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Artesian springs support important wetland ecosystems comprising highly specialised, endemic animal and plant species. Understanding the hydrogeological processes that sustain springs is critical to their management and characterisation of the connected groundwater system. This paper focuses on the springs of the Eulo and Bourke supergroups in the south-central part of the Great Artesian Basin in Queensland and New South Wales, Australia. These springs have been heavily impacted by aquifer drawdown, as a result of uncontrolled discharge of artesian water from pastoral bores. This study uses a stratigraphic model of the aquifer and aquitard sediments, a model of the potentiometric surface, existing information on the coverage of basement and surface structures, and a comprehensive field survey of all known springs to identify the association of springs with the edge of the geological basin, basement outcrop, basement highs, surface and deep basement faults, and a thinned aquitard. Many springs in the Eulo Supergroup are associated with basement highs and many springs are associated with deep faults, suggesting that reactivation of these faults may be an important cause of spring discharge. Springs in the Bourke Supergroup previously thought to be outcrop springs (gravity-fed), by virtue of their surface characteristics, are more likely to be artesian springs based on the interpretation of basement geology, surface faults and water chemistry, which align most closely to the Hooray Sandstone, the artesian aquifer underlying the springs.

The Great Artesian Basin (GAB) in Australia underlies semi-arid and arid regions across 1.7 million km2 or one-fifth of Australia. The basin’s groundwater resources were discovered around 1880 and their development allowed pastoral activities, homestead and town water supplies, and petroleum and mining ventures to exist. The GAB is a multi-layered confined aquifer system, with aquifers in Jurassic and Cretaceous continental sandstones and intervening confining beds of siltstone and mudstone of the constituent Eromanga, Surat and Carpentaria sedimentary basins. The basin is up to 3,000 m thick and forms a large synclinal structure, uplifted and exposed along its eastern margin and tilted southwest. Recharge to the exposed aquifers occurs in the eastern margin, an area of relative high rainfall. The western margin in Australia’s arid centre receives minor recharge. Regional groundwater flow is towards the southern, south-western, western and northern margins. Flowing artesian springs discharge in the southwest margin and have produced carbonate mounds. Although lateral groundwater movement dominates, vertical upwards leakage is considered important. Potentiometric surfaces of the Jurassic and Lower Cretaceous aquifers are still above ground level throughout most of the basin, though pressure drawdowns of up to 100 m have been recorded in recent decades in highly developed areas, and consequently some artesian water boreholes and springs have ceased flowing. Government borehole rehabilitation programs have led to groundwater pressure recovery in some areas. This paper outlines the history of GAB exploration, scientific investigations (hydrogeology, hydrochemistry, isotope hydrology, groundwater modelling) and management, summarising the recent knowledge on the GAB.

The Great Artesian Basin (GAB), Australia, one of the largest confined aquifer systems in the world, attracts great attention for groundwater dating. However, there is little information about the confining layers. Therefore, core drilling investigations were conducted to characterize the main confining layer using chloride (Cl) and chlorine isotopes (36Cl, 37Cl) at Marree (South Australia) and Richmond (Queensland), which are near the discharge and recharge areas in the GAB, respectively. Pore water samples were collected from rock cores by squeezing and leaching. The Cl concentration, 36Cl/Cl ratio, and δ37Cl value in the confining layer decreased with depth at both Marree and Richmond. The 36Cl/Cl ratios at the shallower part of the confining layer are significantly higher than the in situ secular equilibrium (36Cl/Clse) calculated from the chemical compositions of the rock. The 36Cl/Cl ratio logarithmically decreased with depth. The calculated 36Cl/Clse is equivalent to the lowest 36Cl/Cl ratio at Richmond. In contrast, the 36Cl/Cl ratios at Marree are higher than the calculated 36Cl/Clse. The 36Cl/Clse was reached at Richmond, but not at Marree. This probably reflects the transport time due to the difference in diffusion coefficients. The δ37Cl value becomes more negative toward the deeper levels, reaching −4.5‰ and −6.1‰ at Marree and Richmond, respectively. These results suggest that the Cl is of meteoric origin and is transported by diffusion in the confining layer. Analytical simulations using diffusion equations were conducted to reproduce excess 36Cl (36Clex), Cl, and δ37Cl. The 36Clex profile, which logarithmically decreased with depth, was reproduced by steady-state diffusion equations with radioactive decay, and the diffusion coefficients derived from the 36Clex profile were equivalent to those from the laboratory experiments. A grid-search simulation using an unsteady-state diffusion equation was conducted to reproduce the Cl and δ37Cl profiles. The Cl and δ37Cl profile could be reproduced; however, the Cl concentration could not be reproduced well at shallower depths. Further studies are required regarding the geological events on the initial and boundary conditions. The diffusive flux from the confining layer to the aquifer is significant in Marree, but not in Richmond; though the δ37Cl can be a good index of diffusive flux. However, it is difficult to evaluate diffusive flux quantitatively when the diffusive flux is significant because it depends on the transport parameters in the confining layer. The groundwater flow in the confining layer is less than 1 × 10−5 m/y because diffusion is dominant.

