TOWARD A NEW PARADIGM FOR THE GREAT ARTESIAN BASIN: HYDROLOGIC MIXING, PARTITIONED SUB-BASINS, AND MANTLE INFLUENCES ON GROUNDWATER QUALITY

The Great Artesian Basin (GAB) underlies 22% of the Australian continent and is one of the largest groundwater basins in the world. Of great national and societal significance for Australia, the GAB is also an iconic example of a continental-scale artesian groundwater system and of international scientific importance as sustainable groundwater management is emerging as one of the great global scientific challenges of the new millenium. Existing models involve recharge in Eastern Australia, relatively simple flow paths, and discharge in springs and bores in the western GAB. New geochemical and hydrologic data suggest these may need modification. Our hypothesis, based on new geochemical data, is that endogenic fluid inputs into the J-K aquifer system leak up faults from below the aquifer and cause degradation of water quality. It has long been known that elevated ³He/⁴He values are present in groundwater from eastern Australia (3 R_A at Mount Gambier corresponding to 37% mantle helium and 0.81 R_A in eastern GAB, 10% mantle helium). New ³He/⁴He data from mound springs in the western GAB also indicate mantle inputs. Warburton Spring has R_A= 0.16 (2% mantle helium) with X=30, and CO₂/³He= 9x10⁹; Bubbler Spring has R_A= 0.72 (9% mantle helium) with X=3.2 and CO₂/³He= 2x10¹⁰. These fluids are also rich in CO₂, Cl, metals, and radiogenic Sr. Additional studies are underway as part of an Australian National Water Commission project to evaluate complexities of mixing models, variable flow rates, variable water quality, and tectonic influences in different sub-basins of the Great Artesian Basin at different time scales. 1) Hydrologic modeling will quantify recharge from western and northern sources, 2) CO₂/³He values can be used to resolve asthenospheric, lithospheric, and surface system CO₂ contributions, 3) hydrochemical modeling, stable isotope studies, and ⁸⁷Sr/⁸⁶Sr will elucidate contributions from radiogenic basement, 4) paleohydrology will be approached via modeling of changes in equipotential surface through time as constrained by U-Series dates from the travertine record, 5) neotectonic studies involve scales ranging from the mound springs fault line to the Australian plate. New understandings of the Great Artesian Basin will require holistic models that merge these hydrologic, geochemical, and tectonic perspectives.