Open-cut mines can leave surrounding trees unable to withstand drought, according to research that may have implications for the controversial Shenhua Watermark coal project.
Researchers have found that underground water tables can fall by up to 20m within two kilometres of an open-cut mine as miners pump out groundwater to keep it out of the mine pit.
Groundwater can fall by up to four metres more than five kilometres away from a mine because of the cone of depression that forms around the excavation site.
Dr Sebastian Pfautsch, of Western Sydney University’s Hawkesbury Institute for the Environment, said changes in groundwater supply affected how well trees grew and how well they could survive in a drought.
Even when trees are not directly tapped into groundwater they could at times use “capillary water” drawn up through the soil to survive when there was little or no rain.
The research was conducted on eucalyptus trees around the Hope Downs iron ore mine in Western Australia but Dr Pfautsch said it raised questions about the impacts of the Shenhua Watermark coal mine that has been approved to be dug next to rich farmland on the NSW Liverpool Plains.
“What we examined in this study is how these trees respond when nearby mine operations start changing underground water supplies,” he said.
The study found changes in groundwater levels affect how much water is used by trees.
“The tight connection between water use and the growth of trees implies that a reduction in water use will lead to a reduction in growth.
“In extreme cases trees will die of thirst.
“Even if the Shenhua situation might be very different, what remains the same is the trees need to access water in some way when the ground is very dry.”
Dr Pfautsch said it is not clear if tree roots are able to quickly “follow” underground water when groundwater falls rapidly.
Federal Environment Minister Greg Hunt approved the Watermark mine with strict conditions in July, sparking anger from his cabinet colleague Barnaby Joyce who called the project “ridiculous”.
A report from the Independent Expert Scientific Committee (IESC) to Mr Hunt identified uncertainties and “information gaps” in the mine proponent’s water impact studies.
The IESC also said it was not satisfied with the robustness of proposed water monitoring for the mine project.
Dr Pfautsch said better monitoring to assess tree health was needed for mine projects.
“You can have no effect for 10 years,” he said.
“Then the eleventh year comes along where you have a bit of a drought and everything just dies instantly.”
Dr Pfautsch said he was not saying “that mining kills trees” but that “the headline should be that mining can kill trees if you are not using an intelligent groundwater management system around the site”.
Dr Pfautsch’s research was jointly funded by the Australian Research Council and Rio Tinto Iron Ore and was published in the journal Ecohydrology in June, 2014.