Groundwater can seem like a bit of a mystery, probably because it rarely comes to the surface, so we can’t see it. People might descend into a cave system somewhere and see an underground river, and maybe think that‘s what groundwater is like. But groundwater rivers in caves are highly unusual – very rare.
There aren‘t huge voids underground holding vast reservoirs of water. It’s more helpful to imagine the ground beneath us as a bucket full of sand, and then you pour water into that bucket. The water occupies the gaps between the grains of sand. And it moves slowly through the gaps beneath the sand grains, which are like the gaps in the rocks beneath us.
But to call groundwater a mystery would suggest we know nothing, when it’s more accurate to say we have imperfect knowledge.
When we‘re trying to understand how groundwater works, we bore down and take samples to give us our information. It’s as if you‘ve got a book, but the only way to read it is to take a holepunch and punch half a dozen holes through that book, right through all of its pages. What you then have in front of you is a whole bunch of letters, and from this you‘ve got to work out what the story is.
But to call groundwater a mystery would suggest we know nothing, when it’s more accurate to say we have imperfect knowledge. In my field, we’re trying to understand how the groundwater is behaving in a particular area, how it‘s moving, and how it interacts with the vegetation on the surface and the surface water system.
It’s obviously a hugely important resource in Australia, because over most of the country it’s the only water resource we have.
A very important question is how deep you have to dig to reach it. Then when you reach it, you have to determine its quality. Is it too saline to use? Is it fresh, potable water? When you pump it out, how quickly does the hole fill again?
We couldn‘t live in many parts of Australia if groundwater wasn‘t there. Do we appreciate it sufficiently? Probably not.
If you have to wait a week, there‘s obviously not enough to develop an irrigation industry out of it. For that, you need it to flow very readily through those grains of sand. You need the material holding the waters in that aquifer to be permeable, so that when you pump it out it just flows back into the hole to give you a good reliable supply. If it‘s not permeable, you‘d probably still have a good enough supply to water your stock because they don‘t drink a lot, but not support an industry.
Of course, the further you have to drill down, the more costly that is, and the more costly it would be to pump to the surface. If it‘s getting down to 100 metres or more, we tend not to use it as it becomes too expensive.
We couldn‘t live in many parts of Australia if groundwater wasn‘t there. Do we appreciate it sufficiently? Probably not. Traditionally, we’ve thought of it as an infinite resource, or at least acted as though it was an infinite resource – you pump it out from the hole and it just comes back like magic. But we‘ve now completely turned that notion on its head, and now acknowledge that every drop you take out was going to go somewhere and probably have some important role.
We now recognise that if we take out groundwater, that‘s water that trees are not getting, or that‘s not flowing into the rivers, and then not flowing to the ocean. Groundwater that flows into the ocean is not wasted. It creates important ecosystems in coastal areas as well. So every drop is doing something, going somewhere.
I guess the big thing is that we’re not able to predict the effects of groundwater extraction very well.
It’s now a matter of us understanding where is it not going that it was going before. Is there a wetland that‘s now going to die? Is this river that used to flow perennially now only going to flow when it rains? These are some of the impacts that we need to understand in order to use groundwater wisely.
There‘s a lot of things we can do better. We looked at some of the issues with managing groundwater in a study that came out late last year, and there‘s a whole bunch of challenges. It’s going to take a lot of incremental changes to manage it.
I guess the big thing is that we’re not able to predict the effects of groundwater extraction very well. We know in principle that if we drop the level of the water, the trees are going to struggle more. Can they still get their roots down and use it enough, or are they just going to keel over tomorrow and die?
We can read that book after it‘s happened. We can see the dead trees – but the prediction, that‘s a really hard nut to crack. That would be very helpful.
As told to Graem Simms for Cosmos Weekly
