When floodwaters recede, where do they go?

By Loren Smith

University of Sydney researchers have modelled water-storing aquifers that perform a natural balancing act: they absorb water during floods and supply water during drought. Yet human intervention is limiting their function.

University of Sydney researchers have identified a groundwater ‘superhighway’ along Australia’s east coast. Stretching from Canberra to Brisbane, the flow of water connects aquifers (underground layers of water-bearing permeable rock) that stretch for thousands of kilometres.

In the case of the recent ongoing floods – among the largest on record – these aquifers store a significant amount of floodwater – up to 20 percent of it. Floodwaters seep through rock layers into groundwater aquifers via stream beds and soil.

The study’s lead author, Dr Ben Mather from the School of Geosciences, said the aquifers are especially useful during drought conditions. “Surface and groundwater sources are connected. Groundwater acts as a buffer during flood and drought, by either soaking up or providing water supplies as the situation demands.”

During flooding or bushfires, groundwater can also be a source of safe drinking water as overhead rock and soil layers filter contaminated surface water.

How fast is a drop of water?

Built from sediment from the Permian (around 300 to 250 million years ago) and Triassic (around 250 to 50 million years ago) geological periods, the superhighway transports groundwater from the Great Dividing Range into Sydney Harbour and off the continental shelf. On average, it takes a droplet of water 300 to 1,000 years to travel from its source to coastal aquifers.

Groundwater extraction threatens water security

The study, published in Nature’s Scientific Reports, also examined the impact of pumping (using groundwater for productive activities) on water reserves and conditions. “Growing human reliance on surface water for urban use and groundwater for agriculture and mining are changing our naturally interconnected water system,” Dr Mather said. “We are now seeing widespread and long-lasting effects from the last 20 years of groundwater pumping.”

Since the year 2000, groundwater pumping has resulted in a seven-metre drop in aquifers across eastern Australia, and up to 17 metres in agricultural regions. This leads to increased pressure on surface water supplies. It can also increase water salinity during drought conditions; as groundwater levels drop and stop naturally replenishing rivers, water evaporation leaves salt behind, with negative implications for rural and urban water users.

Scientific Reports: Constraining the response of continental-scale groundwater flow to climate change

Declaration: This study was supported by the Office of the Chief Scientist and Engineer, NSW Department of Industry, and the AuScope Simulation, Analysis & Modelling node funded by the Australian Government, through the National Collaborative Research Infrastructure Strategy, NCRIS.