Navigating our ancient underground labyrinth
A four-year study of the western margin of the Great Artesian Basin (GAB) has shown the basin to be a much more complex system than previously thought.
A new understanding of the hydrogeology and ecology of the western GAB has been attained by the National Water Commission co-funded research project Allocating Water and Maintaining Springs in the Great Artesian Basin. A comprehensive set of seven reports, by researchers from Flinders University, Adelaide University, the CSIRO, the South Australian and Northern Territory Governments and overseas experts, was released in March.
One of the largest groundwater reservoirs in the world, the (GAB) lies beneath 22 percent of the Australian landmass, from Queensland’s wet/dry tropics, to the arid areas of SA’s Lake Eyre region. It underlies not only much of the Lake Eyre surface catchments, but most of the Queensland portion of the Murray-Darling Basin and a large part of the plains country of northern New South Wales.
The project has revealed that the only modern source of groundwater recharge (inflow) on the western margin occurs beneath specific ephemeral rivers, primarily the Finke and Plenty as a result of monsoonal flow events. But the recharge these rivers yield is insignificant and effectively zero for management purposes. Similarly overland flow sourced recharge in the region has been effectively zero since the last wet phase around 6000 years ago. However, groundwater still discharges via slow upward movement of water through the numerous iconic mound springs in the region. The groundwater system is not in a steady state as previously thought, as the discharge is significantly greater than the recharge and the aquifer water pressure is undergoing a natural long term decline.
The term mound spring is the name given to describe steeply walled domes or gentle inclined shield structures that have formed as a result of degassing of carbon dioxide of emerging spring water, creating a calcium carbonate residue. The springs occur when water reaches the surface through breaks in the overlying rocks created by faulting, along a fault zone from Marree to Dalhousie in the north of South Australia. The springs are aquatic islands in the desert, providing permanent habitat for an array of rare flora and fauna that have evolved within these systems. The isolation has provided the circumstance for a high degree of endemism of aquatic life to develop, with distinct lineages between spring groups and complexes.
The mound springs flow is enabled by pressure resulting from confinement of the water by a thick layer of ‘Bulldog Shale’. The project’s co-chief investigator Associate Professor Andrew Love from Flinders University states that the “key to maintaining flow in the mound springs is to keep the pressure up. Looking at the groundwater levels only, simplifies the issue.’’ To this end, managing the water pressure to maintain the springs is a focus of water management activities across the basin.
A fascinating feature of the spring waters is the presence of primordial Helium 3, indicating a linkage between the Earth’s mantle and the surface. Indeed the mound springs are associated with a zone of weakness in the Earth’s crust that has been present for the last 800 million years since the break-up of the super continent Rodinia. This finding indicates a significant vertical water source in the GAB in addition to the eastern and western horizontal flows previously known.
Associate Professor Andrew Love and collaborators from the University of Bern and Argonne National Laboratories have been trailing a new technique to determine groundwater ages. The method involves the measurement of naturally occurring Krypton 81, a noble gas radioisotope with a half-life of 230,000 years making it an ideal tracer for dating large groundwater systems such as the GAB. In the western margin of the GAB, groundwater ranging in age from modern water up to 500,000-year-old groundwater has been found to occur from the springs, which convert to groundwater flow rates from west to east in the range of 0.25 to 0.5 m/year.
Referring to a new groundwater model of the basin, Associate Professor Andrew Love estimates under current climatic conditions, it would take in the order of 50,000 years for the basin to establish a new water balance, where recharge equalled discharge, albeit at a lower water level than the level resulting from the legacy of the wetter Holocene era (not accounting for human extraction via bores).
It is clear that the basin water store is a legacy of a wetter era. Maintaining water security for the region, including mound spring flow for the foreseeable future, will require management interventions tailored to the labyrinth of vertical and horizontal geology, groundwater levels and pressure of each sub aquifer.
The full set of reports, including those on the spatial science and ecology programs, as well as the hydrogeology program discussed above, can be accessed via archive.nwc.gov.au/library/topic/groundwater