Abstract When environmental tracers are used to determine groundwater age distributions, various factors including measurement error, model structural error, and sample contamination can affect the outcome. In this study, we tried to untangle and explain the uncertainties associated with the inference of age distribution for a set of highly-utilized production wells in a fractured-rock aquifer system. A total of 17 samples were collected at various depths (22–150 m) from production wells. These samples were analyzed for six age tracers, including 3H, tritiogenic 3He, radiogenic 4He, CFC-11, CFC-12, and CFC-113. A Bayesian approach was used for evaluating age distribution for the samples based on several presumed lumped parameter models (LPM)s and shape free models. It was found that the measured data in most wells could be explained by traditional LPMs reasonably well. However, for five of the 17 wells it was found that the simple LPMs were not adequate and the more complex shape-free approaches resulted better based on the Deviance Information Criteria (DIC) measure. This looked attributed to 1) competition between densely located wells, 2) a thick vadose zone layer possibly causing preferential flows 3) fractures allowing a localized production. Furthermore, it was shown that three other factors including tracer contamination, non-uniform 4He production, and the heterogeneity in the travel time in vadose zone added uncertainties to groundwater age-dating. Overall results showed that the combination of Bayesian approach with DIC criterion can help to identify uncertainties associated with the age distribution for hydrologically complicated systems.

中文翻译：

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裂隙岩含水层中高利用农井周围的流动动力学特征：评估地下水测年的不确定性

摘要 当环境示踪剂用于确定地下水年龄分布时，测量误差、模型结构误差和样品污染等各种因素都会影响结果。在这项研究中，我们试图解开和解释与裂缝岩含水层系统中一组高度利用的生产井的年龄分布推断相关的不确定性。在生产井的不同深度（22-150 m）共采集了 17 个样品。分析了这些样品的六种年龄示踪剂，包括 3H、氚源 3He、放射源 4He、CFC-11、CFC-12 和 CFC-113。贝叶斯方法用于基于几个假定的集总参数模型 (LPM) 和无形状模型来评估样本的年龄分布。发现大多数井中的测量数据可以用传统的 LPM 很好地解释。然而，对于 17 口井中的 5 口井，发现简单的 LPM 是不够的，根据偏差信息标准 (DIC) 测量，更复杂的无形状方法会产生更好的结果。这看起来归因于 1) 密集井之间的竞争，2) 厚的包气带层可能导致优先流动 3) 允许局部生产的裂缝。此外，研究表明，示踪剂污染、不均匀的 4He 产生和渗流带旅行时间的异质性等三个其他因素增加了地下水测年的不确定性。