Research aims to predict groundwater recharge

Andrew Baker is Professor in the School of Biological, Earth and Environmental Sciences at the UNSW. He is combining the analysis of cave stalagmites, underground hydrological monitoring, and climate hydrological modelling to identify the replenishment of groundwater.

Groundwater supports so much of the Australian economy. In rural and remote areas, most of the water supply is groundwater. It provides irrigation for agriculture and farming. It provides water for industry and mining. It supports our ecology. But there are fundamental questions about the sustainability of our groundwater that we are yet to answer: we don’t know how quickly it’s recharged, or how it is affected by rainfall.

The actual level of groundwater is quite disconnected from what you see on the surface, so you don’t know that there’s going to be a problem until it’s too late. The classic example here is in California, where the groundwater levels have been depleted so much for use in agriculture that the land surface has subsided.

In areas of Australia, the borehole records show that the water table is tens of metres lower than it was before we started intensive agriculture. But on the surface, you wouldn’t know. That’s part of the problem in understanding how to manage it.

To manage it, you first have to measure it. So you put a water level measuring device in a bore or a well and observe how the water level moves up and down. Sounds straightforward.

The problem is that you don’t know where that water’s come from. Has it come from that location, or has it come from a few kilometres away? Perhaps it has come from hundreds of kilometres away? If it’s groundwater coming from somewhere the size of Australia’s Great Artesian Basin, it could be coming from half a continent away.

You also don’t know how old it is. You don’t know if it was replenished last year, or hundreds of years ago, or even hundreds of thousands of years ago.

Even if the water level goes up in your bore, you don’t know if that’s from a recent rainfall event. It could be pressure effects: if you blow up a balloon, for example, and you put your foot on it, the rest of the balloon will expand where it’s not compressed. Similarly, the water level in a groundwater system might go up and down without the arrival of any new water.

In fact, groundwater bores really aren’t the places to investigate groundwater replenishment.

Just to be clear, groundwater is not contained in a vast system of underground lakes. Groundwater is typically stored within the rock beneath our feet. If the rock is relatively young in geological terms, maybe a few tens of millions of years, there are spaces between the pores of sedimentary rocks. But if the rock is older, like Sydney sandstone (about 200 million years old), those pores have been cemented up, so the water can’t flow through them anymore. In which case the water is stored in cracks.

That’s the rule: young rocks, the water is held in the pores; in older rocks, it’s held in the fractures.

Thankfully for my work, there’s an exception: limestone caves.

If water flows through pores or cracks, and if you have limestone underneath, the water dissolves the limestone and caves are formed.

These caves are rare places of incredible beauty with their stunning colours and amazing formations – and they are ideal for our work. This is because they can be found above the groundwater table but beneath the surface. If we can see water coming into the cave system from the top to the bottom, we know it has to be from a rain event.

This way we can use them as observatories of the water moving from the surface down to the groundwater table. This is the crux of the groundwater mystery.

We are particularly interested in the stalagmites we find in these caves. Stalagmites grow up towards the roof incrementally. If you take a sample and cut it long ways, from top to bottom, it reveals a perfect layer system. And with each layer, you can count the years back in time – a continuous record of the thousands of years of past climate.

They only grow when there’s water moving from the surface to the subsurface. Just by knowing when they’re forming gives us some idea of the wetter phases and when groundwater recharge is likely to be happening. We measure their isotopes, because the natural composition of the water varies with climate. And we’ve worked out you can tell whether it’s going to be a recharge event or not from the water isotope composition.

We’ve spent the last five years or so understanding how the stalagmite isotope records work here in Australia. We’re now going to use this ARC fellowship project to take those stalagmite records, find out when recharge occurred in the past, and then use that to set up some computational models to predict water recharge into the future, through different climate change states.

Water licensing allocations currently vary from state to state. Everyone is doing the best they can but the problem is that they’re working with uncertain data. Our aim is to be able to go back and say we’ve refined our understanding how quickly groundwater is being replenished, and how that relates to climate change and climate events. Hopefully, we can then inform government policy if it needs to be revised to make groundwater use more sustainable.

I’ve just been back from the NSW mid north coast, exploring some wild caves with the local caving club who have been doing lots of monitoring as a citizen science project. There are locations out there that very few people know exist. I’ve been lucky enough to go to caves where no one’s been before – we’ve actually discovered them on overseas expeditions.

Every cave is different. Some of them go down to the groundwater table – you just keep descending until suddenly there’s a horizontal layer of water in front of you, perfectly still. Because the water’s so rich in calcium carbonate, it forms these delicate calcite rafts on the surface. It’s pure blue as well, because the water’s so pure.

It’s such a privilege to see. That’s my laboratory right there.

This article was originally published in Cosmosmagazine.com.