2.6.2.2.2 Office of Groundwater Impact Assessment groundwater model for the Surat cumulative management area

The primary purpose of the Office of Groundwater Impact Assessment (OGIA) model is to predict regional water pressure and water level changes in aquifers within the Surat CMA in response to the depressurisation of the coal seams for CSG production. Specifically, the model is used to identify the immediately affected areas and the long-term affected areas where water pressures are predicted to decline by 2 m for unconsolidated aquifers (such as sand aquifers) and by 5 m for consolidated aquifers (such as sandstones). The model is also used to identify potentially affected spring sites where the long-term predicted impact on water pressure in the underlying GAB aquifers due to extraction of water for CSG production exceeds 0.2 m (QWC, 2012).

The OGIA model covers a spatial extent of 662 km × 548 km, overlays the entire Surat CMA and includes the coal seam formations and potentially connected aquifers within the Surat, southern Bowen and Clarence-Moreton geological basins. The model is discretised into uniform grid cells of 1.5 km × 1.5 km horizontally and vertically into 19 layers of variable thickness supported by relevant geological and hydrogeological data (QWC, 2012). The OGIA model grid discretisation is comparable to model grids (100s to 1000s m) developed for other subregions in the BA of a similar size.

The OGIA model is set up to run in a predictive mode from 1995, where two runs, a Base Run and a Petroleum & Gas (P&G) Production Run, are used to estimate the groundwater impacts resulting from the CSG operations in the Surat CMA. The Base Run involves running the model with water extraction from 1995 onward accounting only for non P&G groundwater extraction (QWC, 2012). In the P&G Production Run, water extraction from current and proposed P&G activities is added to the Base Run water extraction. The difference in the predicted water levels between these two runs provides an estimate of the water level impacts or groundwater drawdown that are attributed to P&G activities. This is consistent with the approach taken in BA, where rates of groundwater extraction for non P&G groundwater extraction are modelled as constant and equal to the rates after a specified date (the last quarter of 2012 in BA).

The first-generation OGIA model was built in 2012 and has the best available representation of CSG development in the Surat CMA for cumulative groundwater impact assessment. Tenure holders provide updated CSG development pathway information to revise the P&G Production Run each year (QWC, 2012; OGIA, 2013, 2014). This information is generally consistent with the information in the environmental impact statements prepared by tenure holders (QWC, 2012). The OGIA model uncertainty analysis gives probabilistic estimates of the hydrological changes associated with coal resource development, which is a requirement for groundwater modelling in BA. However, hydrological changes resulting from the operation of the coal mines in the subregion are not currently represented in the OGIA model.

The focus on deeper regional aquifers that are targeted by CSG development, means that evapotranspiration in areas with shallow watertables is not represented using a depth-dependent boundary condition. This means that the OGIA model is not on its own suitable for assessing hydrological changes in surficial aquifers (outside of the Condamine Model area) that are important in representing impacts to groundwater-dependent ecosystems, such as water-course springs and terrestrial vegetation.

Queensland’s regulatory framework provides that a new underground water impact report (UWIR) is prepared at least every three years. The revised Surat Underground Water Impact Report (UWIR) was released for public comment in early 2016 (OGIA, 2016). As a consequence, the Assessment team are using the 2012 OGIA model with the CSG development pathway reported in the 2014 Annual report (OGIA, 2014).

In conclusion, the OGIA model meets the fit-for-purpose criteria for groundwater modelling in BA, with the exception of criteria related to integration with surface water numerical modelling and receptor impact modelling that are related to the representation of water fluxes in surficial aquifers, as discussed in Section 2.6.2.1 .