|A palaeosol in the Alstonville Basalt|
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.