The water supply in Perth, Australia
by Jim Heath
Things have moved on since I wrote this article. The Gnangara Mound is no longer such a giant sponge of groundwater and the desalination plants have taken some of the strain off the Perth water supply. But the maps of the aquifers need no change, there are the same dams, and groundwater is just as vital. So I'm leaving the article for its historical background and educational use.
11 Feb 2013
Facts at a glance
Before we start...
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Drawings by Mary Heath
The nine great aqueducts of ancient Rome poured water into the Imperial city at about the same rate water pours into modern Perth from its eleven reservoirs. The Roman aqueducts supplied public baths, fountains and the occasional mansion. Perth's water system supplies every last house with clean water.
The beaut dams and reservoirs
The Darling Ranges (Perth people correctly call them 'hills') are the rough edge of an ancient -- and gigantic -- granite slab. Large areas of these hills to the east are still covered with eucalypt forests. The rivers that rise in those forests flow with water that's about as pure as such water ever gets: not perfect, but OK.
It's also a water supply that can't be expanded much more. There are only so many rivers worth putting a dam across, and the most rewarding dams have all been built: 11 dams and reservoirs, strung over 75 km of the Darling Ranges, and all connected.
There's one other reservoir in the hills that doesn't supply Perth: the Mundaring Weir. The water from that one is pumped inland, to the east. It is dry country there, and gets drier the further you go. You end up in a desert. The 550km pipeline irrigates a huge area that's used to grow wheat. The wheat covers 27,000 square kilometres (164km by 164km, if it were a tidy square). The pipeline ends in Kalgoorlie, a gold-mining town and as dry a place as you can picture.
This long pipeline was built in 1903. It was cutting-edge technology. The design called for a great steam-powered pump at the base of the Mundaring Weir dam. There were lots of doubts whether it would work. Many people scoffed. The engineer-designer for the project bore so much criticism and mockery he broke down and shot himself -- ten months before the pipeline was finished. (Which makes me wonder how smoothly things went with the first Roman aqueduct.)
When the water reached Kalgoorlie, Sir John Forrest, the Premier of Western Australia, sampled a glass of it and was ready with a quote from the Bible: "They have made a way in the wilderness, and rivers in the desert."
Other dams and reservoirs in the hills don't have such colourful stories. They are utilitarian, but some are strikingly scenic. If they were filled to the top, they could supply Perth through three and a half completely rainless years -- even if the reservoirs were Perth's only water supply. And they are not, as I mentioned at the beginning.
Not much else needs to be said about the dams and reservoirs, unless you're an engineer and want construction details (if so, please refer to the references). To most people, the more interesting topic is probably the water that doesn't sparkle openly in reservoirs, but collects in the darkness underground.
Groundwater is everywhere
About 40% of Perth's mains water comes from underground. If you first come to Perth in summer, you might think: "Water?! Under this?" It seems improbable, looking at any parched area that doesn't have a sprinkler system. Rain itself seems impossible -- as your skin withers under the hot sky.
But there's about 150 cubic kilometres of water under Perth, in various layers. Some of it has been there for 40,000 years.
This water is so important for Perth -- even more for its future -- that it's worth taking this slowly and going into detail.
The Perth Basin
The map shows a few of the "basins" in Western Australia. The technical term is "sedimentary basin". It means a depression in the earth's crust where sediments accumulate -- like a huge natural washbasin, full of sand, pebbles, clay, mud, dust, old seashells, and anything else that may have fallen in, been blown in by wind, washed in by flowing water, or whatever. All this material was compressed by its own weight and acted on by time to make most of it into layers of rock. So the "basins" are full of rock. They are no longer big dents in the landscape.
The Perth basin formed when a chunk of Western Australia slipped along a fault line (the Darling Fault). The basin area dropped 15km -- yes 15 kilometres, but not all at once. This impressive event happened 300 million years ago, even before dinosaurs were there to watch.
So here was this very deep hole between the Darling Ranges and the continental shelf. Down the ages, sediments fell in and filled it. The drawing shows the cross-section of the top part of the Perth Basin as it is today. To the right, there's the bulk of original Darling Ranges granite, now topped with State Forests, reservoirs, and some houses.
