Blog Update #10 - Pain and a paper

Eleanor Faith Holland

The blog has been on hold for a while. But I have always intended to keep on writing. There has been some major changes in where I live and work and most recently our lovely 6 year old daughter Eleanor passed away. There has been much pain but there is much celebration too for our special girl. My wife Becky and I are in feelings of loss. That said, we want to celebrate the life of our daughter who demonstrated so much strength in so many ways. A service will be held at St Peter’s Cathedral in Armidale at 2pm on Monday (19th December).

 

Coming to Christmas it is a subdued feeling at our home and my birthday was yesterday as well so I didn’t feel like celebrating. However, there was something I was excited to see. A confirmation that a paper that I was a co-author of has been accepted to the journal Science of the Total Environment (Santos et al 2016). I was only a minor co-author but there is something about having your name up there that caters for ones ego.

The highlights of the article are:

• We assess groundwater recharge through a pervasive layer of floodplain muds.
• Modelled groundwater flow paths were consistent with tritium dating.
• The clay layer did not prevent recharge because of macropores and cracks.
• Fine-grained floodplain soils do not necessarily protect underlying aquifers from pollution.
• Combining multiple techniques gives more confidence in recharge estimates.

The article can be found here:

 

http://dx.doi.org/10.1016/j.scitotenv.2016.11.181

References

*Santos, I.R., Zhang, C., Maher, D.T., Atkins, M. L. Holland, R. Morgenstern, U & Li, Ling. 2016. Assessing the Recharge of a Coastal Aquifer using Physical Observations, Tritium, Groundwater Chemistry and Modelling, Science of The Total Environment, Available online 15 December 2016, ISSN 0048-9697, http://dx.doi.org/10.1016/j.scitotenv.2016.11.181.

Baseline CSG methane in groundwater

A friend recently let me know that a paper that one of his students wrote for the Journal of Hydrology had been published. I had a very minor involvement in the formative stages of the paper which came about indirectly as a result of the protests of many local people about potential coal seam gas (CSG) and other natural gas types in the region. The paper (Atkins et al 2015) is essentially the results of a data collection exercise but has some interesting techniques and findings about the baseline concentrations of gas in groundwater bores in the Richmond Valley area.

Methane concentration for different geological environments
(after Atkins et al 2015)

91 water samples were collected from government and private bores in geological units overlying the target CSG geological formations in the Clarence-Moreton Basin (e.g. the Walloon Coal Measures). These units were quite diverse and ranged from sedimentary rocks of the Piora Member of Grafton Formation and the Kangaroo Creek Sandstone (recently reclassified as the Orara Formation), basalt lava flows of the Lismore, Astonville and Kyogle Basalts and Quaternary aged alluvium including coastal sands and riverine sedimentary environments.

Special glass water sample containers were used to collect the samples. These were then injected with a carbon dioxide and methane free gas to create a clean “air bubble”. The methane and carbon dioxide naturally dissolved in the water will then come into equilibrium with the “air bubble”. The resulting gas from the bubble can then be extracted and the concentration and isotopic composition of the carbon in the two compounds determined by an electronic analyser. The isotopic signature can then be assigned to recent biological formation (biogenic) or geologically derived (thermogenic) origin.

The end result was annoyingly quite not straight forward. The concentration of methane showed no obvious relationship to the chemistry of the groundwater. However there was a relationship between geological units. Methane concentration was very low in the basalt aquifers and relatively higher than the Clarence-Moreton basin sedimentary rocks and much higher in the Quaternary alluvium of the Richmond River valley floodplain and coastal sands systems. So there was more methane in some of the aquifers that were the less likely to be connected to any CSG formations! Quite counter-intuitive.

The isotopic signatures did not really help clear up this confusion very much. There appeared to be a large thermogenic component to the coastal sands and flood plain aquifer systems sometimes at concentrations greater than the formations that should be the thermogenic CSG source. Why? It was noted by some CSIRO scientists working in the Great Artesian Basin that sometimes biogenic gas can be oxidised and then be chemically reduced back to methane and this process favours the thermogenic isotopes (Day et al. 2015). So, It gives the impression of thermogenic gas.

