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4He as a dating groundwater tool in shallow crystalline aquifers

Abstract : Groundwater dating methods have been widely used during the last decades for studying subsurface water hydrological and hydrochemical processes. Estimation of groundwater residence time is essential for resource preservation, contaminant studies or groundwater recharge rates and flow velocities assessments. Due to the complexity of groundwater flow, the joint use of several environmental tracers has been often promoted as it offers integrative information on the structure of complex aquifers. Anthropogenic gas tracers as CFC, SF6, 85Kr, 36Cl or 3H have been widely used to study shallow groundwater with residence time of less than 70 yrs. For longer groundwater residence time (100- x1000 yr), 39Ar, 14C, 36Cl and 4He have been used. Although it informs mainly on residence times from several thousands to hundreds of thousands years, 4He can also cover an age range of 10 to thousands years. The residence time is estimated by taking into account all 4He fluxes from atmosphere, crustal and mantellic, but also taking into account diffusion processes in fractured media. The main difficulty is to estimate the crustal production rate through U and Th decay and its homogenity in the aquifer and the others 4He fluxes: atmosphere, crust and mantellic, and diffusion processes in fracture media. In many cases U-Th production deduced from U and Th concentrations is not sufficient to explain the 4He concentrations observed in the aquifer. Other 4He fluxes can then be estimated through the use of other tracers such as 14C, 36Cl or modeling. Fracturing may also enhance 4He concentrations in groundwater through diffusion processes from the matrix to the circulating water and should also be evaluated. We present here the evaluation of 4He in a crystalline fractured aquifer in the Northwest of France (OZCAR – H+ national hydrogeological network), in order to investigate the range of groundwater residence time in this complex shallow aquifer. Previous studies on this aquifer reveal mixing between young (<70yrs) and old waters (>1000yrs) based on 14C. The Helium radiogenic production rate is then evaluated through in situ production (U, Th) and calibration with CFC and 14C. Mixing processes are estimated through a lumped parameter model approach and diffusion processes are discussed through an estimation of fracture aperture and fracture interval. Apparent ages are compared and uncertainties discussed. Once 4He production calibrated and diffusion processes characterized, 4He gives access to groundwater ages from decades to several centuries, and thus completes the range of groudwater ages obtained by the other tracers.
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Contributor : Isabelle Dubigeon <>
Submitted on : Friday, April 10, 2020 - 11:03:38 AM
Last modification on : Sunday, April 12, 2020 - 1:11:30 AM

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  • HAL Id : insu-02539575, version 1

Citation

Thierry Labasque, Eliot Chatton, Luc Aquilina, Virginie Vergnaud-Ayraud. 4He as a dating groundwater tool in shallow crystalline aquifers. European Geosciences Union General Assembly, Apr 2020, online, Austria. pp.EGU2020-7558. ⟨insu-02539575⟩

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