To the left of the granite are sediments that formed into rock (or sometimes still just sediments). A lot of the rock is shale -- ancient clay that became rock. There is also sandstone formed from -- yep, sand.
Two 'formations' are also shown (more about those in a minute). And there are fault lines, where the rock fractured and slipped. Near the top, there's a layer of clay. And at the very top, there's sand and some limestone. This heap of sediments and rock is now very stable, after all this time.
Here's the point of all this: there's a lot of water in the two 'formations' and in the top layer of sand and limestone. Most of the groundwater Perth uses comes from the sandy layer (up to 100m thick) at the very top. The reason: you don't have to drill far to get the water.
In the summer, the top few metres of sand dry out. But below that, the sand stays saturated from the winter's rain, and from winters before.
If Perth people left this water alone, it would move according to natural hydraulic laws. The water near the sea would slowly seep out into the ocean. Inland, and higher up in the sand, the water would seep downward and sideways. All these seepages are pretty slow.
Any place the sand dips lower than the 'water table' (which is the top of the underground water), water seeps out into the dip. That makes 'wetlands' (today's word for swamps). Or sometimes even lakes.
You can see these wet places on a map of Perth. Some are called 'lakes'. Some are called 'pools'. Some are called 'swamps'. There are two noticeable groups of them: the Wanneroo chain and the East Beeliar chain. They form along lines -- like long grooves -- where the sand lies lower.
During summer, the underground water still keeps seeping downward and out to sea, and evaporating from the wetlands and lakes. So the water table retreats further below the surface.
I'd also better stress something that isn't too obvious: the water table doesn't have to be flat. If rain falls on a hill, then the water table will resemble the shape of the hill. The water would look like a mound, if you could see it. Two of Perth's choice supplies of fresh water are underground mounds like that.
But what happens if you interfere with Nature by pumping water out of the wet sand? First, the water table may drop, whether it's winter or summer. It may drop a bit -- depending how much water you pump out. The wetlands and lakes could even dry up if you overdid it. And Perth could lose attractive wildlife (such as the spotted duck). Also, some trees depend on their roots reaching the groundwater. Too much pumping, and they'd die.
The Water Corporation keeps an eye on the water table in the metro area: they monitor it with hundreds of bores . They don't want more groundwater taken out than the rain puts back. And they need to allow for evaporation from the wetlands. Also for seepage to the ocean -- important, because if the seepage rate got too low, then ocean water could seep inland and mix with the fresh underground water.
And there's another complication. About 80,000 houses and industries have their own groundwater bores. More water is pumped from these bores than the Water Corporation takes for the mains system. So private bores have to be licensed.
With all the houses and industries on top of the sand, it isn't only fresh rainwater that goes into the ground. Pesticides can sometimes be detected in the water, and engine oil, fertilisers, bacteria, industrial chemicals and much else. Which means that private bores are used for lawns and gardens, or industrial purposes. Not for drinking. The Water Corporation mainly uses groundwater from areas with few houses and no industry: especially from those prized sources of underground water -- the Gnangara Mound and the Jandakot Mound.
The Gnangara mound is the single most important water source for Perth. It stores about 20 cubic kilometres of fresh water. If that doesn't sound like a lot, then compare it to Perth's yearly water use: about 0.2 cubic kilometres for the mains supply. That means the Gnangara mound -- considering the volume alone -- stores enough water to keep Perth going for a century. (Sketch, Gnangara and Jandakot Mounds.)
But that doesn't mean that if all rain stopped it would be possible to drain water out of the mound for a century. Before then, the dwindling flow of freshwater out to sea would let seawater seep inland and it would pollute the groundwater. Even if that didn't happen, it would still be impossible to get all the water out of the mound: the flow would get slower and slower, so you'd never drain it all.
To be practical, Perth can take the amount of water that's replaced by rain each year -- allowing some extra to keep the spotted ducks in their wetlands, and make sure there's enough to flow seaward. Even with those restrictions, the Gnangara mound is still a whopping source: half a cubic kilometre of new rainwater soaks into the mound each year. That's two and a half times Perth's mains-water requirement. The Water Corporation and private users now take about 17% of the yearly rainwater input. There is plenty of scope to take more.
