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The Limits Of Groundwater Modelling


Modelling the impact of mine developments on groundwater is critical for protecting ecosystems and agriculture, but it can go astray if we don’t recognise that all models lack certainty.

By Emma White, University of Melbourne

Mention the word modelling and minds tend to drift towards washboard abs, perfect teeth and catwalks ... the flow of groundwater, not so much.

But modelling is a critical tool in managing supplies of underground water and assessing the potential impacts of large resource developments like coal mines. If we get it wrong, bad things happen – groundwater is contaminated, river flows decline, wetlands dry out, and ecosystems can lose the water they depend on.

The problem is that a model is only as good as the necessarily limited data it is based on. “Garbage in, garbage out” as my Hawaiian-shirt wearing professor used to say sagely.

Best available practice therefore relies on what we call “uncertainty analysis” which quantifies the likelihood that something may happen, but in Australia analysis like this isn’t legally required. As a result, decision makers are not equipped with an accurate representation of risks and consequences.

At best this leaves them blind to risk. At worst, it provides a false sense of security against more negative, but equally plausible, outcomes.

HIGH STAKES

For example, a 2015 decision by the Queensland Land Court to uphold approval for Adani to build the Carmichael Coal Mine despite concerns about the impact on groundwater, particularly nearby Doongmabulla Springs, has been re-examined in a recent paper by researchers involved in the court challenge.

The authors discuss modelling deficiencies and highlight the existence of an alternate and equally likely geological interpretation to the one used by Adani.

Adani proposed that the source of the springs is above a geological layer called the Rewan Formation, while the alternate interpretation places the source below it. This is significant because if the source is below the Rewan Formation, Adani concedes the springs will be lost. And yet, Adani chose to use the best case scenario in their modelling and failed to explore the alternate interpretation.

So, how can this happen?

In hydrogeology, we measure things like the porosity (how much fluid a rock can hold) and hydraulic conductivity (how well fluid flows through the subsurface) at only a few spots in a study area, like in wells, drilling core samples, and at exposed rock outcrops. These spot locations are then used to extrapolate what is underground across the whole area – a bit like a dot-to-dot puzzle.

But sometimes it can be like trying to sketch a whole person when all you can see is their little toe. That is to say, we just don’t know for sure.

As a result, geologists must use professional judgement to make an interpretation of the subsurface that’s consistent with the limited field data.

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Source: https://pursuit.unimelb.edu.au/articles/the-limits-of-modelling-knowing-we-don-t-know