4He as a dating groundwater tool in shallow crystalline aquifers

⁴He as a dating groundwater tool in shallow crystalline aquifers

Labasque T., Chatton E. , Aquilina L., Vergnaud V.

OSUR-Géosciences Rennes, Université Rennes1 – CNRS; Rennes France

 

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.