This all means that instead of Perth getting 40% of its mains supply from groundwater and the other 60% from hills reservoirs (as it does now), the percentages could be reversed some day: 60% of the water may come from the ground, 40% from the hills. The groundwater is there to be tapped, while the hills water can't be increased much more. (The chief suggestion for increasing the hills water is to thin the forests. That would increase the runoff to the reservoirs, but could also increase the salinity of the water.)
Before you release coloured balloons and start a Gnangara Mound Fan Club, I'd better point out two drawbacks: first, the water is much more expensive than hills water. It costs something to pump it and it costs a lot to remove the undesirable stuff in it: colour, odour, and iron.
The other drawback is possible pollution. Large areas of the Gnangara mound are covered by pine forests and natural vegetation. But as Perth grows, there could be more and more houses and industries perched on the mound. All kinds of stuff could seep into the groundwater. So it is in everyone's interest to keep the mound as 'native' as possible.
The Jandakot mound (Sketch, Gnangara and Jandakot Mounds) holds 15% of the amount of water in the Gnangara mound. But that's still twelve years supply for Perth, by a crude calculation that assumes the mound could be drained dry.
Jandakot Airport is on the mound, so great care is taken to avoid leaks of aviation fuel and oil. And there's a patchwork of farms. The farms of course use fertilisers and chemicals, and there's possible pollution from animals (viruses and bacteria). If suburbia keeps crawling up the mound, then other things could also seep into the groundwater: fine particles of tyre rubber, pest poisons, car-wash mixtures -- who knows what all?
My handsome map of Groundwater Contamination Sites shows the top of the Jandakot mound surrounded with a band of blue dots: they mark the boundary of an underground water-pollution control area. But inside that, we see possible contamination sites: an abandoned landfill, a poultry-processing plant, manure-bagging and processing industries, and several piggeries.
A wool-scouring business caused serious pollution for awhile, but that was downstream of the public water-supply bores.
Since 1975 the Water Corporation has had powers to make bye-laws to protect the purity of the ground water from the Jandakot mound. It is in Perth's interest that they succeed.
The farmers on the mound use the water mainly for horticulture. The Water Corporation draws water for use in the mains. The total water taken could be doubled without over-using the supply.
The Yarragadee and Leederville formations
The Yarragadee formation (sketch) holds something like 1000 cubic kilometres of fresh water. If your mouth didn't drop, compare that to the 20 cubic kilometres in the giant Gnangara mound. In places the Yarragadee formation is 3km thick. It's about half sandstone, and the rest is siltstone and shale.
Part of the formation is right under Perth. It's so far down the water is warm. Back in the 1890s, one of Perth's old (and now expensive) suburbs had a bore that took water from the Yarragadee formation -- the 'Dalkeith hot spring' (it isn't there any more, to save you the trouble of looking.)
The huge soaked slab of the Yarragadee formation extends north of Perth all the way to Geraldton, where the formation is tapped as a water supply.
South, it extends about 50 km from Perth, then stops. There's a gap (like one of those suburban roads that ends for no obvious reason, just to start again a few streets later) and the rest of the formation in the south is about half as thick as the northern part. It's mainly sandstone there, and the water quality is very good.
The Water Corporation taps the Yarragadee formation. It adds about 5% to the Perth metro supply. Most of it is taken from under the Gnangara mound. The bores go down as far as 1500m. This is tapping very old water -- from rain about 40,000 years ago.
Rain replenishes the northern part of the Yarragadee formation through a couple of 'outcrops.' But they aren't ordinary rock outcrops that show at the surface: they're parts of the Yarragadee formation that rise to about a hundred metres of the surface. Rain seeps through the ground, then into these outcrops. Then very slowly, it seeps into the other parts of the formation. There's also some direct trickle-down through the Gnangara mound, again very slow.
