Friday, 4 November 2011

Ground water in the Alstonville Plateau

A palaeosol in the Alstonville Basalt
Ground water is a valuable source of water for stock watering, domestic uses, irrigation and town water supply in the area of the Alstonville Plateau. For example both Ballina Shire Council and Rous Water operate ground water bores as sources of water for municipal use. The reason for the popular use of the ground water from this source is its yield and also freshness. The quality of the water in the aquifers is excellent and the quantity good. In fact the popularity of ground water from the Alstonville Plateau is such that it threatens to be over used with many aquifers being badly drawn down and for this reason the NSW state government has put in place a water sharing plan that prohibits new water extraction licenses from some areas of the plateau and all ground water bores in the area require a license.

So what is the Alstonville Plateau ground water source anyway. Where does the water come from? Well, in short, the Alstonville Plateau ground water source is a series of aquifers that occur in the Cenozoic basalt that defines the area of the Alstonville Plateau. The plateau extends from beyond? Lismore in the west almost to the coast at Lennox Head, past the little village of Newrybar in the north (almost to Bangalow) and south almost to the Richmond River at Broadwater. According to Brodie and Green (2003) there are several aquifers with the upper most being an unconfined source of water within the upper weathered and/or fractured zones of the basalt. Below this is at least one confined aquifer which flows through permeable layers such as paleosols (old soil horizons) or through fractures in the basalt. An example of a paleosol from the Alstonville Basalt is shown above (not acting as an aquifer in this case).

The unconfined aquifer is usually able to be intercepted within several metres of the surface but this depth can vary wildly depending on the depth of soil weathering zones and local topography. This shallow source is usually easy to find but yields are usually low and are often subject to drying out during periods of drought due to the local surface water influence on these aquifers. In general when it rains the streams tend to recharge the aquifers and when the weather dries out the aquifers tend to return base flow to the streams (until the aquifers run out of water).

The deeper aquifers are confined between layers of basalt. The layers that the water is found in is either made from substantially fractured rock or paleosols that were developed on lava flows and were subsequently covered up by new lava flows (i.e. are directly related to the eruptive conditions during the formation of the basalt). Interestingly, the dip direction of the aquifers is generally from east to west which is somewhat inconsistent with the idea that these rocks were sourced from the Tweed Volcano which is the established theory since Duggan and Mason published their paper on the volcanic rocks of the area in 1978.

The interesting thing about the importance of this ground water source is that despite the area being mapped as Lismore Basalt  most other areas of the Lismore basalt away from the Alstonville Plateau are not in as high demand for ground water as the Alstonville Plateau. Why is this? It is possible that there are peculiar features of the plateau such as extensive paleosols but it is possible that it is related to the plateau being derived from an older basalt unit that was identified by Cotter (1998) but has not been followed up in detail by any other authors since. See my older posts on this subject here and here.


*Brodie, R.S. & Green, R. 2002. A Hydrogeological Assessment of the Fractured Basalt Aquifers on the Alstonville Plateau, NSW. Australian Bureau of Rural Sciences, Australia
*Duggan, P.B., Mason, D.R. 1978. Stratigraphy of the Lamington Volcanics in Far Northeastern New South Wales. Australian Journal of Earth Sciences V25.
*Cotter, S. 1998. A Geochemical, Palaeomagnetic and Geomorphological Investigation of the Tertiary Volcanic Sequence of North Eastern New South Wales. Masters Thesis, Southern Cross University.


  1. Hi Rodney, the east-west dip of the aquifers is very interesting because Cotter seemed to think there was a northerly dip associated with the Alstonville plateau, however this based on only geomorphic features such as drainage patterns. Your observations are also based on drainage patterns, but these are confined by local rock types and associated sediments. These observations are exciting as they may yield some clues as to what underlies the Lismore Basalt and/or what the landscape looked like before the Lamington eruptions.

  2. Hi Dylan,

    Brodie and Greens hydro-geological assessment is available online if you don't have a copy. It is a very good assessment. I'm glad you mentioned the paleodrainage. If the dips in the paleosols are unchanged since their development (and there is little reason to think that they would have changed anyway) then that has interesting implications.


  3. Thankyou for sharing so much of the information you have studied and collated.

    I have a question: on the South-Western slopes of the Alstonville plateau not far from Whyralla there is a sandstone ridge. Under the loose sandstone along its bedrock are veins containing what appears to be some sort of metal. I've not seen any like it. Have you identified any metals in the sandstone during your study of the plateau. I would be happy to email a picture for you

    1. Hi Galilean,

      Feel free to email me a picture. My email address can be found on the page "about this blog" - the tab is along the top of this page.

      I must say that I would be surprised if there were any metals or even metal ores in that area. But I'd love to have a look and figure out what it is you are looking at!



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