Geochemical evidence for groundwater mixing in the western Great Artesian Basin and recognition of deep inputs in continental-scale flow systems

Mound springs of the western Great Artesian Basin (GAB), Australia, represent a significant proportion of the discharge of the continental-scale confined aquifers of the region. They also provide unique ecological niches, and they are important historical and cultural sites in an austere landscape. Fed by confined aquifers within the GAB, these spring systems are at risk due to anthropogenic drawdown and increasing demand on scarce hydrologic resources. New water and gas geochemical data indicate that they record hydrologic mixing and complex, fault-influenced flow paths within the western GAB. Elevated 3He/4He gas values, termed “xenowhiffs”, with RA up to 0.09 (Bubbler Spring) provide evidence for mantle-derived fluids introduced through fault conduits into the groundwater system in the last several million years and hence an active mantle-to-groundwater fluid linkage. We apply multiple tracers to understand mixing. Major and trace element data show distinctly different water chemistries for Dalhousie versus southern mound springs suggesting different flow paths and mixing proportions. The source of the C for the CO2 -rich springs is evaluated using water chemistry and C-isotope data. Carbon isotope values range from -9 (Bubbler) to -16 (Strangways). Mixing models allow us to distinguish contributions from dissolution of carbonate in the aquifer (Ccarb=Ca+Mg-SO4 and δ13C= 0), from biological/organic sources (δ13C= -28), and from endogenic sources (deeply derived; δ13C= -3). Results show that all of the springs contain appreciable (many > 50%) endogenic CO2, with Dalhousie showing less endogenic CO2 than the southern mound springs and Paralana hot spring system. CO2/3He values of 4 to 8 x 109 (Bubbler and Jersey Springs) are close to MORB end member values of 2 x 109 whereas other springs have values strongly enriched in CO2 (up to 1013 at Elizabeth Spring). Elevated but highly variable 87Sr/86Sr values up to 0.718 at Dalhousie and up to 0.76 at Paralana Hot Springs record different degrees of fluid-rock interactions in granitic crust and small volume, but geochemically potent, crustal contributions to the endogenic fluids. U-Series dates indicate persistent deposition of travertine mound springs (conceptualized as “chemical volcanoes”) at discrete vent sites for millions of years. The geochemistry of the active travertine-depositing mound springs, coupled with geochemistry of the associated mound and platform travertine rock record, thus collectively provide a rich record that can be used to link the present hydrologic system to paleohydrology of the GAB over the last several million years at the continental scale. The overall goal is to test a model for interactions between mantle and deep crustal fluid inputs, neotectonic pathways, groundwater mixing, groundwater quantity and quality, and unique microbiology in the near-surface hydrologic systems.