As long as no more water is pumped out than goes in, plus a bit to allow seepage to the sea, then the Yarragadee formation can be used as a water source for as long as people live in Western Australia. The northern part, under Perth, could supply up to 70% of Perth's current needs without any risk to the natural flows or water quality in the formation. But care has to be taken to guard those 'outcrops' against polluters.
Nevertheless, the Yarragadee formation isn't going to be used while there's cheaper water in the Gnangara mound, or in the smaller formation just beneath the mound -- the Leederville formation.
The southern part of the Yarragadee formation is an attractive thirst-quencher because of its high-quality water. Even untreated, straight out of a bore, the water compares with mains water supplied to Perth. But to pipe the water to Perth would cost about four times the usual price of mains water. So the southern Yarragadee formation will probably only be a used as a local supply (for Bunbury, for example).
Meanwhile, there's that smaller Leederville formation that's much closer to the surface. The water quality varies a lot. In places, it's brackish and hopeless. Even the best sources from it usually need mixing with fresher hills water to keep the salinity down.
But even now the Leederville formation is important for Perth's northern suburbs. It adds about 10% to Perth's mains-water supply there.
So why does Perth sometimes have water restrictions?
If the hills reservoirs get low, it isn't an easy matter for the Water Corporation to run the Gnangara pumps faster to make up the difference. Groundwater is more expensive than hills water, and people wouldn't like seeing extraordinary water bills. Also, extra groundwater supplies are going to be added over many years, and engineered and paid for in orderly stages. So the pumping and pipe capacity may not even be there to make up very low reservoir levels.
And the reservoirs do get low. In fact, the levels dropped for a decade because average rainfall dropped. The trend line in the graph of runoff into the hills reservoirs shows it. The reservoirs were built assuming something like 250 million m3 average inflow. In the last twenty years, it's been down to 150 million m3.
That was just a dry patch in the weather, probably. If you look at the hundred-year average rainfall, in round terms we're getting only two-thirds of that. In 1996, the rainfall perked up again and this began to boost the reservoir levels some. Mundaring Weir, a quick responder, even overflowed (first time in 24 years).
After a reservoir gets low, it usually takes a long time to fill -- even with above-average rainfall. Most Perth reservoirs would take about two years of average rainfall to fill, even if no water was being taken out. One reservoir (South Dandalup) would take eleven years. They were built big to store extra water in above-average rainfall years, and slowly use it in drier years. But if the average rainfall drops year after year, there's a danger of getting into serious water-supply problems unless everyone conserves what's there. Hence, restrictions.
Anyway, what is in Perth's drinking water?
After you've lived in Perth for a while, you'll probably notice that the taste of the tap water changes. It's because the percentage of groundwater in it changes. There's more groundwater in the summer. The hills water and groundwater taste different.
Groundwater has more dissolved solids in it than hills water. If you have your own groundwater bore, then you wouldn't like to drink it and shouldn't: it's probably murky from peaty deposits, has a lot of iron in it, and has that classic underground smell (a drink that might appeal to trolls.) Depending where you live, your garden bore water might also have traces of bacteria and viruses, fertilisers, pesticides, and chemicals -- including heavy metals.
In the mains system, both hills water and groundwater are treated, but treated in different ways.
Hills water is treated with chlorine to kill bacteria. Fluoride is also added for dental reasons (a legal requirement since 1968).
Similarly, groundwater is treated with chlorine and fluoride. (The chlorine helps kill odour, as well as bacteria. And helps get rid of dissolved iron: it encourages particles to join into clumps that will settle out, or can be filtered out.) But there are other groundwater treatments as well:
There's nothing wrong with all that. You end up with good-quality drinking water (in a chemical and health sense). But as I said at the start, it tastes different from the hills water.
Anyway, hills water doesn't exactly pour from some heavenly fountain either. It flows along forest floors before it gets to the reservoirs. The Water Corporation has not been able to ban insects, small animals, mud, mould and bacteria from the forests. Hills water can actually be more cloudy than some of the underground water: it has some dissolved solids in it, and some suspended particles and some bacteria. But no industrial wastes, or farm chemicals. (For the chemical lowdown, scroll down a few pages.)
