About the subregion

The Namoi subregion, within the Northern Inland Catchments bioregion, covers approximately 29,300 km² in the Murray–Darling Basin in NSW (Figure 2).

The subregion is home to about 27,000 people, with the main centres of Gunnedah and Narrabri located along the Namoi River. Agricultural land covers 77% of the subregion, with irrigated agriculture a large component. The Namoi alluvium supports highly valuable agricultural development that includes cropping of cotton and grains, and livestock grazing on the less arable soils.

The assessment extent (the total area investigated in this assessment) is about 35,660 km² (Figure 2).

The main near-surface aquifers in the Namoi subregion are associated with the alluvial sediments along the Namoi River and its larger tributaries, the Mooki and Peel rivers, and the Coxs and Pian creeks (Figure 2 and Figure 3). Other aquifers occur within the Great Artesian Basin (GAB) and the Gunnedah Basin.

The Namoi River is the subregion’s main surface water resource. It drains about 42,000 km² from its headwaters in the Great Dividing Range. Except for the perennial Namoi River, all streams are temporary.

The Liverpool Plains contain endangered native grasslands and riparian vegetation with dominant river sheoaks and willows, and river red gum communities along the larger streams. A wide range of aquatic habitats, including large areas of anabranch and billabong wetlands downstream of Narrabri, add to the ecological importance of the Namoi subregion.

To study potential impacts on these ecosystems, the assessment developed a landscape classification to categorise the main natural and human-modified ecosystems in the assessment extent (Box 7), based on the subregion’s geology, geomorphology (physical features), hydrogeology (the way water moves through porous rocks), land use and ecology. See ‘What are the potential impacts of additional coal resource development on ecosystems?’ for more information.

The assessment also investigated potential impacts on assets based on their ecological, economic or sociocultural values (O’Grady et al., 2015). These include ecosystems such as the state forest of the Pilliga (the largest remaining area of dry sclerophyll forest west of the Great Dividing Range in NSW) and the Pilliga Nature Reserve in the upper catchment of Bohena Creek (the largest reserve in the region). See ‘What are the potential impacts of additional coal resource development on water-dependent assets?’ for more information.

Coal resource development

When the CRDP was finalised in December 2015, it included ten additional coal resource developments that were proposals for new or expanded operations (Figure 2 and Figure 3). Of these, the hydrological modelling incorporated eight additional coal resource developments:

The remaining two mines, Vickery South Coal Project (open-cut) and the Gunnedah Precinct (underground and open-cut), did not have sufficient information to model. The impact and risk analysis provides a commentary on these two developments (Section 3.6 in Herr et al. (2018a)). The assumed timeline of construction and production for each additional coal resource development is shown in Figure 4.

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 Project remained part of the modelling even though it is no longer proceeding. As of July 2017, the Shenhua exploration licence for the Watermark Coal Project was reduced by 51.4% to exclude the Liverpool Plains. However, this did not change the operational mining area and therefore the hydrological modelling of the Watermark Coal Project remains relevant.

The coal resources under development in the Namoi subregion are primarily in the Gunnedah Basin, with the main economic coal seams located in the Black Jack Group and the Maules Creek Formation. Two regional-scale block diagrams illustrate the geology associated with baseline and additional coal resource developments (Figure 5 and Figure 6). The Werris Creek Mine is located in the Werrie Basin, adjacent to the eastern side of the Gunnedah Basin. The target coal seams for the Werris Creek Mine are in the Willow Tree Formation (Figure 7). The Hunter-Mooki Thrust Fault System isolates the groundwater in the Werrie Basin from the Gunnedah Basin. For this reason, groundwater modelling did not include the Werris Creek mine.

More information about the coal and CSG resources, mines and proposed developments are in the coal and CSG resource assessment (Northey et al., 2014).

Figure 2 Coal mines and coal seam gas tenements in the coal resource development pathway and the major modelled streams

The coal resource developments in the coal resource development pathway (CRDP) are the sum of those in the baseline coal resource development (baseline) and the additional coal resource development. Three developments are both baseline mines and additional coal resource developments (ACRD): Boggabri, Tarrawonga and Narrabri North are baseline mines, while Boggabri expansion, Tarrawonga expansion and Narrabri South are additional coal resource developments (ACRDs). The Vickery South Coal Project and the Gunnedah Precinct, both ACRDs, did not have sufficient information for hydrological modelling.

Data: Geoscience Australia (Dataset 2), Santos (Dataset 3)

Figure 3 Close-up of part of the Namoi subregion, showing named watercourses in the vicinity of baseline and additional coal resource developments

The coal resource developments in the coal resource development pathway (CRDP) are the sum of those in the baseline coal resource development (baseline) and the additional coal resource development (ACRD).

Data: Geoscience Australia (Dataset 2), Santos (Dataset 3)

Figure 4 Timelines for modelled coal resource developments in the coal resource development pathway

These timelines were used for the hydrological modelling. The coal resource developments in the coal resource development pathway (CRDP) are the sum of those in the baseline and the additional coal resource development. Two additional coal resource developments (Vickery South Coal Project and Gunnedah Precinct) could not be modelled due to insufficient data, hence they are not shown on this timeline.

Figure 5 Schematic diagram of the south-east Namoi subregion from Quirindi to Gunnedah showing underlying geology relative to coal resource development

The coal resource developments in the coal resource development pathway (CRDP) are the sum of those in the baseline and the additional coal resource developments (ACRD).

Figure 6 Schematic east-west diagram of the Namoi subregion from Gunnedah to Wee Waa showing underlying geology relative to coal resource development

The coal resource developments in the coal resource development pathway (CRDP) are the sum of those in the baseline and the additional coal resource development (ACRD). CSG = coal seam gas

Figure 7 Cross-section of the Werrie Basin

The location of the cross-section is shown as an orange line in the inset map. Fm = formation

Source: derived from Pratt (1996)