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

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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.