This means that the methane gas concentration is related to the biological activity in and around the aquifer. The shallowest groundwater systems are the most connected with surface water and biological processes and therefore these have the highest concentrations of methane. The Clarence-Moreton Basin sediments are not connected with the CSG and natural gas rich formations.

This means that if companies like Metgasco do commence gas operations in the area there is a statistical background that can be used to compare if anyone becomes concerned about methane in their water bores. Interestingly, it also shows that methane in groundwater is probably not a good method to search for natural gas in the region. It might apply to other areas like the Great Artesian basin but apparently there are good barriers between CSG and non-aquifers in the Northern Rivers. This is good news since if something does go wrong it is now more easy to identify if it has impacted upon any groundwater.

References/bibliography:

Atkins, M.L., Santos, I.R. & Maher, D.T. 2015. Groundwater methane in a potential goal seam gas extraction region. Journal of Hydrology: Regional Studies. V4.
Day, S., Ong, C., et al. 2015. Characterisation of regional fluxes of methane in the Surat Basin, Queensland. CSIRO report EP15369

This is what one aquifer looks like

In some amazing places you can immerse yourself in an aquifer. These places are rare and dominated by a rock type that does not occur in any substantial amounts in our region. However, people dive in the sub-terrainian waters of the limestone caves of the Nullabour in South Australia. The best aquifers in our region do not contain large caves compared with limestone areas. They are hosted in riverine alluvial sediments, fossil soil horizons in volcanic rocks, or fractures in hard metamorphic and volcanic environments. The main aquifers being on the coastal river flood plains, Alstonville Basalt and the New England areas respectively. However, volumetrically the sources that are very large are those in coastal sands.

Auger containing saturated sand from the Woodburn Sands

This post is an illustration of how one of those coastal sands aquifers looks. I've covered the Woodburn Sands in several previous posts but a quick summary is still needed. The Woodburn Sands are beach and dune sand that was laid down during the last significant interglacial. This was around 130 000 years ago during the Pleistocene period. The sea level was much higher than now and this meant that beach systems were often formed a significant way inland.

From the picture you can actually see what the medium that hosts an aquifer looks like. The Woodburn Sands are just that, sands. The sand grains are mostly quartz but there are also some grains made from volcanic and metamorphic rock fragments. Occasionally you can see grains of heavier minerals that were mined until the 1980's. The sand grains are very similar in size which is typical of wave and wind sorting. There is a very small fine fraction of clayey material.

Where the clay content is higher the ability of the water to flow through the aquifer is reduced. This is why some bores can only produce a small amount of water compared to the huge volume that is in the whole aquifer. This is an example of why aquifers tend not to behave as underground lakes. You can pump water out of one end and run out because the hydraulic conductivity (flow velocity) is not high enough to allow the water at the other end of the aquifer to flow in.

The Woodburn Sands is not the only important coastal sands aquifer in the region. Another very important water source include the Macleay sand coastal aquifers. These aquifers were formed in a similar way to the Woodburn Sands and are used for similar purposes. Usage includes irrigation, stock, domestic use and town water supply for places such as Kempsey and Evans Head. There are also some interesting arsenic contamination issues in one aquifer system (Stuarts Point) in the Macleay area which I will post on in the near future.

The similar characteristics of the coastal sands aquifer systems in the North Coast area has motivated the NSW state government to develop a Water Sharing Plan for these systems as a whole. The Water Sharing Plan is expected to be formally adopted this year (2014). Local governments regard groundwater from the coastal sands aquifers as very important. Rous Water has recently adopted its future water strategy which identifies coastal sands as the main source of additional information in the medium to long term and Mid-coast water have recently increased their production of groundwater for drinking too.

