Caves’ climate warning to south-west: groundwater at 800-year low

A watery wonderland south of Perth, Lake Cave is evocative of Jules Verne’s classic adventure Journey to the Centre of the Earth. But its crystalline delights and cool waters also foretell a drier future for Australia’s south-west under climate change, and a challenge or two ahead for water planners.

Make no mistake, caves can teach us a lot about human endurance. But they can also be dangerous. That didn’t stop 6 scientists keen to gauge how a decline in rainfall in south-west Australia since the 1960s has affected groundwater recharge.

The team, led by ANSTO, found that rainfall recharge may “no longer be reliably occurring in the region, which is highly dependent on groundwater resources.”

According to the results published in Nature Communications: Earth and Environment, groundwater sources still meet 75% of the water needs of Western Australia’s south-west and provide water for the wheatbelt zones bordering the coast.

The researchers measured water flows at 7 sites across 4 Late Quaternary-aged caves in the Leeuwin-Naturaliste National Park.

The limestone caves are sited along the Leeuwin-Naturaliste Ridge and adjacent to the Perth Basin, which holds WA’s largest and most significant source of fresh water in the sedimentary aquifers that supply water to Perth.

Why are regional aquifers drying out?

According to the research, rainfall has decreased by 10–25% overall across the region since the 1960s owing to human-induced changes in stratospheric ozone and atmospheric greenhouse gases. This is 100 to 230mm less rainfall each year since 1971–2010 compared to 1911–1970.

“The rainfall decrease, which caused the reduced rainfall recharge in our study area, is widespread throughout southwest Australia,” says Dr Stacey Priestley, a hydrogeologist and geochemist at the University of Adelaide who led the research effort.

Research demonstrates recharge will decline further throughout the 21st century, and that the present situation is unprecedented in the past 800 years.

“While we cannot say for certain from our study alone that the rainfall decrease has caused a rainfall-recharge decrease in the Perth Basin aquifers, it is highly likely this is the case.”

Of course, the south-west’s water woes are nothing new; a recent report on its water security by Infrastructure Australia recognised the region faces “considerable risk … from climate change, population growth, ageing assets and changing needs and expectations from water users”.

To help bolster the region’s supplies over the past 2 decades, the WA government has commissioned ocean water desalination plants, surface water storages, and water recycling schemes. Yet groundwater is forecast to remain an important pillar of water security in the region.

Looking for answers underground

Caves can be a window into the ways rainwater moves from the earth’s surface through the intervening layers beneath – the vadose zone – to storages of groundwater below, such as aquifers or underground lakes like Lake Cave.

Water moves through the vadose zone in two ways: using pathways such as fractures or cracks (preferential flow, faster), or through the soil/rock matrix (diffuse flow, slower).

The researchers used documented water level changes in cave lakes and ponds, measured the amount of water dripping from cave roofs, and tested the chemistry of the various waters.

Fortunately, water levels at some caves were already being monitored. Discounting recharge from sources other than rain was important and meant the researchers could concentrate on a link between water table changes and fluctuations in rainfall.

“A couple of the caves in southwest Australia have [lakes or ponds],” says Priestley, “but we couldn’t include most of them in the study because they didn’t have sufficient long-term measurements.

“Jewel Cave had good long-term water table/lake level measurements … and showed a 2.5m decrease from the 1960s to 2012. Lake Cave, [which] also has lake level measurements, showed a rainfall-recharge relationship with higher rainfall in 2013, the… water table also rising approximately 0.5m.”

And the water dripping from cave roofs?

The allure of the crystal formations in Lake Cave and others is for more than mere aesthetic appeal. Scientists call such formations speleothems, a term that includes stalactites and stalagmites, where mineral deposits accrue from water dripping over time; and from which much can be learned by analysing the chemical characteristics of the water.

Scientists wearing a headlamp sit next to two funnels that are collecting drop water for monitoring.
Scientists monitor drip waters in WA caves. Credit: A Baker.

The work builds on research of more than a decade by co-author Dr Pauline Treble, who wants to understand past rainfall-recharge relationships by decoding the mineral deposits formed from groundwater in underground caverns.

Specifically, Treble wants to understand flow paths – or how water flows from the surface to the cave – in nearby Golgotha Cave and others.

The main players in both investigations have been isotopes of oxygen present in the drip waters, an isotope being an atom containing the same number of protons and electrons as ordinary oxygen, but a different number of neutrons. The isotopic content of drip water varies with location and the original rainwater, but an isotopic fingerprint may also remain in the water from whichever pathway it takes on its way underground: preferential or diffuse.

Preferential flow paths yield higher drip rates than diffuse, and so contribute more rainfall recharge to groundwater. Such differences help scientists deduce which mechanism has predominated at a site, and what effect climatic fluctuations have had on rainfall volumes.

Measurements across the 4 caves in the region showed an increase in the presence of oxygen isotopes during the late 20th and early 21st centuries.

While similar results were reported earlier from Golgotha Cave, this study confirms the results from other caves in the Cape Leeuwin-Cape Naturaliste region.

Priestley and her colleagues used this to assess how the recharge of groundwater responded to rainfall during recent climate changes compared with pre-industrial times.

“We are building on this research to extend the stalagmite hydroclimate record… to 10,000 years ago, to better understand when groundwater was recharged and under what past climate scenarios.”

That might help WA’s Department of Water and Environmental Regulation (DWER), which manages and regulates the state’s water resources. “Growth in water demand and the impacts of climate change across the Perth basin is a problem and opportunity of national significance,” a DWER spokesperson told Cosmos in a written response.

“The implications of climate change on water supply and water-dependent ecological, cultural and social values are well known to DWER as we have been accounting for climate change in our water planning for decades. Planning…. over the past two decades has reflected the effects of reducing rainfall due to climate change on decreasing availability of surface water and groundwater resources and the growth of the state’s population and economy.”

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