My current work focuses on springs that well up from the Great Artesian Basin in arid Australia. In this first springs-themed post I am just going to give a general overview of what springs are, how they form and what that means for the animals and plants that live there.
The Great Artesian Basin (G.A.B.) is an extensive underground aquifer that collects water at its recharge zone along the east coast of Australia and transmits it along porous sandstone through to its’ discharge regions . The sandstone that forms the basin acts as a large permeable sponge, transmitting water across the arid interior. The sandstone that forms the G.A.B. is exposed at its eastern edge which allows water to permeate, and thanks to gravity, this water is pushed down into the subterranean parts of the aquifer deep under ground. From here, this water travels westward into the discharge zones, where it is pressurised beneath a confining layer that does not allow the water to transmit into over or underlying geology. However, this water can find its way to the surface via weaknesses in this confining geology. Such points of escape can be faults or fissures, or areas where the G.A.B. is shallow and close to the surface. Artificial bores capitalise on this pressure and act in a similar fashion, allowing water pressurised within the G.A.B. to reach the surface. The map below shows the extent of the G.A.B. and its recharge and discharge zones.
Most springs in Australia are connected to this system, or are found emerging from local aquifers that overlay the G.A.B. but act in essentially the same way. Hydrogeologically they have diverse origins, many of which affect their outward appearance and ecology. Recharge springs (shown as green markers in Figure 1b. and c.) form when water is ejected before it enters any confined, subterranean parts of the aquifer. This can happen in the G.A.B. when water emerges within the recharge area, usually when some localised confining layer captures water and funnels it to the surface. This can also happen in areas overlaying the discharge zone, when local aquifers exist above the G.A.B.s confining layers, creating their own mini systems. In contrast, discharge springs emerge when water travels up along weaknesses from the confined portions of the G.A.B. where the water has already travelled from the recharge zone. Below is an example of a discarge (left) and a recharge (right) spring (Figure 2).
Supergroups, complexes and springs
Because springs are associated with particular geologies and their interaction with water, we find in most situations they cluster together. Pioneering springs researchers like Haebermehl  began categorising springs based on their spatial clustering and to this day we still use this system of categorisation. Nationally, springs are found to fall into 13 ‘supergroups’ which are often named after predominant towns in the region (e.g. the Barcaldine or Eulo supergroup)(Figure 1a.). Within each supergroup, springs are then further clustered into what are called ‘complexes’. A complex can consist of a small scattering to thousands of springs, depending on the particular region. Complexes may also consist of different types of springs depending on the local geology – for example, over 100 complexes are grouped within the Barcaldine supergroup, of which the ‘Edgbaston’ complex is a cluster of almost 200 discharge springs (Figure 3).
Different water, different story
Due to the different origins of the water emerging, each spring and region of springs has its own special characteristics. Generally, recharge springs have water that is relatively similar to rain water – the pH is close to neutral, the salinity is very low and the amounts of trace elements (e.g. carbonate) are minimal. In contrast, springs in the discharge portions of the G.A.B. start taking on particular characteristics linked to the flow path of the water they receive. Most are characterised by water that has very low dissolved oxygen and is rich in calcium carbonate (a residue from the aquifer). Some are extremely sulphurous, some are rich in trace metals, some are particularly salty but in each case the water that is exuded from the spring holds the key to its origin as it carries with it trace elements from its path. Because of this, we can trace which springs are connected below the ground and which are not. For example, recent research from the western margins of the G.A.B. in springs from northern South Australia show that particular regions have distinctive water chemistry overall (Figure 4 ).
Endemic or cosmopolitan
Springs are not the only waterbodies in the arid zone – waterholes, rockholes and these two types of springs are all available along with the great rivers of the channel country when they are flowing. However, springs are special amongst these waterbodies due to their subterranean water source. Unlike waterholes and rockholes, they do not rely on rainfall for their water but instead on a source constantly being recharged from the wetter coastal regions. Because of this, springs are generally permanent with consistent flows constantly refreshing the water they hold. The water that emerges in springs is also filtered by the G.A.B., particularly of fine particles like clays, which leaves the water crystal clear. Springs are also clustered into groups that, in some cases, are completely unconnected to surrounding waterways.
This affects the animals that live in them. Generally, aridzone water bodies are occupied by a relatively cosmopolitan suite of animals and plants – these organisms are found across the arid zone and, in some cases, even in coastal wetlands and watercourses. In contrast springs, particularly discharge springs, are home to a suite of organisms that are primarily endemic – they are not found in any other type of waterbody and, in many situations, are only found in one spring complex (Figure 5 ). This means that, if that particular complex of springs, which in some situations can be as small as a cattle property, were to be excessively disturbed or drained of water these particular suites of species will be lost forever.
1) Wikipedia’s article on the G.A.B. is relatively good (http://en.wikipedia.org/wiki/Great_Artesian_Basin) as is this short video produced by the Great Artesian Basin Co-ordinating committee (https://www.youtube.com/watch?v=VB4HFHDdzUc)
2) Habermehl, M. (1982). Springs in the Great Artesian Basin, Australia: their origin and nature. Canberra, Australia, Australian Government Publishing Service for the Bureau of Mineral Resources, Geology and Geophysics.
3) The Great Artesian Co-ordinating Committee releases numerous reports from conferences summarising cutting edge G.A.B. research. These can be accessed here: http://www.gabcc.org.au/12091/GABCC-Meeting-Outcomes/
4) This paper, by some of my lab members, is a CRACKER and highly recommended! Fensham, R. J., et al. (2011). “Four desert waters: Setting arid zone wetland conservation priorities through understanding patterns of endemism.” Biological Conservation 144(10): 2459-2467.