Tracing arsenic threat to groundwater

A project in West Bengal, India, to remove arsenic from groundwater. Credit: Ravakrishna Vivikananda Mission
A project in West Bengal, India, to remove arsenic from groundwater. Credit: Ravakrishna Vivikananda Mission

In the driest inhabited continent on earth, underground water accounts for a large portion of Australia’s most precious resource – freshwater.

But, under certain circumstances, groundwater can be threatened by naturally occurring arsenic which is one of the most serious threats to groundwater quality on a global scale.

This very problem has prompted Ilka Wallis, a PhD candidate in Flinders University’s School of the Environment, to investigate exactly how arsenic may leach into underground water.

Groundwater is found in aquifers which sit deep below the earth’s surface and are made up of porous rocks and gravel such as sandstone which saturate and hold the water, almost like a sponge. Arsenic can be present in these rocks, but how it can leach into groundwater is still not fully understood.

As part of her research, Ms Wallis has studied the geology, hydrology and chemistry of affected sites in the US and the Netherlands to work out how arsenic made its way into those systems, and the extent of the problem.

“Naturally occurring arsenic is present in rocks and it normally doesn’t get into groundwater but sometimes there are circumstances when it’s released and when that happens it can be potentially devastating,” Ms Wallis said.

Previous laboratory studies have found that common naturally-occurring geological and chemical processes can play an important role in controlling arsenic in groundwaters.

As these processes are often complex and interlinked, Ms Wallis’ research has focused on “numerical modelling tools” to integrate diverse data sets, including geological, chemical and hydrological information, to understand why arsenic may have been released into the groundwater.

She said her research was one of the first studies where the extent of the leakage of arsenic was quantified on a field-sized basis using numerical modelling.

The modelling tools helped to quantify information including the mass of arsenic being released and also lead to discussions on proposed engineering solutions to mitigate the problem.

Ms Wallis said factors such as the pH and oxygen concentration of the groundwater, as well as certain geological influences such as mineral occurrence and initial arsenic concentration in the rocks, were among reasons why arsenic was found in the groundwater.

She said that in Australia – where groundwater is the second most important freshwater resource – arsenic is generally not a problem however “we need to be aware that under specific circumstances arsenic can be released into groundwater and then concentrations can become elevated”.

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