- Bioregional Assessment Program
- Namoi subregion
- 2.6.2 Groundwater numerical modelling for the Namoi subregion
Coal resource development can potentially affect water-dependent assets through changes in groundwater hydrology. The bioregional assessment (BA) groundwater numerical modelling provides probabilities of groundwater drawdown and changes in the surface water – groundwater flux due to coal resource development in the Namoi subregion.
This product describes the model development and presents the modelled hydrological changes due to coal resource development in the Namoi subregion. Results are reported for the difference in model outputs between the two potential futures considered in a BA:
- baseline coal resource development (baseline): a future that includes all coal mines and coal seam gas (CSG) fields that are commercially producing as of December 2012
- coal resource development pathway (CRDP): a future that includes all coal mines and CSG fields that are in the baseline as well as those that are expected to begin commercial production after December 2012.
The difference in model outputs between CRDP and baseline is due to additional coal resource development – all coal mines and CSG fields, including expansions of baseline operations, that are expected to begin commercial production after December 2012.
Baseline coal mines and additional coal resource development were defined in Section 2.3.4 of companion product 2.3 (conceptual modelling) for the Namoi subregion. The baseline includes six coal mines: five open-cut coal mines: Boggabri Coal Mine, Rocglen Mine, Sunnyside Mine, Tarrawonga Mine and Werris Creek Mine; and one underground longwall mine: Narrabri North. The ten additional coal resource developments include nine coal mines and one CSG development: Boggabri Coal Expansion Project, Caroona Coal Project, Gunnedah Precinct, Maules Creek Mine, Narrabri South, Tarrawonga Coal Expansion Project, Vickery Coal Project, Vickery South Coal Project, Watermark Coal Project and Narrabri Gas Project. Due to insufficient information regarding the location and depth of mining, two of the additional coal resource developments, Vickery South Coal Project (open-cut coal mine) and the Gunnedah Precinct (open-cut and underground) are not being modelled. Of the baseline coal mines, Werris Creek Mine is not modelled because it belongs to a geologically separate basin (Werrie Basin). Analysis of the potential impacts and risks of these developments will be restricted to commentary in product 3-4 (impact and risk analysis).
BHP discontinued the development of the Caroona Coal Mine in August 2016. The NSW Government bought back BHP’s Caroona coal exploration licences on the Liverpool Plains in August 2016. This occurred after the finalisation and modelling of the CRDP, thus the Caroona Coal Mine was included in the modelling even though it is no longer proceeding. Similarly, there have been changes to the mining licence for the Watermark Coal Project. Nevertheless, for reasons outlined in the companion submethodology M04, this development forms part of the CRDP.
There have been many groundwater models developed in the Namoi subregion. A review identified two models of sufficient scale and complexity; however, neither model was able to be re-purposed for use in BA. The Namoi Catchment Water Study model did not have the stability required for the uncertainty analysis due to problems with numerical convergence, and the Gunnedah Basin Regional Model had limitations with the conceptualisation of the alluvium and Great Artesian Basin (GAB) layers. For BA purposes, the MODFLOW-USG regional-scale numerical groundwater model was built to evaluate hydrological changes due to additional coal resource development in the Namoi subregion.
The groundwater model consists of up to nine hydrostratigraphic layers with the alluvium, Pilliga Sandstone, Hoskissons Coal and Maules Creek Formation explicitly included, and with other formations combined into interburden layers. These interburden layers comprise mostly aquitard formations and occasional aquifer formations that are not largely used for beneficial water use. The modelling domain spans an area of about 59,000 km2 with depths exceeding 1500 m. It is represented by a variable Voronoi mesh with a resolution of 300 m around coal resource developments and streams and increasing to 3000 m in areas remote from the coal resource developments and streams.
