Evaluation of dissolved 4He as a groundwater age tracer in shallow fractured aquifers

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.