Roadshow 3.2 Characterising the connectivity between permanent waterholes and groundwater

Andrew R Taylor: Good afternoon. Today, I'm going to give you a brief summary of the work that we did on characterising the connectivity between permanent waterholes and groundwater. Just some brief acknowledgments before I start, so I just want to acknowledge CSIRO colleagues and collaborators at Geoscience Australia who contributed to this work, acknowledge the traditional owners past and present of the Cooper GBA region, acknowledge the pastoral station owners who assisted us with access to field sites, and obviously acknowledge the communities that hosted us during field sampling campaigns. So look, some very brief background on the Cooper GBA region, it's around 130,000 square kilometres, has a very subtle topography, so mostly flat as shown on the image on the right there. But it has a very unique and well-known fluvial geomorphology.

So a great picture here of what's known as Channel Country. So ephemeral drainage basically shapes the landscape as floodwaters move from the north through the region. Geologically, it's diverse. So we've got three nested geological basins, the oldest being the Cooper Basin. The Permian sediments of the Cooper Basin shown in the purple or grey here. Overlying that, we've got a very thick sequence of Jurassic Cretaceous sediments of the Eromanga Basin and the Great Artesian Basin. So that's shown in blue to mustard. And at the surface, we have a much younger Cenozoic Lake Eyre Basin. Groundwater across the region is sourced from the shallow sub-artesian supplies in the Cenozoic sediments as well as the top of the Winton Formation where it's economically viable to intersect.

So why the focus on waterholes? Well, potentially just because any future water resource development poses, may pose a threat to what is key refugia in the region, an example just shown on the image of the right here of Cullyamurra Waterhole with a beautiful riparian vegetation and the pelican on the water there. So ecologically, they are significant supporting a range of terrestrial and aquatic species. Some are even Ramsar-listed such as Coongie Lakes. Culturally, they have a rich history too, and economically, they support the pastoral industry as well as the towns and communities in the region.

So if we want to evaluate any potential future hydrological impacts to these, it's really important to understand the water sources, particularly, for the permanent waterholes because they're the ones that provide the long-term refuge during prolonged dry periods. So I'll just quickly take you through the approach to the assessment used, which included conducting some desktop analysis, doing some targeted field investigations, and then integrating that data to support our findings.

So in terms of understanding which waterholes are permanent, one of the first things we did is look at some Earth observation data. So there's water observations from space data. So that comes from 30 years of LandSat data that's been collected by the LandSat satellites. Essentially, there's an algorithm that allows for those observations to map the percentage of the landscape that's wet, when there's cloud-free or shadow-free imagery available. I've just got an example shown here on the alluvium. So yellow to orange or red is very low. And then the high areas that are wet most of the time are blue to green, so not surprising. It's the in channel areas where the waterholes are that are the most wet. In terms of identifying the permanent waterholes across the GBA region what we did was look at where it's most prospective for development of unconventional gas. So it's the grey hatched area, probably difficult to see. And we use two thresholds. So we identified 334, semi-permanent waterholes so they're waterholes inundated greater than 70% of the time, they're shown in black. And then there's around 112 permanent waterholes. So these are the waterholes that are inundated more than 90% of the time in that 32-year period and coincide with the area perspective for unconventional gas. And these are the red waterholes so mostly in the mid to lower Cooper.

So how do they compare to depth to groundwater in terms of understanding groundwater connectivity. So on the map on the right here, we've plotted over 1,300 bores that have groundwater level data representing the unconfined aquifers. We combine the Queensland and SA and a small database from Santos to get this data. And interestingly enough, we plotted them greater than the GBA region because it's not the hydrogeologic groundwater flow boundary.

So we expanded the analysis to the entire Cooper catchment, compared them to water levels in waterholes, and just overlaid it on the hydrogeology just to see what was happening. So 71 permanent waterholes had a bore within 5 kilometres that had a groundwater level. Now 5 kilometres is a reasonable distance, it's important to keep in mind there's not that many bores within a kilometre or two of the Cooper just because of the flood inundation that happens. But 63 of the permanent waterholes had a groundwater level below the water level of the waterholes. And then the black sites that are shown there, nine had a groundwater level above, suggesting potential groundwater discharge to waterholes, and there the red sites so none of them were on the Cooper. They are actually off the Cooper. So we had one on the Wilson River about 20 kms off the Cooper. A few on Kyabra Creek, which is around the outcrop of the Winton Formation about 100 kms off the Cooper and a few up in the outcropping areas of the GAB.