If those comments upset you, where does any city's water ever come from, if not from surface runoff to reservoirs, or from underground?
If you don't like the taste (or don't like it at times during the year), you can drink bottled water, or use a filter. Or go even further (like I do) and use a reverse-osmosis system.
As Choice Magazine said in a report about bottled water (Feb 1996):
"If you're looking for a clean, safe water supply, generally you need look no further than your tap. There are only two reasons to choose bottled water over tap water:
But you might like to read the Choice article on bottled water first. (Parts of it may make your eyebrows go up.) Not all bottled water is the same.
You can usually get up-to-date reports on bottled water by searching the Choice website.
And what's the water situation outside of Perth?
Any Western Australian town where there's less than 750 mm of rain a year will probably be getting its water from the ground. The exceptions would be towns in a broad path between Perth and Kalgoorlie: they're supplied by that famous pipeline from the Mundaring Weir.
A lot of Western Australia is so flat there'd be no way to dam a river, even if the river was a reliable water source and big enough. Instead, bores are drilled into river-bed areas and the underground water pumped out. This is done, for example, along the Gascoyne -- Western Australia's longest river. It's about 800km long -- even if it's 'intermittent' in some parts.
There's underground water in about 40% of the State. Anywhere there's a sedimentary basin, there's likely to be lots of groundwater, even under the deserts.
For example, the Great Sandy Desert roasts on top of what may be the largest underground water source in Western Australia: the Canning Basin. Much of its groundwater is drinkable. But only just, because it's near the limit in salinity.
The same applies to the Gibson Desert and the Great Victoria Desert. Both sit on top of a lot of water because they're on top of sedimentary basins. Part of the (pretty meagre) rainfall in these regions seeps down through the sand and accumulates in the huge pore space below.
Another immense, dry -- and very flat -- place is the Nullarbor Plain. It's a raised seabed, famous these days for almost boring drivers to death. Before there were cars, travellers sometimes did die: from thirst, not boredom.
Yet the Nullarbor is on top of a deep limestone formation that holds enormous amounts of just-drinkable water. At Eucla, a settlement on the eastern edge of the Nullarbor, water is desalinated to use for washing dishes and such. Most people there drink water they collect in rainwater tanks.
Deep sedimentary basins also border the Western coastline of the State. Again, there's a lot of water in them, but it's often too salty to use. In two coastal towns -- Useless Loop and Denham -- the underground water is desalinated to make it drinkable.
In the rest of the State, the 60% that isn't sedimentary basins, the landscape is little more than dressed-up bare rock. In the western part, there's a precambrian shield -- an ancient plateau of stable rock. No sedimentary basins there. If water collects anywhere, it collects in rock fractures and in superficial deposits of sand, gravel and limestone. Any towns or farms are built in places like that and rely on bores for water. And even that's only possible in places well north of Perth, because the underground water is too saline in the south. If it weren't for the goldfields pipeline, mining at Kalgoorlie couldn't happen (unless they made so much money they could afford desalinated water for everything). Nor could farms and towns exists in that expanse of forbidding and lightly-covered ancient rock.
There's another broad chunk of solid rock in the upper right-hand corner of the State: the Kimberley. It doesn't have sedimentary basins to collect water, but doesn't need them. There's so much rainfall that fresh water -- in theory -- shouldn't be a problem. Anyone looking at the annual rainfall figures might conclude there has to be a fine opportunity there somehow: the tropical climate, lots of rainfall, and plenty of land (much of it pretty good). But the curse is that the rain comes down hard in a short period. It's like having a swimming pool dumped on you. There are floods. People get trapped by water. Then suddenly it all goes away -- except for water collected in the famous (formerly infamous) Ord River Dam project. The water in it is used for irrigation, in increasingly successful projects. Unfortunately, the huge artificial lake probably covers some wonderful diamond-prospecting country.
If you're keen: details about dams, water chemistry, regional supplies
Some of the following sections assume you have a map in front of you, or you know some of this geographic material some other way.
Things that can affect people's health:
Note, July 1998: There was nothing in the technical literature I referred to about giardia, cryptosporidium or other parasites. If you have factual information about these health risks in WA water, I'd be glad to see it.