Available here is a presentation by the NSW Office of Water on the overall coastal sands systems in North East New South Wales

Where Does the Groundwater Flow?

There has been renewed interest in groundwater resources in the Northern Rivers of late. In part this is due to peoples concern about "unconventional" gas exploration and production in the area. Surprisingly, less known is the release of Rous Water's Future Water Strategy which includes groundwater as first on the list for new water sources. Rous Water is a major bulk drinking water supplier in the region. I've previously covered an area within the coastal sands groundwater source called the Woodburn Sands but this was a cursory look and I'd not covered where the groundwater actually goes.

Groundwaters do not exist as an underground lake in our region
Image courtesy of International Association of Hydrologists

Groundwater is often seen as a bit of an unknown, a black box, or some kind of underground lake (see the cartoon). It is quite difficult to observe and therefore people can get the wrong idea of what goes on underground.

One area that is not understood is that groundwater usually discharges somewhere. Sometimes groundwater discharge is obvious through springs. But where it intersects with permanent surface water it is much less obvious. The Evans Head area is a good example of where discharge from the Woodburn Sands aquifer and broader Coastal Sands aquifers is concealed.

Spring-fed creek on Chinaman's Beach.

While walking along Chinaman's Beach south of Evans Head during a recent long dry spell, I couldn't help notice the dark coloured water flowing over parts of the beach. This is one of those discharge areas I'm talking about (most people might be more used to seeing freshwater flowing over a beach from contaminated urban stormwater drains). The coastal sands above Chinaman's Beach holds groundwater and slowly discharges it at the beach. The dark colour of the water is from dissolved humic matter from coastal vegetation soaking into the sand. Tasting the water it was apparent there was no salt in it and understanding the groundwater area I knew it was clean. The springs I observed on Chinaman's Beach were obvious areas of groundwater discharge. The vegetation in the springs was lush and clearly reliant on the groundwater. This is formally known as as groundwater dependent ecosystems.

The lesser known discharge is not all through visible springs like those on Chinaman's Beach. Much of the discharge from the coastal sands aquifers is actually concealed by the sea. It might be a surprise to many in some areas just off the coast there are zones with freshwater. The amount of water that can be discharged underground into the sea can exceed the discharge from terrestrial springs (e.g. Santos et al. 2009). These are the undersea equivalent of the Chinaman's beach springs. This is interesting from a aquatic ecology point of view because it may mean that there are ecosystems in the ocean that are dependent on freshwater! That is, groundwater dependent ecosystems in the sea.

Groundwater is an interesting feature of our region. It is a source of drinking water, irrigation water and even industrial water. It is often important as some ecosystems are dependent on it. It is also surprising since ecosystems can be dependent on fresh groundwater even when out to sea.

Postscript: about a month after this blog post a story emerged in the local newspaper about sinkholes or zones of quicksand on Chinamans Beach. These quicksand 'pits' look just like typical groundwater discharge areas. The Northern Star article can be found here.


References/Bibliography:
*Santos, I.R, Burnett, W.C., Chanton, J., Dimova, N. & Patterson, R. (2009). Land or Ocean?: Assessing the driving forces of submarine  groundwater discharge at a coastal site in the Gulf of Mexico. Journal of Geophysical Research. vol114.

An excellent outcome from atmospheric atomic bomb testing

Human ingenuity surprises me again and again, especially the efficiency in which we can annihilate each other. During the 1940’s and 1950’s the superpowers were focused on increasing the efficiency in the way they could destroy everyone on the planet. It was a very worthy goal (yes that was a joke) and to achieve maximum efficiency they needed to conduct atmospheric tests of their bombs. Sometimes, unforeseen obvious benefits other than the benefits of death and destruction of humanity can arise.

