Very Old Groundwater : Eastern Great Artesian Basin Case Study

Introduction The Great Artesian Basin (GAB) of Australia occupies one fifth of the Australian continent and 1.7 x 10km. The region typically receives order 100mm of rain annually with a maximum of 600mm near the eastern margin recharge areas. Average annual runoff is much less than 10mm and most commonly less than 5mm. As a result, the land surface is marked with few perennial rivers and many ephemeral rivers; rivers draining into Lake Eyre in central Australia (60m below sea level) often dry up or are lost to infiltration before reaching the terminus in Lake Eyre. This dry, harsh climate of interior Australia limited the ability of the early settlers to explore and exploit the resources of this region. Beginning in the 1880s, artesian water was discovered in this vast interior region and eventually over 4700 wells were drilled into the primary artesian lithology, the Cadna-owieHooray aquifer (Habermehl, 1980). In addition, the overlying non-artesian Cretaceous aquifer was tapped with 20,000 pumped wells. This degree of exploitation led to significant drawdown of the water and by the1970s only 3100 wells remained flowing under artesian pressure and many of the non-flowing artesian wells were augmented by wind-powered pumps (Habermehl, 1980; Radke et al 2000). Recent estimates have placed the storage of water in the artesian system at 8.7 x 10 Mliters making it one of the largest artesian aquifers in the world (Radke et al., 2000). The last 30 years have seen a significantly increased scientific effort to understand the capacity of this system and its recharge although some efforts considerably predate this time period. In the following sections, a case study is developed for how the Eastern GAB was studied, quantified and evaluated. The case study is guided by hindsight that may or may not accurately reflect scientific thinking at the time. However, it is hoped this case study will enable the structure and components of this scientific study of very old groundwater to be placed in a logical framework so the precedents and the lessons of the GAB can be applied most successfully to other systems. The GAB is comprised of multiple subsystems (Fig. 1) with each system subject to its own developmental pressures and often its own unique recharge flow and discharge conditions. Jack (1923) recognized the convergence of two separate flow systems near Lake Eyre characterized by the high alkalinity waters from the east and high sulfate waters from the west. Here, the focus will be on the primary, long time-scale flow within the Central Eromanga Basin. The primary water use in this Central Eromanga Basin system is agriculture with an emphasis on cattle for beef and sheep for wool. The driving economics of the Basin is utilization of a sustainable resource, with conservation, rehabilitation and/or removal of some wells. As water in this Central Eromanga Basin has some of the longest flowlines and some of the slowest flow rates, it provides an endmember for the application, analysis and evaluation for dating very old groundwater. Habermehl (1980) sets the stage for detailed scientific investigation of the GAB system and the individual studies discussed below. To a large extent, the discussion in the following sections is an overview of the information contained within the individual studies with a historical context to elucidate how dating methods and field sampling were guided by existing information. The reader is referred to these primary literature sources for greater detail. Radke et al. (2000) provide detailed interpretation of the combined structural, hydraulic, geochemical, radiochemical and isotopic analyses and is an example of how information can be combined, integrated and utilized for the fullest understanding of very old groundwater basins.