We tested two methods for dating groundwaters that cannot be reliably measured by 36Cl dating alone, one based on groundwater flow velocity plus distance along a flow path and the other based on 4He accumulation rates calibrated with 36Cl dates. We sampled groundwaters along six inferred regional groundwater flow paths in the Great Artesian Basin (GAB) of Australia. We selected three groundwater paths where the decrease in 36Cl was largely controlled by cosmogenic 36Cl radioactive decay without a significant increase in chloride concentration. The extrapolated groundwater velocities were 0.133 ± 0.018 m/y to 0.433 ± 0.140 m/y. The estimated residence time of 1.06 × 106 y at the discharge area around Lake Eyre was comparable to the estimate of (1–2.2) × 106 y in previous studies. On the other hand, our estimated 4He accumulation rates for the selected three groundwater flow paths (1.85 ± 0.31 × 10− 11 to 1.51 ± 0.63 × 10− 10 ccSTP/cm3∙y) were approximately 2–15 times lower than previously reported rates for the central GAB. Our estimated rate of 1.51 × 10− 10 ccSTP/cm3∙y− 1 in the western GAB is compatible with previous estimates based on 81Kr ages. The groundwater residence time estimated from the 4He accumulation rate was approximately 7 × 105 y near the discharge area at Lake Eyre. Finally, both estimations were mutually compatible with a 30% error.

Artesian spring mound deposits in the southwestern part of the Great Artesian Basin arise from groundwater discharge from flowing springs in the southern and western margins which reaches the surface through faults and weaknesses in thin confining beds overlying the Mesozoic artesian aquifers. Carbonate in solution in the artesian groundwater is deposited by many springs as tufa, building ‘spring mounds.’ Active flowing and dry inactive spring mounds occur in a variety of sizes and shapes. The ages of a representative selection of spring mound deposits have been found by luminescence dating of quartz sand grains that have been incorporated in the mound deposits. The spring deposits of the active flowing springs, Big Bubbler, Blanche Cup and Beresford Spring have ages of 15.1 ± 2.2, 10.9 ± 1.5 and 13.9 ± 1.0 ka, respectively. Spring complexes with both active flowing and dry extinct mounds, Strangways and Beresford Springs, have ages of 60 ± 8 and 219 ± 35 ka, respectively. Spring deposits of springs which...

The ratio of dissolved excess 4 He concentration to the 4 He accumulation rate is equivalent to the residence time of groundwater if 4 He is accumulated at a constant rate. Groundwater contains 36 Cl from cosmogenic and subsurface production sources. The 4 He clock can be applied by using the correlation between cosmogenic 36 Cl decay or the subsurface 36 Cl production and the excess 4 He increase. We calibrated the 4 He clock in the Great Artesian Basin (GAB), Australia, considering dissolution of chloride from the rock matrix. Cosmogenic 36 Cl is dominant over 36 Cl produced in the subsurface in the Coonamble Embayment of the GAB, because the quartzose sandstone aquifers have a low density of thermal neutrons. The 4 He accumulation rate was estimated to be 9.6 × 10 - 11 ccSTP/g H 2 O · y - 1 in the GAB. We then applied this method to estimation of the groundwater residence time in the Tokachi Basin, Japan, where both cosmogenic and subsurface 36 Cl are present in the groundwater. The longest estimated residence time was 2.3 million years, based on a 4 He accumulation rate of ( 1.2 - 2.1 ) × 10 − 12 ccSTP / cm H 2 O 3 · y - 1 , assuming that ion filtration occurred as pore water was squeezed by the deeply subsiding sedimentary strata in the basin.

Groundwater springs are significant landscape features for humans and the biota that occupies their habitat. Many springs become inactive where groundwater exploitation by humans has lowered the water table or artesian pressure. In order to assess this impact, it is important to identify and locate active, and with more difficulty, inactive springs. Using a variety of archival, environmental and field-based data, this study presents a protocol for the determination of the location and status of springs across the Great Artesian Basin of Australia. This protocol underpins a database of springs, which is not only important for the assessment of spring ecosystems, but also contributes to understand groundwater extraction impacts and hydrogeological processes. The database indicates that 30.0% of discharge (artesian) springs in the Great Artesian Basin are entirely inactive and another 11.8% are partially inactive. For the outcrop (gravity) springs of the Basin, only 1.9% are entirely inactive and 7.4% partially inactive, and for the outcrop springs in the Tertiary sandstone overlying the Basin 30.9% are inactive and 18.2% are partially inactive.

