Evaluation of dissolved 4He as a groundwater age tracer in shallow fractured aquifers
Résumé
Anthropogenic gas tracers such as CFC, SF6, 85Kr, 36Cl or 3H have been widely used to study shallow groundwaters with residence time of less than 70 yr. For longer groundwater residence time (100- x1000 yr), 39Ar, 14C, 36Cl and 4He have been used. 4He can cover a dating range of 10 to thousands of years (Solomon et al., 1996). The main difficulty is to estimate the production rate through U and Th decay and the others fluxes: atmosphere, lithosphere and asthenosphere. In many cases U-Th production 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: 14C, 36Cl or modeling. Fracturing may also enhance 4He concentrations in groundwater.
We present here the evaluation of 4He in a crystalline fractured aquifer in the Northwest of France (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 (<70 yrs) and old waters (>1000 yrs) (Ayraud et al., 2008). The Helium radiogenic production rate is then evaluated through in situ production (U, Th, porosity), calibration with CFC and 14C, and modelling of the diffusion processes affecting 14C and 4He through physical characteristics of the aquifer (porosity, fracture spacing and aperture). Young groundwater residence times estimated by 4He agree with those estimated by CFC and 3H/3He. In this fractured media, old groundwater residence times (> 100 yr) are better estimated through the integration of the mass transfer between the fractures and the porous rock matrix through diffusion processes. 4He proves to be a valuable tool to characterize groundwater mixing processes and groundwater residence times from a decade to thousands of years.
Solomon, D. K., Hunt, A., & Poreda, R. J. (1996). Source of radiogenic helium 4 in shallow aquifers: Implications for dating young groundwater. Water Resources Research, 32(6), 1805-1813.
Ayraud, V., Aquilina, L., Labasque, T., Pauwels, H., Molenat, J., Pierson-Wickmann, A. C., ... & Fourre, E. (2008). Compartmentalization of physical and chemical properties in hard-rock aquifers deduced from chemical and groundwater age analyses. Applied geochemistry, 23(9), 2686-2707.
