Seven Wonders of the Hydrogeological World (in Australia)

In an effort to raise the national profile of hydrogeology, the Australian branch of the International Association of Hydrogeologists (IAH Australia) launched a competition in 2010, challenging interested parties to nominate Seven Wonders of the Hydrogeological World (in Australia).

With a $1000 cash prize on offer for the best submission, entries were impressive, varied and reflective of the diverse and abundant hydrogeological wonders Australia has to offer. Submissions were assessed on the basis of scientifc merit, interest to scientific media, visual amenity, quality of submission and public interest. Of the wonders nominated, there were seven clear standouts.

Number 7: Gins Leap Gap, Upper Namoi Valley, New South Wales

The Gins Leap Gap is located approximately 9 Km north of Boggabri on the Kamilaroi Highway in New South Wales. The major tributaries of the Namoi River, including Cox’s Creek, the Mooki, Peel, Cockburn, Manilla and McDonald rivers all flow through The Gap as well as the groundwater associated with the unconsolidated sediments of the alluvial aquifer. The Gap also forms the major constriction in the Upper Namoi Valley and represents the northern extent of the Liverpool Plains.

Recent hydrogeological investigations at The Gap demonstrate how geological controls, such as faulting, fracturing and volcanic events, can constrict the alluvial aquifer and limit groundwater flow. A better understanding of major constrictions in hidden valleys is strategically important to measure and monitor groundwater. The Namoi CMA Gins Leap Gap Project revealed some surprises and geological problems that make us wonder at the features of the underground landscape.

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Number 6: Miocene Wine Glass Aquifers of the Ellendale Diamond Field, Kimberley, Western Australia

During the Miocene epoch over 150 volcanic pipes erupted in the West Kimberley Province; about fifty of these intruded Permian and older sediments, collectively forming the Ellendale Diamond Field.

The volcanic pipes are generally carrot-shaped at depth, but near the surface they frequently erupt outwards with the uppermost section forming a “wine glass” shaped structure. During the eruptive events which occurred 18 to 20 million years ago, the sediments were super-heated and then, as they cooled, the sandstone adjoining the pipes recrystallised to form a hard quartzite rim, which commonly form an aquiclude. The surrounding sandstone of the Grant Group is moderately to highly permeable, with hydraulic conductivities ranging from 12 to 20 m/d.

Many of the pipes consist of magmatic lamproite cores with tuffaceous lamproite margins. The diamonds are mainly found within the tuff, which can be vuggy, and where calcified, moderately permeable. Streams and high rainfall runoff during the wet season recharge the tuffaceous lamproite forming low salinity aquifers. In many pipes, the lamproite has been fractured and calcretised near the surface and calcified at depth. Calcium rich groundwater originating from the nearby Devonian limestone rocks can infiltrate the tuff forming calcrete near the surface. The weathering of magmatic minerals such as diopside (calcium-magnesium pyroxene), which forms in the fine-grained groundmass under rapid cooling conditions (i.e. along chilled margins), may have contributed to calcification.

Mining of the diamond-bearing pipes has required dewatering of the wine glass aquifers.

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Number 5: Canning Basin, Western Australia

The Canning Basin of Western Australia is a magnificent hydrogeological entity, representing Australia’s second largest sedimentary basin (after the Great Artesian Basin) and containing a fascinating ensemble of extensive aquifers and doubtlessly large volumes of groundwater beneath the Great Sandy Desert. The underexplored, pericratonic Early Ordovician to Early Cretaceous Canning Basin occupies about 506,000 km2, of which 430,000 km2 are onshore. It has a maximum sediment thickness of over 15,000 metres concentrated in two NW trending depocentres. The northernmost of these is the Fitzroy Trough-Gregory Sub-basin complex, while the southernmost is the Willara Sub-basin-Kidson Sub-basin complex.

The Canning Basin is also exceptional because of the unique cultural history and significance to remote desert dwellers and early settlers. The latter is well-represented along the Canning Stock Route where an unusual variety of aquifers supply natural wells and watering points.

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Number 4: Blue Lake, South Australia

The hydrogeological wonder that is the Blue Lake has many unique features. It is a window into the unconfined aquifer of the region, has storage of about 30,000 megalitres and has the most brilliant unique colour change twice a year.

Through the winter months the lake is a dull grey colour, but as soon as the weather starts to warm, it undergoes a spectacular change to vivid cobalt blue and holds this until the air temperature begins to cool again in April.

While many Blue Lakes exist in the world, this lake stands out because of the unique colour change which for many years kept many people theorising as to what may be the cause. Some of these theories are quite humorous. Science has now explained the change mechanism; however the local tourist bodies would rather explain how the colour change is still a mystery.

The Blue Lake along with the volcanic crater has had a human connection since its formation, as it has been part of the local Buandig (also known as Boandik) Aboriginal dream time legend. The Lake is the municipal water supply to the people of Mount Gambier.

