Author: Anthony Barich
Date Published: 08 November, 2017
CSIRO's Gas Industry Social and Environmental Research Alliance' recently issued report demonstrated that CSG development would result in some changes to the Pilliga Sandstone's groundwater flow and water balance in that recharge area of the GAB.
Based on 500 simulations constrained with observations, preliminary results of the groundwater modelling showed an expected maximum water loss from that GAB aquifer due to CSG development of around 85 megalitres per year - equivalent to about 0.3% of the long term annual average extraction limit of 29.68 gigalitres per year from that part of the aquifer.
"Given wide variation in input parameters to the model [GISERA] used, the corresponding 95% confidence of predicted maximum water loss from the Pilliga Sandstone ranged between 0.28 and 2299 megalitres per year," the report said.
"This indicates that the likely impacts are relatively small compared to estimated recharge to and groundwater use from the Pilliga Sandstone for other purposes.
"The potential increase of groundwater flow from the Pilliga Sandstone to deeper formations was also accompanied by increased rate of water flow into the Pilliga Sandstone from the Namoi alluvial aquifer and surface streams overlying it.
The report also confirmed that the GAB aquifer is not connected with the coal seams to be accessed for CSG production, as there are thick layers of rock formations between them that generally have low permeability.
The report pointed out that water extracted for gas development is not taken from the GAB aquifer, but from the coal seams which are part of the Gunnedah Basin that underlie this GAB formation resulting in a reduction in pressure in the coal seams.
CSIRO estimates 0.89ML of water to flow into the Pilliga sandstone from the Namoi Alluvium, which is a major agricultural resource for the region.
That's nearly 0.001% of the average annual extractions from the Namoi.
Santos' proposed CSG development in the Pilliga forest in northern New South Wales has raised several environmental concerns as the Pilliga Sandstone acquifer is an important fresh water source used for irrigation, stock and domestic uses.
The forest is also part of a broader recharge region for the aquifer, which is part of the Great Artesian Basin aquifers, and activists and some landholders have expressed concern that coal seam depressurisation during gas production could impact groundwater pressure in the Pilliga Sandstone aquifer and affect the quality of water flow in the GAB aquifer.
The major groundwater sources around the project include key aquifers in the alluvial cover of the Namoi River and its tributaries.
GISERA sought to refine the conceptual understanding of the hydrogeological system in the Narrabri project area by analysing existing and new hydrogeological data, including environmental tracers and quantifying confidence in estimating the potential GAB flux and water balance changes in the region caused by CSG development.
A final report - which will cost $453,287 in total funded a quarter each by CSIRO, GISERA, the NSW government and the Commonwealth- is due in mid-2018.