Floods crucial for ongoing access to groundwater

Posted 14 February 2015

UNSW honours student Lucienne Martel and Dr Dioni Cendon sampling irrigation bore water.

New research in south-east Queensland is developing a clear understanding of the significance of flooding for sustainable groundwater use.

Researchers Dr Bryce Kelly from The University of NSW and Dr Dioni Cendon from the Australian Nuclear Technology and Science Organisation are analysing the chemistry of groundwater in the Condamine Catchment to map where and when the Condamine River Alluvial Aquifer is being recharged.

In the Condamine Catchment and other catchments throughout the Murray-Darling Basin, floods over millions of years have created the level landscape ideal for broadacre farming. Flooding rains transported weathered material from the volcanic and sedimentary rocks that surround the Condamine plain and deposited the sedimentary grains that formed the fertile grey and black cracking soils, known as vertosols. These soils are unique in their capacity to hold water throughout our harsh dry spells and to sustain high-yielding crops.

Bryce says over the past 50 years there has been considerable debate about the amount of groundwater that can be allocated to sustain irrigated agriculture, and there has been a desire to have a single figure for groundwater allocations.

“It is clear from our research that ongoing access to groundwater will be related primarily to flood frequency,” Bryce said.

“How many floods will we have in the next few decades? Nobody knows: we can make an estimate by looking at past rainfall and stream flow records, but our climate is dynamic and will continue to change.

“The climate throughout the 1960s to today will not exactly match what we experience in the next few decades or centuries.

“We will always need to monitor groundwater levels, and to manage allocations in balance with our economic and environmental goals during times of both rising and falling groundwater levels.”

By studying the isotopic composition of the oxygen and hydrogen atoms that form water molecules and analysing the salts that are dissolved in rain, surface and ground water, the origin of the water can be traced.

“The chemistry of groundwater from irrigation bores throughout the Condamine Catchment indicates that recharge to aquifer depths from which groundwater is pumped occurs only following rainfall of at least 400 millimetres per month - yet this occurs on average once every four years,” Dioni Cendon said.

“Such rainfall is usually associated with extra-tropical lows in spring and autumn, and the remnants of tropical cyclones in summer.

“Meanwhile, contributions to groundwater recharge from irrigation deep drainage, rainfall over the wider landscape or river leakage under normal streamflow conditions are small and recharge from hard rock aquifer systems, in particular the Great Artesian Basin, is small.

“Floodwater is the primary, and in some places only source of groundwater recharge.”

Dating and tracing water movement using Tritium

Interestingly, by measuring levels of tritium, a byproduct of nuclear bomb testing, scientists can measure whether recharge has occurred before or after tests were undertaken in the 1950s and ‘60s.

The concentration of tritium increased in the atmosphere after these nuclear tests, and this tritium is incorporated into raindrops. Higher tritium activity (an indirect measure of concentration) in groundwater indicates recharge must have occurred within the last 70 years.

Tritium activity measurements from groundwater samples throughout the Condamine Catchment indicate that recharge to the Condamine River Alluvial Aquifer system is primarily along the river corridor.

“This corridor must be protected from developments that may hinder aquifer recharge during floods,” Dioni said.

“We can refine the mapping of recharge zones and enhance the recharge during major floods,

“While some may argue that the floodwater is needed further down the Darling River system, skimming a small portion of the floodwaters and enhancing recharge to the alluvial aquifer would have both economic and environmental benefits.

“We are underutilising the dams already created by nature, and unlike new surface dams, we would be storing water in an evaporation-free environment.

“There are some studies exploring managed aquifer recharge as part of optimising catchment water management, but much more needs to be done.

“Groundwater is critical for the economic viability of many farms throughout the Condamine Catchment and many other cotton growing catchments in Australia.

“Groundwater geochemical studies, such as this project just undertaken in the Condamine, will help us to sustainably manage groundwater and we aim to investigate other catchments of interest for the cotton industry using similar techniques.”

Source: Spotlight on Cotton R&D Summer 2014-15

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