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

使用 He 作为自然示踪剂来了解地下水系统的动态：一种定量方法

• 批准号: 0308707
• 负责人: M. Clara Castro
• 金额: $ 28.31万
• 依托单位: Regents of the University of Michigan - Ann Arbor
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2003
• 资助国家: 美国
• 起止时间: 2003-06-01 至 2007-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0308707&HistoricalAwards=false
• 关键词: Using; He; Natural; Tracer; Understand

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.

0308707的旧地下水在很大程度上仅限于14C约会时可以访问的时间段。一种有希望的新方法是使用保守的示踪剂（例如4HE）的应用，该示踪剂对确定地下水停留时间的年龄尺度没有限制。 4HE方法的困难是使用14C和4HE同位素计算的水年龄之间的明显不一致。这些差异可能是由于对4HE在水中和水中如何运输的处理不足所致（对流，分散和/或分子扩散）。这项研究将调查并希望解决14C -4HE差异，目的是进一步开发4HE示踪技术，以便它在水文研究中具有更广泛的适用性。德克萨斯州的Carrizo含水层及周边地层为比较14C -4HE水居住时间提供了绝佳的环境。定量探索和理解需要系统中水流和质量传输的数值方法。将开展广泛的高贵气体和14C采样活动，为4HE运输模型的模拟和校准提供基础。水流和质量运输的模拟将在具有有限元代码的稳定和瞬态状态下同时进行。将采用两种方法。首先，将在观察到的4HE浓度上校准水流模型。 4HE浓度的变化模式将产生有关液压电导率空间变化的信息，从而限制了地下水流量模型，并有助于更好地定义这种异质地下水流量系统。其次，将使用基于液压头和透射率数据的地下水流量模型进行独立的校准。测得的4HE浓度将通过显示与HE数据兼容的可能解决方案的数量。两种方法都将评估4HE作为日期地下水的工具的可靠性，这些方法有望带来相似的结果。然后，以这种方式估计的地下水年龄将与14c浓度的计算年龄进行比较。用4HE获得的水流模型也将用于模拟3HE，14C，CL，BR和SR的运输，以进一步减少与地下水流量模型相关的不确定性。最初的2-D模型随后将扩展到3-D表示形式中，以评估通过将三维系统表示为二维系统引入的潜在误差。这项研究将使许多不同的领域受益，尤其是水文地质学和古气候学。