STAGNANT AQUIFER CONCEPT .1. LARGE-SCALE ARTESIAN SYSTEMS - GREAT-ARTESIAN-BASIN, AUSTRALIA

Artesian systems, 1000-2000 m deep, occurring in the one and half mill ion km(2) Great Artesian Basin (GAB), are described by most researcher s as through-how aquifers that are recharged at eastern outcrops in th e Great Dividing Range. A simplified mathematical model, assuming basi n-wide through-how in two aquifers - of Jurassic and Cretaceous rocks, has been applied by previous researchers to calculate hydraulic trave l limes. The latter were described as agreeing with groundwater ages o f 10(5) to 10(6) years, computed from Cl-36 data (assuming part of the observed Cl-36 was atmospheric) and He-4 ages (assuming the bulk of o bserved He-4 diffused from the crust). An alternative conceptual model is discussed in this paper, suggesting that artesian wells, drilled i nto the deep Jurassic rocks of the GAB, are tapping stagnant aquifers that are hydraulically isolated from each other. The stagnant aquifers are envisaged as mega fluid inclusions, formed during the subsidence of the basin. The age of the water in the stagnant aquifers is expecte d to be high - between the age of the host rocks (connate water) and t he age of the subsidence stage at which the aquifer was sealed off by covering sediments and burial to beneath the active base of drainage. Isotopic ages were calculated to be in this range of very old groundwa ters: the chlorine-36 data were interpreted as being beyond the dating limit of this method (i.e. over one million years, suggesting that th e bulk of the observed Cl-36 was produced in situ), and the He-4 ages were computed to be 5-10 million years or more (suggesting that all th e He was produced in the aquifer rocks alone). The groundwater in the artesian systems is concealed and hence the through-flow and stagnant aquifer models are both based on interpretation of indirect observatio ns. Mathematical modeling is not feasible either, as too many paramete rs pertaining to the number of distinct aquifers, their rock propertie s, and other components of the groundwater cycle are unknown. Hence, a phenomenological approach is adapted, and the through-flow and stagna nt aquifer models are weighed up in the light of first principles of p hysics, geology, hydraulics, chemistry and isotopic compositions.