And this is pretty consistent, even though there's sparse hydrological data with one previous study that put in a transect of piezometers between two permanent waterholes, and just show that they're clay-lined and there's an unsaturated zone of 5 to 10 metres between the waterholes.

So what we did next is then use this data to design field sampling campaign where we sampled for a range of chemistry environmental tracers, we did this three months after ceased to flow in the Cooper around three years since the last flood. So in November 2019 and February 2020, was when the sampling took place. We targeted 30 sites, but we only got access to 17 from the pastoral station owners. They're shown in red here. And we sampled for a whole range of different environmental tracers to look at the connectivity between the waterholes and groundwater things like radon, where concentrations can be a few orders of magnitude higher in groundwater, stable hydrogen and oxygen isotopes, strontium isotopes, and dissolved noble gases, but particularly helium because that's a good indicator of regional groundwater. And this was compared to sampling that was done by others in the assessment in groundwater. This is just an example of Russell and I out in the field, sampling the waterholes for different chemistry tracers.

So quickly looking at the chemistry radon and salinity. Essentially, look there's some very distinct hydrochemistry for the different water types. So waterholes pretty much of a calcium bicarbonate composition. The shallow sub-artesian groundwater is more of a calcium, magnesium sulphate type composition. And then the groundwater from the Great Artesian Basin has a clear evolution, not surprisingly from a sodium bicarbonate composition to a sodium chloride composition. If we have a look at radon and salinity, again, we've got some clear trends here so waterholes very fresh, very low salinity, hardly any radon, not indicating any groundwater inflow. Shallow sub-artesian groundwater in yellow huge range in salinity from fresh to saline and radon concentrations varying by two to three orders of magnitude higher than waterholes. Great Artesian Basin, deep groundwater, slightly higher salinity than waterholes, so more in the brackish range and again, low radon but still radon one to two orders of magnitude higher than waterholes. Just looking a quick look at stable hydrogen and oxygen nitrogen composition as well as dissolved helium.

Again, we're seeing some clear trends. So one of the interesting things to note on this plot is we've got a grouping of much more depleted waters where groundwater from the deep, Great Artesian Basin has been localised recharging the aquifer outcrop. We've got a huge range in compositions for the shallow, sub-artesian groundwater, so most of it has been evaporated, but we've got some more depleted compositions ranging to more enriched compositions, and that probably just shows the history of the diffuse recharge that happens to those aquifers across the region. And then of course, we've got much more enriched isotopic composition for waterholes showing the different water sources as water flows in from the north but also the different amounts of time for evaporation to occur after sampling. Looking at the hydrogen isotope composition and the dissolved helium composition, again, we've got distinct groupings so we're not really seeing much connection with groundwater. We've got virtually no dissolved helium at all in waterholes and a very enriched hydrogen isotopic composition. Again, for the sub-artesian groundwater, we've got a huge range. So we've got a partially enriched, partially depleted signature in hydrogen, oxygen isotopes. We've got a huge range in dissolved helium, so two to three orders of magnitude.

And then we've got the grouping for the deep artesian groundwater from the GAB, again, not surprising significant residence time. So depleted hydrogen isotope composition but more importantly, a much more higher concentration of dissolved helium indicating the long residence time. So three orders of magnitude higher than atmospheric concentration. So just in conclusion quickly, from all the data that we've gathered, permanent waterholes in the mid-to-lower Cooper seem to be perched above the regional watertable as shown on the conceptual diagram here, they appear to be surface water fed as demonstrated by this distinct chemical composition and salinity. They're enriched isotopic composition, and they're very low radon and dissolved helium composition. And appears from the data that's similar to other findings, they're conduits for ephemeral recharge following floods, so the clay gets scoured in the channel, and then you get some infiltration to recharge groundwater before the waterhole seals again, and just to give you an example is how turbid some of the waterholes are.