South Siste St. Marys, Tasmania

South Sister

geology & hydrology

GPO Box 320
Hobart, TAS. 7001
January 23, 2005


As you have requested, I have reviewed such information as I could find about the geology and hydrology of South Sister – including the material you obtained under Freedom of Information – in order to assess the possible effects for forest operations.

A summary of my opinion about the nature of local water supplies, and the risks attached to disturbance of local systems, follows. Should this matter proceed further then I would be pleased to prepare, as explanation, a full Proof of Evidence (with complete CV) from my notes of record.


It is my professional opinion that the decision by the Forest Practices Board to approve any forest operations on South Sister – without proper consideration of groundwater hydrology and the broader environmental implications – is to act without due care in relation to the probable loss of water supplies local to South Sister, possible significant damage to supply at St Marys, and the impacts of hydrological changes on the general environment and land stability.

Although Forestry Tasmania and the Forest Practices Board admit that the nature and origin of water supply for residents around South Sister and the township of St Marys – and its maintenance – is not understood, there is a preparedness to act as though there is no risk to such supplies and to abrogate any responsibility to the community and environment implicit in such presumption. It is disturbing to find so many claims of 'too little information', 'no evidence', or assurances that 'no significant damage is likely', followed by approval of actions in the absence of any attempt to remedy this perceived situation, or even follow the limited expert advice sought and received, when the needs of a community and its local environment are at stake.

Domestic water supply to towns and individuals is rated as the highest priority usage in most jurisdictions – including Tasmania’s new general principles for Water Management Plans.


Contrary to the expressed views of those administering the Forest Practices Act, it is possible to assemble a likely explanation of the nature and origin of the local water supply systems from available data: much of which is unique and locally anomalous, but all of which denies the presumption of entire catchment involvement and its corollary that the coupe area, by being only a small portion, is consequently insignificant in hydrological terms. This is demonstrably false.

There are two types of water system involved:

The first is a part of the second.

High groundwater flows and yields are regionally restricted to the Gardiners Creek area with anomalous flows derived from Triassic basalts and porous Permian limestone (one instance only). Other lithologies, whether in or near the fracture zone related to local faulting, are not critical for town supply and could not provide the adequate supply. The basalt is critical and it alone provides the exceptional basis for flows.

Although the relationship between the Gardiners Creek Fault and the basalt is poorly defined (the basalt may be thickest along this axis) the fault has been active since eruption of the basalt and increased fracturing is certain along the eastern margin of the main outcrop.There may also be some local relationships between the deposition of the older Permian limestone and the history of the fault.

A tongue of basalt extends from the Esk Highway to mid level on South Sister 9about 400 m asl). This is both an original formational and a more modern erosional effect. Nothing like it occurs anywhere else in the area and, indeed, the basalt is itself restricted only to the German Town, South Sister and St Marys region. The geology is exceptional and unique.

The Jubilee Mine, which operated in coal measures above the basalt, and beneath the eastern half of South Sister, was noted for its broken working and water problems: the standard feature for all local mines being poor roof conditions.

Geological mapping east of Derricks Marsh on South Sister provides patchy information due to the extensive and irregular cover of dolerite talus but the unit relationships indicate the existence of several small faults between the Gardiners Creek Fault (and Gould Fault continuation in the east) and the Marsh, above the level of the basalt on the mountain. Some of these structures trend nearly east-west and it is probable that some instability in the upper coal measures and overlying talus is related to them. Water almost certainly entered the mine, from the talus, along this primary network of fractures and the multiple lesser features created by them.

This information allows an explanation of the town supply.
The supply is a fluky curiosity since it depends on recharge through the talus cover on South Sister, accumulation and storage in the fracture network in the coal measures about the Jubilee Mine area, and transfer via the basalt (and perhaps limestone) along the fault-related fracturing to the town bore. Only bores which fulfill these conditions have any chance of high yields (there are two).


Every element in this linkage is essential: high storage, capacity for high volume transmission, and effective widespread recharge. Two of these conditions cannot be met by the regional catchment, or the rocks in it, and the catchment for the town supply is, therefore, quite small with a very particular ground transmission system (fractured basalt). Its regional augmentation from the entire catchment can only be a very small part of the total local supply to the town bore.

More local spring supplies, above basalt level, depend only on the talus cover and its geometry – or the disposition of the faults, large fractures, and intersections with the land surface. Springs occur at critical points and their flow rate and permanence reflects involvement with the mine area fracture system and the large storage involved.

Some crude calculations can be made for this very effective water system. Annual demands are of the order of 200 ML, likely recharge potential to both local surface and subsurface systems from South Sister is of the order 700 ML or more which means the system is not being overdrawn at current rates, and the storage capacity of the coal measures and fractured basalt may be about 13000 ML., Total storage may exceed 20000 ML when the talus is seasonally engaged but the long term behaviour of both springs and town supply on the main rock storages.

Loss of supply can occur due to

A road cutting, which for example worked across slope and led to drainage or transfer of part of the system, would either reduce or terminate the town supply. The shallowness of the basalt aquifer makes this a very real risk and local planning schemes should take note of it – for all sections of the tongue from South Sister. Similar problems may affect the fracture network at the level of the Jubilee Mine.

In the context of current forest operations and proposals with the introduction of roading, working with large machines, and use of fire, there must be some effect on recharge capacity within the talus – either due to compaction and erosion, or changed run-off behaviour. Water quality, especially turbidity, is normally affected by disturbance and changed run-off. Changes in surface conditions and hydrology may transform the stability of the talus deposits – all of which provide ample evidence of past failure, including a 1950 event which disrupted operations at the Jubilee Mine.

Changed vegetation cover, loss of cover or effect of regrowth and planting, will also influence water use, availability and the recharge-storage equation and patterns of recovery. The effect of any alteration cannot be accurately predicted in such varied conditions but new, or forced growth after thinning, will make demands on the water system and its behaviour.


Since the proposed South Sister coupe is possibly more than 20% of the town supply system and perhaps as much as 50% of the critical recharge area (highest rainfall, more porous media) such proposals would seem to involve risks to supplies which are wholly avoidable: better to treat this area as a protected water catchment subject only to natural activity.

The proposal is a high risk activity when one considers the balance of resources and the value of what is being risked. The rarest and most valuable entity in this equation is the water supply, including that for a substantial town, and no actions should be allowed which might risk this, and certainly no actions in the absence of rigorous demonstrations of safety of supply, or alternative supply.

D.E. Leaman B.Sc. Ph.D.
23 January 2005

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