Helium accumulation in groundwater, I: An evaluation of sources and the continental flux of crustal 4He in the Great Artesian Basin, Australia
The rate of accumulation of 4He in the groundwaters of the J-aquifer of the Great Artesian Basin, Australia has been determined using 14C and hydrologic ages. For groundwaters less than 50 Kyr in age, the 4He accumulation rate is 4.6 × 10 -12 HeAU (where 1 HeAU = 1 cm 3 STP 4He · cm H 2O -3 · yr -1) in close agreement with in-situ production rate of 4He (3.95 × 10 -12 HeAU) based on U and Th concentrations of 1.7 and 6.1, respectively, of the sandstone. For groundwaters older than 100 Kyr, the rate of 4He accumulation is 2.91 × 10 -10 HeAU based on hydrologic ages; or 74 × the rate of in-situ production. The rate of 4He "production" due to weathering of the aquifer rock is calculated to be ~10 -16 HeAU, indicating that the weathering input of 4He is insignificant. If the groundwater of the GAB can be considered as a trap for the total crustal production of 4He, the rate of 4He accumulation under a steady-state flux is calculated to be 3.02 × 10 -10 HeAU, in agreement with the measured accumulation rate. It is concluded that over long times the 4He accumulation rate in groundwater aquifers may be controlled by the whole crust flux of 4He.