Helium in deep circulating groundwater in the Great Hungarian Plain: Flow dynamics and crustal and mantle helium fluxes

Observed helium concentrations in deep circulating groundwater of the sedimentary basin of the Great Hungarian Plain (GHP), Hungary, cover a range of three orders of magnitude (≈4 ·10^-8 to 4 · 10^-5 ccSTP g^-1). ³He ⁴He ratios and noble gas concentrations are used to separate helium components originating from the atmosphere, tritium decay, crustal production, and mantle degassing. The characteristic distribution of measured helium concentrations and isotope ratios can be reproduced qualitatively by a simple two-dimensional advection/diffusion model. Other simple models isolating parts of the regional flow domain (recharge, discharge, and horizontal flow) are discussed and applied to derive quantitative information on helium fluxes due to degassing of the Earth's crust /mantle and on the dynamics of groundwater flow. The estimated helium flux of 0.7-4.5 · 10⁹ atoms ⁴He m^-2 s^-1 is lower than values derived from other deep groundwater circulation systems, probably because the relatively young upper few thousand meters of the sedimentary basin (Tertiary to Quaternary age) shield the flux from the deeper crust. The high mantle helium flux of up to 4.2 · 10⁸ atoms ⁴He m^-2 s^-1 is probably related to the Miocene volcanism or to continuing intrusion accompanying extension. By fitting calculated helium depth profiles to measured data in the discharge area, vertical flow velocities of the order of 1.5 mm y^-1 are estimated. Assuming that a flux of 0.7-4.5 · 10⁹ atoms ⁴He m^-2 s^-1 is representative for the entire basin, the turnover time of the regional groundwater flow system is estimated to be about 10⁶ y.