Groundwater contains dissolved He, and its concentration increases with the residence time of the groundwater. Thus, if the 4 He accumulation rate is constant, the dissolved 4 He concentration in ground-water is equivalent to the residence time. Since accumulation mechanisms are not easily separated in the field, we estimate the total He accumulation rate during the half-life of 36 Cl (3.01 × 10 5 years). We estimated the 4 He accumulation rate, calibrated using both cosmogenic and subsurface-produced 36 Cl, in the Great Artesian Basin (GAB), Australia, and the subsurface-produced 36 Cl increase at the Aspo Hard Rock Laboratory, Sweden. 4 He accumulation rates range from (1.9±0.3) × 10 −11 to (15±6) × 10 −11 ccSTP·cm −3 ·y −1 in GAB and (1.8 ±0.7) × 10 −8 ccSTP·cm −3 ·y −1 at Aspo. We confirmed a ground-water flow with a residence time of 0.7-1.06 Ma in GAB and stagnant groundwater with the long residence time of 4.5 Ma at Aspo. Therefore, the groundwater residence time can be deduced from the dissolved 4 He concentration and the 4 He accumulation rate calibrated by 36 Cl, provided that 4 He accumulation, groundwater flow, and other geo-environmental conditions have remained unchanged for the required amount of geological time.

New measurements of the He concentration in bore waters from the Great Artesian Basin (Australia) show: (1) a regional increase in the 4He flux over the Central Shield, where high heat flow has been attributed to increased U, Th and K in the crust; and (2) a reduced 4He flux and enhanced 3He flux in eastern Australia, where high heat flow is attributed to volcanism or underplating during the Cenozoic. These observations suggest correlations between (1) the crustal 4He flux and the component of heat flow attributed to crustal U and Th (and K), as well as (2) the 3He flux and recent volcanic activity. However, the specific relations between the heat flux and the He flux can be considerably modified by transport processes and source fractionation. It is suggested that time-dependent tectonic models of heat and mass (He) transport in the crust may be used to constrain either the components of heat flow or the bulk rate of mass transport in the Earth's crust.

The Great Artesian Basin (GAB), Australia, contains several groundwater types within the Lower Cretaceous-Jurassic confined aquifer system with distinctly different chemistry. The groundwater flow pattern of the GAB has been analyzed with over 40 36Cl measurements from this study and that of Bentley et al. (1986a). The results indicate general agreement between the hydrologie model ages (Darcy's law) and the radioisotopic model ages suggesting that flow conditions over ∼1 m.y. have remained largely unchanged. The results confirm that the chemically distinct western GAB operates as a separate flow system with its own recharge area and its own geochemical evolution. The results indicate an area of groundwater with relatively young 36Cl ages in the central portion of the basin which cannot be explained by flow or mixing along the presently known flow lines. It is hypothesized that an additional source of young water originated from the basin margin region to the north and northwest between the Simpson Desert and Mount Isa. This additional recharge area, active more strongly over the last 500 kyr than at present, is delineated by 36Cl ages which integrate a history and evolution over the age of the groundwater.

Chlorine 36 has many advantages as a dating tool for very old groundwater. These advantages include a suitable half-life (3.01 x 10/sup 5/ years), simple geochemistry, conservative behavior in groundwater, and a general absence of subsurface sources at levels comparable to the atmospheric input. Recent advances in tandem accelerator mass spectrometry have permitted the analysis of /sup 36/Cl at low abundance expected following residence in the subsurface for 10/sup 6/ years or more. In order to test the suitability of /sup 36/Cl for dating very old groundwater, the /sup 36/Cl/Cl ratios of 26 groundwater samples from the Great Artesian Basin of Australia have been measured. Groundwater ages calculated from the /sup 36/Cl data compare favorably with ages computed independently from hydrodynamic simulations.

An Accelerator Mass Spectrometry system has been developed using the 14UD tandem accelerator at the Australian National University. It has been used for 36Cl measurements on groundwater samples from the Murray Basin in southeastern Australia. Measurements of 14C have also been made on the same groundwaters. The information can be combined with stable isotope ratios and other data to illustrate the occurrence of processes such as radioactive decay and local recharge in different aquifers.

A component with approx.6% mantle helium (/sup 3/He//sup 4/He = 5--9 x 10/sup -7/) has been identified in borewaters from the Great Artesian Basin, Australia that is isotopically distinct from previously identified in situ production (/sup 3/He//sup 4/He = 1.6 x 10/sup -8/) and a crustal degassing flux (/sup 3/He//sup 4/He = 6.75 x 10/sup -8/). The occurrence of this component is limited to three identified localities (approx.20.5 /sup 0/S 144 /sup 0/E; approx.25.5 /sup 0/S 146 /sup 0/E; approx.27 /sup 0/S 144.5 /sup 0/E) all within the region of high reduced heat flow in eastern Australia. No evidence for a mantle component was apparent in central Australia. This component is most probably the result of continued intrusive volcanism (