Using He as a Natural Tracer to Understand the Dynamics of Groundwater Systems: A Quantitative Approach

0308707 CastroDating古老的地下水在很大程度上局限于14 C测年的时间段。一个有前途的新方法是应用保守的示踪剂，如4 He，它没有限制其年龄范围确定地下水停留时间。4 He方法的一个困难是使用14 C和4 He同位素计算的水年龄之间存在明显的不一致。这些差异可能是由于对4 He如何在水中和通过水中运输（平流、分散和/或分子扩散）的处理不当造成的。本研究将调查并希望解决14 C-4 He的差异，进一步发展4 He示踪技术，使其在水文研究中具有更广泛的适用性。德克萨斯州的Carrizo含水层和周围地层提供了一个很好的环境来比较14 C-4 He水的停留时间。定量的探索和理解需要一个数值方法的水流和系统中的质量传输。将进行广泛的惰性气体和14 C取样活动，为4 He运输模型的模拟和校准提供基础。水流和物质运输的模拟将同时进行，在稳定和瞬态与有限元代码。将采取两种办法。首先，将根据观察到的4 He浓度校准水流模型。在4 He浓度的变化模式将产生信息的水力传导率的空间变化，从而限制地下水流模型，并有助于更好地定义这种非均质地下水流系统。第二，将使用逆向方法，根据水头和transmittance数据对地下水流模型进行独立校准。测量的4 He浓度将通过显示哪些与He数据兼容来减少可能的解决方案的数量。4 He作为一种工具，迄今地下水的可靠性将通过两种方法进行评估，预计将导致类似的结果。然后将以这种方式估算的地下水年龄与14 C浓度计算的年龄进行比较。用4 He获得的水流模型还将用于模拟3 He、14 C、Cl、Br和Sr的迁移，以进一步减少与地下水流模型相关的不确定性。最初的2-D模型随后将扩展为3-D表示，以评估将三维系统表示为二维系统所引入的潜在误差。这项研究将有利于许多不同的领域，特别是水文地质学和古气候学。

0308707CastroDating old groundwater has been largely limited to time periods accessible by 14C dating. A promising new approach is the application of a conservative tracer such as 4He which has no limitations on its age scale for determining groundwater residence times. A difficulty with the 4He approach is an apparent inconsistency among water ages calculated using 14C and 4He isotopes. These discrepancies likely result from an inadequate treatment of how 4He is transported in and by water (advection, dispersion and/or molecular diffusion). This study will investigate and hopefully resolve the 14C - 4He discrepancy, with the goal of further developing the 4He tracer technique so that it can have broader applicability in hydrologic studies. The Carrizo Aquifer and surrounding formations in Texas provide an excellent setting to compare 14C - 4He water residence times. Quantitative exploration and understanding requires a numerical approach of both water flow and mass transport in the system. An extensive noble gas and 14C sampling campaign will be undertaken to provide a basis for simulations and calibration of a 4He transport model. Simulations of water flow and mass transport will be conducted simultaneously, in steady and transient states with a finite element code. Two approaches will be undertaken. First, a water flow model will be calibrated on observed 4He concentrations. The pattern of variation in 4He concentrations will yield information on spatial variation in hydraulic conductivities, thus constraining the groundwater flow model and contributing to a better definition of this heterogeneous groundwater flow system. Second, an independent calibration of the groundwater flow model based on hydraulic head and transmissivity data will be carried out using an inverse methodology. Measured 4He concentrations will reduce the number of possible solutions by showing which are compatible with He data. The reliability of 4He as a tool to date groundwater will be assessed by both approaches, which are expected to lead to similar results. Groundwater ages estimated in this way will then be compared with calculated ages from 14C concentrations. The water flow model obtained with 4He will also be used to simulate transport of 3He, 14C, Cl, Br, and Sr in order to further reduce uncertainties associated with groundwater flow models. The initial 2-D model will be subsequently expanded into a 3-D representation in order to assess potential errors introduced by representing a three-dimensional system as a two-dimensional one. This study will benefit a number of diverse fields, particularly hydrogeology and paleoclimatology.