Draft Surat Underground Water Impact Report – part 1

May 29, 2012 at 1:26 am Leave a comment

 
Surat Cumulative Management Area
I had much underground water on my mind today.  That was because I attended both sessions today in Roma where the Qld Water Commission were explaining their Draft Underground Water Impact Report (pdf, 8 meg) for the Surat Cumulative Management Area to the public.  The quick and dirty bottom line is that I don’t think the data supports a moratorium of the industry and as a worst-case scenario says the impact is moderate and manageable. However this is the first of several posts that will drill down into the report in some detail.
The Surat Cumulative Management Area is a rough triangle drawn between Emerald in the north, Roma in the west and Toowoomba in the south-east. The geology of the region is complicated as the nature of the water. I had several concepts challenged including what are the Bowen and Surat Basins, what is the Great Artesian Basin and where is the gas stored. The Great Artesian Basin is not a continuous geological formation but a hydrogeological basin across many alternating geological layers. Similarly I used to think the Bowen Basin as the land roughly inland of Mackay including all the big coalmining areas of Emerald and Moranbah. The Surat Basin roughly went from Dalby to Roma.  But it turns out my understanding is that is faulty too. The Bowen Basin lives below the Surat Basin, it is only in the strip-mining areas at Moranbah where its coal formations come to the surface.
Petrol and gas is a different mining process to coal and covered by different legislation.  The Draft Underground Water Report was required because the law allows petroleum tenure owners to explore for petrol and gas on private property and by necessity, there is some interference with the water on those tenures including the removal of the water. This is particularly so in coal seam gas production which works by reducing water pressure in the seams to release the gas.  In the Surat Cumulative Management Area most of the mining is done in the Walloon Coal Measures (Surat Basin) or Bandanna Formation (Bowen Basin) which are geological layers of the Great Artesian Basin which have low permeability rocks alternating with high economic value aquifers and feed important springs.
The problem is that when water is removed, it affects a wide area around the gas well. This is compounded if there are a number of nearby wells also drawing out water. Today most of the groundwater in the Surat Region that comes to the surface is used by agriculture, industry, stock and domestic – some  215,000 megalitres a year. CSG is only responsible for 17,000 ML at the moment but that will rise sharply in the coming years as the four big projects (Santos GLNG, Origin APLNG, British Gas QCLNG and Arrow Surat Gas Project) take off.
When water is removed from the coal formations, water from surrounding aquifers will flow in.  So when the water pressure is reduced, it doesn’t necessarily mean less water. However it does mean there will be a decline in the water level of the bore that taps that aquifer.  The question is by why how much and to answer that question the Queensland Water Commission developed a groundwater model to predict the impacts of the CSG industry. They used vast reams of already known data on water levels and bores which they added to the known plans of tenure holders plus some science about the way underground water moves through the region.
The resulting flow model was complex. There are 19 interacting layers and three million individual cells in the model. It was calibrated to get close matches with known 1995 results from bores giving the team a high degree of certainty they were in the ballpark. They also added ‘uncertainty analysis’ taking the 95 percentile of 200 different predictions for each well. In other words,  they were taking the worst case scenario in 20.
For each well the QWC set a trigger threshold of drawdown.  For consolidated aquifers such as sandstone, the trigger was of five metres for consolidated aquifers, it was two metres for unconsolidated (shallow alluvial) aquifers such as the Condamine Alluvium and just 0.2 metre drop for springs, including watercourses connected to springs.
If the modelling showed the “Immediate Affected Area” (an IAA) of that well exceeded that threshold in the next three years, then the responsible CSG company must undertake restoration measures to restore the bore’s capacity to supply water, or provide the bore owner with an alternative water supply.  This is known in the legislation as “make good” requirements. It could mean adjusting the bore, improving the pressure, drilling a new bore or finding an alternative source. QWC have identified 85 bores in the Surat Region which will exceed the trigger, all of them in the Walloon Coal Measures.
There was also a secondary measure of long-term impact if an IAA exceeded the threshold at any time in the future. This modelling identified 528 bores affected, mostly in the Walloon but some in the Springbok Sandstone (104), Hutton Sandstone (23) and Gubberamunda Sandstone (1).  It is less clear what the Commission expects to happen with these bores though the Roma session talked about gas tenure holders being “proactive” with bore owners in this category.
Part 2 of this will discuss the monitoring regime QWC is putting into place to determine the trigger points.
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Manne bites Australian Surat Underground Water Impact Report – part 2

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