I have recently been thinking about groundwater in the Richmond River area for which I have been consulting sections of a PhD thesis written by Leonard Drury in 1982 (Drury 1982). Drury's comprehensive thesis included qualitative identification on the age of groundwater in aquifers in the Richmond River by using an unstable isotope of hydrogen called tritium. Hydrogen is an atomic component of water (the H in H2O) but hydrogen actually comes in three natural forms based on the number of neutrons are in the nucleus of the hydrogen atom. These different forms are called isotopes. Hydrogen naturally has one neutron or less commonly two neutrons (called deuterium) and very rarely three neutrons (called tritium). In nuclear explosions the third isotope tritium, is created at concentrations much higher than the background. The reason why tritium is rare naturally is that it is only formed in the upper atmosphere but is unstable and loses the extra neutrons to become a smaller isotope over a period of time.

Half of the tritium in a given amount of water (or whatever) decays over a period of 12.5 years (this is called a half-life). Which means that over 25 years there is only a quarter of the original tritium left, 37.5 years one eighth, 50 years one sixteenth etc. Since tritium is not naturally occurring there is no practical use to measure for tritium unless you can introduce it into a system as a tracer and then measure its behaviour. This means that a large ‘slug’ of tritium was created during the 1940’s and 1950’s during atmospheric nuclear testing. Therefore if you can look for tritium in groundwater and if it is not present you can assume that that groundwater has been in existence for more than 50 years, i.e. it was present in the ground before any nuclear tests. If you detect tritium in several locations in an aquifer the relative abundance of the tritium will give an indication of the age of the water and whether mixing is occurring between old groundwater and new groundwater. It won’t give you an exact date but it will let you know a lot about behaviour of an aquifer.

The trouble is time is running out. The half-life of tritium means that as time goes on the ability for us to accurately measure the smaller amount of the isotope means that one day we won’t be able to use this as a technique. I was aware that time was running out on using tritium as an effective groundwater tracer but I was not aware how soon. I have had a few chats with an academic at Southern Cross University one of which was about using tritium, he said we actually only have about 5 or 10 years left to which I jokingly suggested to him that we should reset the tritium clock with some more atmospheric nuclear explosions! To which he informed me that actually there appears to be some more tracers that can be used following the Fukushima Nuclear Accident.

Bibliography/References:

*Drury, L.W. 1982. Hydrogeology and Quaternary Stratigraphy of the Richmond River Valley, New South Wales. University of New South Wales, PhD thesis.
*Moran, J.E. & Hudson, G.B. 2005. Using Groundwater Age and Other Isotopic Signatures to Delineate Groundwater Flow and Stratification. University of Illinois.
*U.S. Geological Survey (USGS), 2004, Stable Isotopes and Radiochemicals, in National Field Manual for the Collection of Water-Quality Data, Chapter A5 Processing of Water Samples. USGS Techniques of Water-Resources Investigation

Rocks named after a creek named after an Australian marsupial

Note that the stratigraphy of this formation has been revised since this blog post. See the this recent post for details.

One of the most widely outcropping rock units of the mesozoic aged Clarence Moreton Basin is the Kangaroo Creek Sandstone named after its type locality at Kangaroo Creek in the Nymboida area. It is also one of the most recognisable stratigraphic units in the basin.

McElroy (1963) showed that the Kangaroo Creek Sandstone consisted mainly of white to cream coloured quartz sand. The texture of the sandstone is saccharoidal, that is, it has a glistening sugar like appearance of the quartz sand grains. This sand glistens more than usual because while buried, fluids in the rock caused extra silica (quartz) to crystallise on the existing sand grains creating new tiny crystal faces that reflect light in a vivid way. The nature of the rock in this formation tends to weather less readily than other units and as a result tends to form prominent topographic features such as hills, cliffs, ridges and the like.

Crossbedding and typical saccharoidal texture in Kangaroo Creek Sandstone

The Kangaroo Creek Sandstone was deposited in a fluvial (river) setting and as a result cross bedding structures are very common in outcrops. Sorting of grains in the unit is very well developed, that is, the grain size is very similar at any particular outcrop. Additionally, the thickness of the beds is very consistent which together indicates that the tectonic setting was relatively unchanged through the period of deposition. Following burial of the sandstone fluids present in the rock caused extra dissolved silica to precipitate out onto the existing sand grains filling in voids and creating the characteristic texture.