Of the 81 parameters in the model, a subset of 37 is allowed to vary stochastically to form the basis for the sensitivity and uncertainty analyses. The observed groundwater levels in the surficial aquifers and surface water – groundwater flux are most sensitive to depth of incision of the streambed, the scaler on diffuse recharge and the hydraulic properties of the alluvium. The main prediction, drawdown due to additional coal resource development, is sensitive to the hydraulic properties of the interburden and the hydraulic properties of the coal-bearing formations. As the groundwater level and streamflow observations are not sensitive to these parameters, these parameters will not be constrained greatly in the uncertainty analysis.
The groundwater numerical modelling allows drawdown due to additional coal resource development to be predicted. In BAs, a conservative 0.2 m drawdown threshold is chosen as it aligns with NSW regulatory thresholds for the protection of springs, and is close to the practical resolution limits of modelled and measured drawdown, within the bounds of seasonal and climatic variability. A zone of potential hydrological change is identified as the area of the regional watertable aquifer where there is a greater than 5% chance that drawdown due to additional coal resource development exceeds 0.2 m. For the majority of the model domain in the Namoi subregion, the median drawdown due to additional coal resource development is less than 0.2 m. The probability of exceeding this threshold is 100% within the immediate vicinity of a mine footprint area and decreases rapidly with increasing distance from a development. This zone of potential hydrological change is generally within 10 km of the boundary of the footprint of the additional coal resource developments. This also means that the zone of potential hydrological change of two developments only overlaps when they are within 20 km of each other. In most cases the drawdown is attenuated at the alluvium boundary due to the high transmissivity and so the largest magnitude drawdowns occur in the consolidated rock rather than the alluvium.
The probability of drawdown exceeding 2 m in the alluvium is very small and very restricted in area. Within 5 km of the mine footprint of the larger mines, there is a 5% probability of exceeding 2 m drawdown due to additional coal resource development, however, this does not propagate more than 2 km from the alluvium boundary.
In the Pilliga forest area, the probability of exceeding 2 m drawdown due to additional coal resource development is limited to the area close to Narrabri South. The probability of exceeding 0.2 m drawdown due to additional coal resource development has a wider spatial extent and is up to 10 km from the planned CSG developments.
An important outcome from BA is identifying the main sources of uncertainty and the opportunities for improving regional-scale groundwater modelling in the Namoi subregion. The qualitative uncertainty analysis highlighted that the implementation of the CRDP has the highest potential to impact on the predictions, particularly in situations where the CRDP becomes out of date if identified coal resource developments do not proceed or new coal resource developments are proposed. Other factors that may have potential impacts on the model predictions include choice of model parameterisation and resolution of the model grid. This regional model is suitable only for quantifying cumulative drawdown impacts on a regional scale. Local models developed using detailed representation of the local geology are more appropriate for making drawdown predictions around individual mines.
Opportunities to improve the groundwater model can be directed to better constraining the assumptions that have the most influence on model results. The Namoi subregion groundwater model is a stochastic regional-scale model: it has a large modelling domain and a relatively coarse model resolution. While it does not provide the level of lithological and hydrogeological information that is represented in local-scale groundwater models built for small areas within the Namoi subregion, it enables a probabilistic assessment of the model results. This provides precisions for model outputs, which makes it highly suitable for a risk analysis.
The results of groundwater numerical modelling for the Namoi subregion inform product 3-4 (impact and risk analysis). Estimates of the probability of hydrological changes due to coal resource development will be used to assess direct and indirect impacts on ecological, economic and sociocultural water-dependent assets, where ecological assets are grouped into landscape classes and incorporate ecosystems such as groundwater-dependent ecosystems.
- 22.214.171.124 Methods
- 126.96.36.199 Review of existing models
- 188.8.131.52 Model development
- 184.108.40.206 Boundary and initial conditions
- 220.127.116.11 Implementation of the coal resource development pathway
- 18.104.22.168 Parameterisation
- 22.214.171.124 Observations and predictions
- 126.96.36.199 Uncertainty analysis
- 188.8.131.52 Limitations
- Currency of scientific results
- Contributors to the Technical Programme
- About this technical product