissolved gas concentrations below equilibrium concentration with respect to atmospheric pressure, indicating degassing. Geochemical and dissolved gas data indicate that saturated zone denitrification caused degassing by gas stripping. Modeling indicates that minor degassing (<10% DeltaNe) may cause underestimation of ground water recharge temperature by up to 2 degrees C. Such errors are problematic because degassing may not be apparent and degassed samples may be fit by a model with a high degree of certainty.
根据溶解惰性气体浓度推断的潜水面温度（惰性气体温度，NGT）可作为末次盛冰期以来气温变化的定量替代指标。尽管它们在古气候研究中很重要，但很少有研究在野外环境中调查NGT与实际补给温度之间的关系。本研究展示了来自一个受农业严重影响的浅层无压含水层的溶解惰性气体数据。考虑到未受脱气影响的样本，根据常见的基于物理的解释性气体溶解模型计算的NGT（该模型针对“过剩空气”校正测量的惰性气体浓度）与测量的潜水面温度（WTT）一致。将数据拟合到多个解释性模型的能力表明，模型的拟合优度并不一定意味着该模型反映了实际的气体溶解过程。尽管NGT很有用，因为它们反映了WTT，但在使用这些解释性模型时建议谨慎。在所研究的两种补给模式（主要在春季和夏季的高通量补给与持续的低通量补给）之间，过剩空气特征（数量和分馏程度）没有可测量的差异。大约20%的样本的溶解气体浓度低于相对于大气压力的平衡浓度，表明存在脱气现象。地球化学和溶解气体数据表明，饱和带反硝化作用通过气提导致了脱气。模型表明，轻微脱气（<10%的δNe）可能导致地下水补给温度低估高达2摄氏度。这种误差是个问题，因为脱气可能不明显，而且脱气样本可能会被一个具有高度确定性的模型拟合。
