Groundwater dating by estimation of groundwater flow velocity and dissolved 4He accumulation rate calibrated by 36Cl in the Great Artesian Basin, Australia

Abstract

We tested two methods for dating groundwaters that cannot be reliably measured by 36 Cl dating alone, one based on groundwater flow velocity plus distance along a flow path and the other based on 4 He accumulation rates calibrated with 36 Cl dates. We sampled groundwaters along six inferred regional groundwater flow paths in the Great Artesian Basin (GAB) of Australia. We selected three groundwater paths where the decrease in 36 Cl was largely controlled by cosmogenic 36 Cl radioactive decay without a significant increase in chloride concentration. The extrapolated groundwater velocities were 0.133 ± 0.018 m/y to 0.433 ± 0.140 m/y. The estimated residence time of 1.06 × 10 6 y at the discharge area around Lake Eyre was comparable to the estimate of (1-2.2) × 10 6 y in previous studies. On the other hand, our estimated 4 He accumulation rates for the selected three groundwater flow paths (1.85 ± 0.31 × 10-11 to 1.51 ± 0.63 × 10-10 ccSTP/cm 3 •y) were approximately 2-15 times lower than previously reported rates for the central GAB. Our estimated rate of 1.51 × 10-10 ccSTP/cm 3 •y-1 in the western GAB is compatible with previous estimates based on 81 Kr ages. The groundwater residence time estimated from the 4 He accumulation rate was approximately 7 × 10 5 y near the discharge area at Lake Eyre. Finally, both estimations were mutually compatible with a 30% error.

A multi-tracer approach to constraining artesian groundwater discharge into an alluvial aquifer

“…For R se a value of 5.7 (×10 −15 ) was used, which is appropriate for aquifers dominated by sandstone (this secular equilibrium value can vary according to the dominant lithology). This R se value has been applied to 36 Cl / Cl calculations elsewhere in the GAB (Moya et al, 2016) and is similar to that calculated from drill-core samples recovered in the GAB by Mahara et al (2009).…”

Fluids and Ore Formation in the Earth's Crust

“…Land topography, more precisely the lateral difference in water table elevation, leads to differential pressures of hundreds of bars, which drive fluid flow through fractured basement rocks or sedimentary basins and give rise to springs and artesian wells. In terms of time-integrated fluid flow, Australia's Great Artesian Basin, the world's largest system of continental sub-surface water flow (Mahara et al, 2009), exemplifies the characteristics of topographydriven fluid flow through rocks. Flow is large-scale and dominantly lateral, over thousands of kilometers.…”

Detecting inter-aquifer leakage in areas with limited data using hydraulics and multiple environmental tracers, including 4He, 36Cl/Cl, 14C and 87Sr/86Sr

“…1b). The GAB is one of the largest aquifer systems in the world, and has been the subject of numerous investigations, which focused largely on the eastern artesian and north-western sections of the basin (Habermehl 1980;Herczeg et al 1991;Love et al 2000Love et al , 2013Mahara et al 2009;IESC 2014;Moya et al 2015). Few studies focused on the unconfined south-western part of the GAB, but some have highlighted the potential connectivity between the GAB and the Arckaringa Basin (Smerdon et al 2012;Keppel et al 2015).…”