The Kangaroo Creek Sandstone is considered by some authors (Wells and O'Brien 1998) to grade into the Woodenbong Beds in the north west of the NSW portion of the basin. However, it is noted that others (Willis 1998) consider the Woodenbong Beds the equivalent to the McLean Sandstone Member of the Walloon Coal Measures (but more about this in future post). The Kangaroo Creek Sandstone underlies the Grafton Formation but the contact with this formation is gradational. According to (Wells and O'Brien 1998) it also sometimes shows a conformable boundary with the underlying Walloon Coal Measures, however, in most areas the boundary is shown by an unconformity. It is easy to tell the difference however, because compositionally any sandstones in the Walloon Coal Measures are composed of feldspar and lithic grains rather than the quartz of the Kangaroo Creek Sandstone.

Outcrop of Kangaroo Creek Sandstone on the Clarence River near Grafton

It is interesting to note that the recrystalisation of quartz in the Kangaroo Creek Sandstone means that this unit is now essentially dry with respect to Ground Water. There is very few spaces left for the water to travel through. for example O'Brien et al (1998) shows that most other sandstones in other basins such as the Great Artesian Basin, is where most ground water is obtained. In fact, in the whole of the Clarence Moreton Basin the only unit to have useful ground water bores is the Grafton Formation which is recharged from rainfall. The Kangaroo Creek Sandstone does have some bores that produce a very little water in the upper most portion of the unit (probably rainwater recharging fractures in these locations (Kwantes 2011), like the overlying Grafton Formation) but it appears that no other bores obtain water from the Kangaroo Creek Sandstone because the formation actually behaves like an aquiclude or aquitard. Water is not obtained from aquifers below the Kangaroo Creek Sandstone because the water quality is generally poor.

It is interesting to note that according to some gas exploration results it is apparent that areas of the Kangaroo Creek Sandstone (assuming this is not mistakenly identified McLean Sandstone) that are directly overlying the Walloon Coal Measures contain substantial areas of conventional natural gas. This is gas that has migrated from the underlying Walloon Coal Measures and been trapped in either pore spaces or fracture zones. I understand that several companies in the area such as Metgasco and Red Sky Energy intend to exploit these reserves.

Pollen spores in drill holes give an age of middle to late Jurassic for the Kangaroo Creek Sandstone (Wells and O'Brien 1998).

References/Bibliography:

*Kwantes, E. 2011. Future Water Strategy: Groundwater Options - Position Paper. Report for Rous Water by Parsons Brinkerhoff.
*McElroy, C.T. 1963 The geology of the Clarence-Moreton Basin. New South Wales Geological Survey, Memoir 9, 172 pp.
*Moran, C., Vink, S. 2010 Assessment of impacts of the proposed coal seam gas operations on surface and groundwater systems in the Murray-Darling Basin. The University of Queensland.
*New South Wales Government. 2010. State of the Catchment Report: Groundwater. Northern Rivers Region. Department of Environment, Climate Change and Water.
*Wells, A.T. , O'Brien, P.E. 1994 Lithostratigraphic framework of the Clarence-Moreton Basin In Wells, A.T. and O'Brien, P.E. (eds.) Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Australian Geological Survey Organisation. Bulletin 241.
*Willis, I.L. 1994 Stratigraphic Implications of Regional Reconnaissance Observations in the Southern Clarence-Morton Basin, New South Wales In Wells, A.T. and O'Brien, P.E. (eds.) Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Australian Geological Survey Organisation. Bulletin 241.

Top of the Basin: The Grafton Formation

The Clarence Moreton Basin covers a large proportion of the catchment areas of the present day Clarence and Richmond Rivers in northern New South Wales and extends a significant distance more into south east Queensland. The portion of the basin which is most well known is the Queensland section but slowly we are learning more about the southern areas. The basin consists of many individual stratigraphic units which were deposited in slightly different environments at different times. The youngest unit is called the Grafton Formation and is thought to have been deposited during the Mesozoic era called the Cretaceous period which could be as young as 65Ma but it may be as old as late Jurassic.

