Testing times for groundwater

jordi-batlle-aguilarIn South Australia’s arid landscape, rivers, creeks and streams are vital sources of groundwater.

But National Centre for Groundwater Research and Training (NCGRT) researcher Dr Jordi Batlle-Aguilar (pictured) says there is virtually no accurate way of knowing how much water seeps from river beds into the ground because traditional testing methods are largely unreliable.

Streams in arid and semi-arid areas only flow after heavy rain and storms, and their water rapidly infiltrates through to underground permeable rocks, such as sandstone, or unconsolidated materials including gravel and sand.

These permeable rocks and unconsolidated materials form a natural underground water storage system – known as an aquifer – which saturates and holds the water, similarly to a sponge, so that it can be extracted in the summer months when water is scarce and replenished in winter.

Dr Batlle-Aguilar said the standard method to measure infiltration did not give reliable estimates at the scale of interest because it simply provided an average for the whole river, even though infiltration “varies greatly” from site to site depending on soil type, gravel content and root presence.

“The conventional way to test infiltration is to put a cylinder filled with water, known as infiltrometer, 5- 10cm under the streambed and measure how much water sinks down over a period of time,” Dr Batlle-Aguilar, based at the NCGRT at Flinders University, said.

“But the results can be completely different at different points because the soil is highly heterogeneous, so if there were big bits of gravel in one part of the river the water would infiltrate faster than another area, so it’s not representative of the true infiltration rate,” he said.

“One value can be very different from another just a metre away, therefore it’s not appropriate to extrapolate one measured value, at the centimeter scale, to a total value for a whole stream.”

As part of his ongoing research, Dr Batlle-Aguilar took his team to Pedler Creek, an intermittent stream in the Willunga Basin, in 2011 to find a better way of measuring river infiltration that would provide more reliable and representative values.

The team isolated an area of the creek using two steel panels placed 7m apart and pumped water continuously into the dammed section of the creek, maintaining three constant water levels over five days. This allowed the team to measure infiltration over a much larger area of the river bed than the standard approaches would normally permit.

By knowing the pumping rate required to maintain a constant level of water in the stream, the researchers could deduce how much water infiltrated from the stream and potentially recharged the aquifer.

“The water must be going somewhere if it’s staying at the same level so we recorded how much water we were pumping into the isolated stream to maintain that constant level, which gave us an accurate value of infiltration,” Dr Batlle-Aguilar said.

He said it was crucial to understand the infiltration rate from rivers to prevent “overexploitation” of aquifers.

“South Australia in particular has lots of dry streams which only flow after storms but it’s really important to know how much water infiltrates from them.

“Knowing how much recharge occurs from rivers to aquifers helps us to determine how much we ‘earn’ and this in turn can guide how much we should be ‘spending’ from our ‘water budget’.”

The research, which has been spearheaded by Professor Peter Cook, Deputy Director of the NCGRT, is expected to be published in a scientific journal later this year.

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