Karst is terrain with distinctive landforms and drainage characteristics resulting from the relatively high solubility of certain rock types in natural waters. Limestones and dolomites are the dominant karst rocks in Tasmania, but karst is also known in magnesite, a magnesium-rich rock which occurs in north-west Tasmania. In some circumstances, karst-like features can develop other rock-types, such as sandstone, granite, dolerite and other rocks.
Karst often drains through natural subterranean conduits, some of which become caves, large enough for humans to enter.
Some 300 areas likely to have karst bedrock, underlying about 277,000 ha or 4.4% of Tasmania, have been documented. Subterranean drainage is known to be widespread, but karst aquifer characteristics have been studied in detail at only a few sites. The presence of subterranean drainage through karst systems can have important implications for water management. In particular, protection of groundwater quality can be a important challenge due the nature of subterranean drainage in karst.
Subterranean Drainage
Geological structures, rather than surface contours, are likely to control directions of flow in karst, and there are many examples in Tasmania of major topographic drainage divides being breached by subsurface karst streams. Karst drainage can pass underneath ridges and others landforms that would block drainage at the surface. As a consequence, specialised techniques, including water tracing, are often required to map flow directions and catchment boundaries in karst.
In comparison to other groundwater environments, flow through karst systems is often rapid due to the presence of conduits such as caves. These can provide efficient pathways for the movement of water from one part of the karst to another, with little purification if the water is contaminated. If water entering the karst system is polluted, this may spread rapidly to a much wider area.
Significant Karst Systems in Tasmania
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A major karst spring at Junee Cave in the Tyenna River catchment near Mt Field National Park is fed by as many as 80 tributary streamsinks (surface streams draining into underground systems via sinkholes), including some up to 13.2 km away in the topographic catchment of the Florentine River. As a result the total catchment of the Junee River encompasses an area of about 60 km2.
The Junee Cave aquifer is one of the most extensive and hydrologically complex karst systems in Australia.
In the Mole Creek area, the boundaries of many karst drainage subsystems have been delineated with the aid of water tracing. Some catchment boundaries have been found to vary dramatically due to conduits that operate only when flows are high. This important karst is renowned for its spectacular caves, two of which are currently open for tourism. At Marakoopa Cave, a karst stream flows through the cave and allows glow worms to survive, which is part of the cave's attraction for tourists.
Other important karst water systems in north-west Tasmania are at Gunns Plains, which also has a tourist cave, and at Loongana, Mount Cripps and Smithton. At Smithton, karst water emerges at mound springs which have built up over long periods of time due to minerals dissolved in the water. These unusual landforms can be damaged by inappropriate developments.
In the Ida Bay area, water tracing has revealed the presence of an extensive karst aquifer associated with the large karst spring at Exit Cave, in the Tasmanian Wilderness World Heritage Area. Rehabilitation of a limestone quarry within the catchment of this karst spring has helped restore water quality within this very important cave system. At nearby Hastings, a karst water system has formed Newdegate Cave, which is open for tourism. The Hastings thermal springs is a karst water system that may be linked to that at the cave.
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Photograph of Marakoopa Cave, Mole Creek by George Apostolidis (Courtesy Tourism Tasmania)