The extent of the Grafton formation is small by Clarence Morton Basin standards because the majority of the unit appears to have been removed by erosion. Exposures can be found as far as 30km south of Grafton to about 10km north of Casino. The full remaining thickness of the formation has been estimated at up to 442m but is probably less with the best estimate of 267m obtained from a drill hole at Grafton.

Grafton Formation lithic sandstone near Casino

The formation is comprised of interbedded lithic to quartz arenites (sandstones), clayey siltstone, claystone and minor coal, sometimes 2metre thick conglomerate layers are present too. The lithic fragments frequently include the volcanic rock andesite implying active volcanism upstream at the same time as the sediments were being deposited. The bedding can be thin to thick and commonly a ferruginous (iron rich) lateritic weathering profile is present creating a very red coloured soil. This is particularly evident in the hills just to the north of Grafton such as Junction Hill. The sandstones are fairly characteristic in that they are usually tough and green-grey in colour.

One author (Wells and O'Brien 1994) suggests that the Grafton formation (and the Kangaroo Creek Sandstone) may also be equivalent to the Woodenbing beds (located between Urbenville/Woodenbong and Kyogle) and even though they are lithologically (rock composition) different this is still possible. An alternative by Willis 1994 is that it is the equivalent of the McLean Sandstone Member of the Walloon Coal Measures. But this will be discussed in detail in a future post.

The formation overlies the Kangaroo Creek Sandstone and is gradational meaning that the Kangaroo Creek Sandstone grades into the Grafton formation. Thankfully, recognising the difference is not hard on the basis of lithology (rock type) because the Kangaroo Creek Sandstone is very consistent in appearance (saccharoidal texture and abundant cross bedding) and consistent rock composition (quartz sandstone). The top Grafton formation has been eroded and is overlain by the more recent Cenozoic volcanics.

The Grafton formation was deposited in a mainly fluvial (riverine) environment with the more common siltstones and mudstones in the south probably being deposited in a lacustrine (lake) environment. This led to an idea that the source of the rivers and lakes that laid down the sediments in Grafton Formation was from the north but recent revisions of the probable mountain chains that existed at the time means that this many not necessarily be the case. Wells and O'Brien (1994) give the maximum age of the Grafton Formation as late Jurassic.

Interestingly, Grafton Formation is the only rock unit in the Clarence-Moreton Basin that has any significant or active ground water sources. The basin has proven to be a very poor source for water because of the lack of volume. In fact the only volume of water obtained from the Grafton Formation is really only unconfined aquifers recharged from surface water and overlying alluvium.


Note: Since writing this post it has been suggested in a new paper that the Grafton Formation appears to be made up of two members. The new paper by Doig & Stanmore (2012) significantly increases our knowledge of the Grafton Formation. I will endeavour to do a new blog post with the updated details.




References/bibliography:

*McElroy, C.T. 1969 The Clarence-Moreton Basin in New South Wales. In Packham G.H.(ed) The geology of New South Wales. Geological Society of Australia. Journal 16.
*New South Wales Government. 2010. State of the Catchment Report: Groundwater. Northern Rivers Region. Department of Environment, Climate Change and Water.
*Wells, A.T. , O'Brien, P.E. 1994 Lithostratigraphic framework of the Clarence-Moreton Basin In Wells, A.T. and O'Brien, P.E. (eds.) Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Australian Geological Survey Organisation. Bulletin 241.
*Willis, I.L. 1994 Stratigraphic Implications of Regional Reconnaissance Observations in the Southern Clarence-Morton Basin, New South Wales In Wells, A.T. and O'Brien, P.E. (eds.) Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Australian Geological Survey Organisation. Bulletin 241.

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.

References/Bibliography:


*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.