Roadshow 2.2 Groundwater sources for the Mataranka Springs

Groundwater sources for the Mataranka Springs

Click here to open transcript

My name is Sebastien Lamontagne. On behalf of my co-workers, it's my pleasure to present to you today our work, looking at the origin of groundwater feeding the Mataranka Springs.

First, I would like to acknowledge the support that we had from the local community for this work. The sampling was approved through Elsey National Park and the Northern Lands Council. We also had cultural monitors from the Yangman and Mangarrayi communities present at the time of sampling. Last but not least, both the Mangarrayi rangers and the Elsey National Park rangers provided support for our field sampling program that we had at Mataranka and I'll show you some of these results today.

The Mataranka Springs is one of the many outlets for the Cambrian Limestone Aquifer. Part of this aquifer is shown in green on the map and this is the real deal. This is one of the largest regional aquifers in Australia and it is one of the major water supplies for the Northern Territory.

Now, the conceptual model for the springs is that they're currently fed by two flow paths from the CLA, one from the south, or Georgina flow path and that's the one that flows over the Beetaloo Sub-Basin where we have potential unconventional gas reserves. There will also be another flow path feeding the springs, one from the north, the so-called Daly flow path.

There are numerous springs at Mataranka and the whole landscape is a groundwater discharge zone to trees and other features, but the ones we'll ... The springs we'll focus on today are the two major one, Bitter Spring and Rainbow Spring and two of the minor ones, Fig Tree and Warloch Pond.

There are three questions that we sought to answer through this research. The first I've already alluded to, is to distinguish the relative importance of the Georgina and Daly flow paths of the CLA to the spring flow, but also, we wanted to see if there could be some more local recharge to the springs. The area around Mataranka has a lot of karst. That is a lot of sinkholes and conduits underground and this would provide an avenue for local recharge of the groundwater and quick flow through the system. In addition, we're interested to see if, in addition to the CLA, there could be other geological formations, other, deeper aquifers contributing groundwater to the springs.

Now, the tools that we have used to look at these questions are environmental tracers. These are compounds dissolved in groundwater or part of the water molecule itself that gives us clues about its origin. Where does it come from and especially how long this groundwater's been travelling underground?

This figure shows some of those tracers that we can use to age groundwater. Notice that the axis is logarithmic. We're looking at age of groundwater from days to potentially hundreds of thousands of years. The short story there is, although there are numerous compounds that we can use to put an age on groundwater, none of them can age any groundwater. They all have a particular window in time where they are effective. So, it's quite typical in these studies to use several compounds, which we did.

In particular, for today's purposes, I'd like you to remember tritium, which is a tracer that we can use for young groundwaters, recharged since the 1950s or later, carbon-14, which is useful for intermediate groundwater source, thousands or tens of thousands of years and finally, helium-4. This one works a little bit differently. It tends to accumulate over time in groundwater, but it does so very slowly, so it's useful to look at very old groundwaters, tens, hundreds of thousands of years or greater.

We've collected a lot of information through this work, so I can only give you a flavour of what we did. I'll give you three examples, one of a fingerprinting exercise where we look at the source of groundwater, one looking at our age dating tracers and finally, a quick look at some of the monitoring work showing some evidence for quick flow in one of the springs.

For the fingerprinting aspect of the work, I'll use the example of the stable isotopes of water. Now, this measurement is quite simple. You may remember that almost all elements have different isotopes of different weights and its simply a measurement of the relative abundance of one of the heavy relative to the one elements, in this case, hydrogen and oxygen in the water molecule. Now, we measured this tracer and numerous others in the springs and several groundwater bores at Mataranka. That's summarised in figure A, but in this case, for the stable isotopes of water, we also look at results found by others for groundwater, regional groundwater in the Georgina and Daly Basin of the CLA. That's figure B.

So, I'll ask you to look at figure B. Now, what really stands out, it has this difference between the salmon colour and the green coloured sample. That's groundwater from the Daly in salmon coloured and Georgina flow path in green. Now, I don't have the time to explain why we have such a difference. It has to do with the climatic gradient from north to south in the Northern Territory, but suffice to say that there's a distinct signature for the stable isotopes of water, for groundwater, in the Daly and in the Georgina Basin of the CLA. More importantly, for the springs, we can see clearly, if you look at the up-right triangle, that Rainbow Spring has a signature very similar to the Daly Basin groundwater, whereas the other springs, the other symbols in blue, are more similar to the Georgina flow path groundwater.

Now, springs are typically seen as a meeting place for people, for animals, for all sorts of things, including groundwater. If you look at the various age dating tracers that we collected, they show that we have groundwater of various ages at the springs, not only one source of groundwater with one age. There are many, many sources and water has been travelling for different amount of times before reaching the springs.

In general, most of the groundwater at the springs is fairly old, thousands of years or more. This can be seen if you look at figure A, comparing one of our young tracers, tritium, with carbon-14, an intermediate tracer. We only found tritium, the young tracer, above background in one of the springs, Fig Tree. The other springs were mostly tritium-free.

In addition, if you look at figure B, comparing helium and tritium, there's additional evidence there that we have some potentially very old groundwater in the system. So, if you look at helium-4 on the X-axis and notice that it's also on a logarithmic scale, the amount of helium is well above what you would expect from a background. It's true for many groundwater samples at Mataranka, as well as in Rainbow and in Bitter Springs. Very unlikely that this is from the CLA because the travel time in the CLA is too short, tens of thousands of years, thousands of years, for much helium to accumulate. There's a very good evidence for a deeper groundwater source also sustaining the springs.

Very briefly, I promised I'd show you a bit of this, but we had a monitoring program over the last two years, despite the pandemic, thanks to our intrepid rangers. The short stories there is that, at least in one spring, we found evidence for quick flow, very quick response to rainfall because of flow through those large karst cavities around Elsey National Park. One spring, that is Fig Tree. No evidence for this quick flow in the major springs, Rainbow and Bitter.

So, in conclusion, what we found in some respect is very consistent with the existing conceptual model for the springs. They are fed largely through two regional flow paths from the CLA, the Daly and the Georgina. However, we also found evidence for local recharge, probably through karst, at Fig Tree Spring. Finally, we ... That was certainly new. We found some evidence of much older groundwater at the springs.

Now, unfortunately, we cannot say at present how significant this source is, how much of it sustain the spring flow, simply because we don't know how much helium and other tracers there is in any of the groundwater from formations below the CLA. They're simply not instrumented, or very lightly instrumented at present. So, this is something for the future. However, it does demonstrate that the system has some degree of natural hydraulic vertical connectivity, which is what you would expect in a regional system of that size. That's probably something that should be looked at in further detail in the future. Thank you very much.

Related factsheet

25. Groundwater sources to the Mataranka Springs Complex  

An investigation of the origins of the water at the Mataranka Springs complex. Spring and groundwater samples were collected in October 2019 (end of the dry season) and environmental tracers were analysed. Tracers provide information on groundwater flow paths, age and source. 


View all GBA factsheets

About the presenter 

Dr Sebastien Lamontagne 


Sebastien is a Principal Research Scientist in the CSIRO groundwater hydrology group. He has over 20 years of experience studying groundwater – surface water interactions using environmental tracers in a range of environments across Australia.

Last updated:
15 November 2021