Stagnant aquifer concept Part 1. Large-scale artesian systems— Great Artesian Basin, Australia

Abstract

Artesian systems, 1000–2000 m deep, occurring in the one and half million km 2 Great Artesian Basin (GAB), are described by most researchers as through-flow aquifers that are recharged at eastern outcrops in the Great Dividing Range. A simplified mathematical model, assuming basin-wide through-flow in two aquifers — of Jurassic and Cretaceous rocks, has been applied by previous researchers to calculate hydraulic travel times. The latter were described as agreeing with groundwater ages of 10 5 to 10 6 years, computed from 36Cl data (assuming part of the observed 36C1 was atmospheric) and 4He ages (assuming the bulk of observed 4He diffused from the crust). An alternative conceptual model is discussed in this paper, suggesting that artesian wells, drilled into 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 expected to be high — between the age of the host rocks (connate water) and the 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 groundwaters: the chlorine-36 data were interpreted as being beyond the dating limit of this method (i.e. over one million years, suggesting that the bulk of the observed 36C1 was produced in situ), and the 4He ages were computed to be 5–10 million years or more (suggesting that all the 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 observations. Mathematical modeling is not feasible either, as too many parameters pertaining to the number of distinct aquifers, their rock properties, and other components of the groundwater cycle are unknown. Hence, a phenomenological approach is adapted, and the through-flow and stagnant aquifer models are weighed up in the light of first principles of physics, geology, hydraulics chemistry and isotopic compositions.