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Number 3: Tufa Dams of Northern Australia, Northern Territory

In the north of Australia, karstic carbonate rocks, with extensive aquifers occur in Lower Palaeozoic and Proterozoic aged carbonates. Their greatest extents occur in the Daly, Wiso and Georgina Basins of the Northern Territory. In many areas these aquifers are overlain by Cretaceous aged sedimentary rocks which form a semi-confining layer. This is the case for a broad area extending from the Daly Basin to the northern Georgina Basin.

The carbonate formations are hard and fractured. Fractures are commonly enlarged by solution of the carbonate rock often to the extent that cave systems are locally present. A more pervasive secondary porosity can be formed by selective dissolution of dolomite crystals. Crystalline dolomite is partially silicified by weathering. Crystals that have not been replaced by silica are then dissolved by acidic recharge waters, leaving a porous rock that resembles “sugary” sandstone. This can form a sheet like aquifer which overlies and is in hydraulic connection with the carbonate rock below.

These karstic aquifers are capable of yielding quantities of water, sufficient for town supplies and major irrigation. The flow systems in these aquifers are of regional to intermediate scale. One extreme is the Georgina Basin with groundwater flow paths of the order of several hundred kilometres. These aquifers discharge via springs commonly with individual flow rates up to several hundred litres per second. Prominent examples include the springs of the Flora, Roper and Gregory Rivers.

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Number 2: Microbialites of South-west, Western Australia

The south west of Western Australia is one of the worlds Biodiversity hotspots. This is due to not only the large number of ecologically significant sites but the fact that they are under threat from anthropogenic forces. One of the most weird and wonderful groundwater dependant examples of this biodiversity is the Microbialites of Southwest WA.

The Microbialites encompass a number of spectacular, incredibly diverse algal communities, which typically biomediate, via respiration, CaCO3 precipitation from groundwater discharge. They occur in coastal freshwater to hypersaline lakes as well as in groundwater springs and some types are widespread in the early fossil record. Each community has a different distinctive morphology of limestone produced and are often visually spectacular. They have been classified into stromatolites, thrombolites and tufas based on texture and mode of occurrence.

Modern stromatolites occur in Shark Bay and Pink Lake in groundwater discharge areas and are distinguished by the presence of laminations in their structure. These laminations are believed to be representative of annual sedimentation or precipitation rates. See Figure – Stromatolites in Pink Lake.

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Number 1: The Great Artesian Basin

The Great Artesian Basin (GAB) of Australia is one of the largest groundwater basins in the world covering 22 % of the Australian continent. The GAB is an iconic aquifer system of both national and international significance. The hydrogeology of the Basin supports the world famous GAB Springs. There are more than 600 springs and spring groups mostly around the northern and western margins of the Basin where the water bearing aquifers and aquatards are nearer the surface. These springs range in size from small soaks to spring complexes with large pools and hundreds of individually flowing vents. Some springs provide base flows to rivers during the dry season. Natural discharge from the Basin through springs supports natural communities containing a wide variety of endemic species in isolated water dependent ecosystems surrounded by an otherwise largely waterless landscape.

The isolated nature of the GAB Springs has resulted in the preservation of many endemic, rare and relict species of great ecological, evolutionary and biogeographical significance both to the nation and the world. The lake Eyre Basin (LEB) is a surface water catchment that overlies the GAB. Many water holes, lakes and rivers within the LEB are supported by upward leakage form groundwater from the GAB aquifers. This interconnection between groundwater from the GAB and surface water in the LEB is of great environmental significance though largely unknown and unexplored.

As well as their ecological significance, these natural springs are culturally very important. Historically the GAB springs have provided the only reliable source of fresh water for human all activity in arid parts of Australia. GAB springs were the only reliable water source for Aboriginal people in central Australia for thousands of years. These vital water sources set the boundaries for dreaming lines and trade routes and remain important sights of cultural significance for local indigenous groups. The string of springs along the western boundary of the Basin in central Australia also guided European exploration and development through the central inland during the 19th and early 20th centuries; beginning with early explorers to afghan trading routes, the overland telegraph and the Ghan railway.

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Summary

1. Great Artesian Basin. Submitted by: Lynn Brake, Rachael Wroe, David Allen and the Australian Local Government Association

2. Microbialites of South-west, Western Australia. Submitted by: Ryan Vogwill

3. Tufa Dams of Northern Australia. Submitted by: Peter Jolly and Steve Tickell

4. Blue Lake, South Australia. Submitted by: Jeff Lawson

5. Canning Basin, Western Australia. Submitted by Pauline English

6. Miocene Wine Glass Aquifers of the Ellendale Diamond Fields, Kimberley, Western Australia. Submitted by: Grant Bolton

7. Gins Leap Gap, Upper Namoi Valley, New South Wales. Submitted by: Ken Crawford

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