Starting from the far north, and going down the west coast and then east across the south border of the State:
Water references (with comments about them)
The Water Corporation has a large collection of free literature about its operations and about the water-supply situation throughout the State. One series of information leaflets is called Water Topics. Here are some in that series that I used in writing this chapter. (The number in front of the topic title refers to the Water Topic sheet number used by the Water Corporation):
 Water resources of the Perth-Bunbury region. The river system, coastal plain wetlands, confined and unconfined groundwater, estimates of water that can be diverted from different sources without environmental problems, and how to protect future supplies.
 The Gnangara Mound. Describes the mound, has an illustration showing the water flows in it, discusses the issues about wetlands and saltwater intrusion, and outlines the Water Corporation's schemes for the mound.
 Western Australia's groundwater resources. Talks about groundwater resources in the whole State, after defining and illustrating the different types of groundwater (confined and unconfined). Has a map of WA showing estimates of groundwater supplies throughout the State.
 The Jandakot Mound. A map shows where the mound is and illustrates the flows of water through it. The report talks about the environment at Jandakot and the challenge of keeping this supply clean.
 Groundwater of the Perth Basin. Discusses the geological formation of the Perth Basin, and illustrates the different aquifers in it, focussing on the Leederville and Yarragadee formations. Outlines the groundwater cycle, and discusses the use of groundwater from the different aquifers.
 Groundwater from the Yarragadee Formation. Describes the geology of the formation, how it is recharged and discharged, its size, links to the environment, and potential as a water source.
There are many other Water Topics leaflets. You can ask the Water Corporation for a complete list. Their phone number: 9420 2420.
If you're interested in future water supplies, here are two documents:
A Water Supply Strategy for Perth and Mandurah. (A Water Corporation publication -- and free.) Covers the strategy, has a graph of the surface water and groundwater supplies needed to meet the projected demand until 2021, gives a table of sources that need to be tapped -- and when. There are maps comparing the existing water supply scheme with the proposed one for 2021.
WA 2029: Stage II, Environmental Implications. A think-tank look at the future of Western Australia until 2029. One section is on water supplies, and there is a comparison of the costs of different options, including some of the far-out possibilities (towing ice bergs from Antarctica). This is a free publication from the Department of Commerce and Trade. Phone: 9327 5666.
And a few other references:
Inventory of known and inferred point sources of groundwater contamination in the Perth basin, WA. A long list of map references, referring to contamination from piggeries, dog pounds, cemeteries, hide processing plants, and on and on. There are four large maps folded in, and they are a lot of fun if you like this kind of thing. You can look up your own suburb and see if there are any charmers nearby. You can buy this from the Department of Minerals and Energy.
Water Authority Annual Report. If you want to know what's been happening in the last year, this summarises things like the amount of water stored in the reservoirs, the rainfall, and much other information about how different development schemes are coming along. (Free. Ask the Water Corporation for their latest Annual Report.)
Perth Urban Water Balance Study. This is a May 1987 study, and in some parts it's out of date. It may also be hard to get a copy, because I think I may have grabbed one of the last ones they had. But it would be available in their library, and if you want lots of detail about how water moves around in the Perth metro area, this one is a beaut. It's essentially an engineering report about a computer-model of the water flows (in aquifers, up bores, out to sea, and all the rest). The illustrations are superb and worth a look on their own, even if you don't get stuck into the text (of which there's plenty -- 106 pages).
Hydrogeological Map of Western Australia. This thing is gorgeous. It's about a metre square and shows the whole picture of underground water in the State. You can tell where the water is salty, how salty, what kind of formation it's in, where there are the most bores, the surface drainage, whether towns use borefields for water supplies, and even the groundwater temperature. All this for only $11 from the Department of Minerals and Energy. (You may have to go there to get it. I'm not sure if they send it out by post.)
New stuffI've considered expanding this one water chapter into a whole book. Here are lots of organised notes and technical details for it . You'll find plenty of things that aren't mentioned above. And it's more up-to-date
By the same authorWhy it's hard to have a good-looking lawn in Perth.
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