- Bioregional Assessment Program
- Namoi subregion
- 2.3 Conceptual modelling for the Namoi subregion
This product presents a conceptual model for the Namoi subregion in the Northern Inland Catchments Bioregional Assessment. In bioregional assessments (BAs), conceptual models are developed to describe the causal pathways, the logical chain of events ‒ either planned or unplanned ‒ that link coal resource development with both potential changes to water resources and ecosystems at or near the land surface. Conceptual models are abstractions or simplifications of reality. The essence of how key system components operate and interact is outlined in the conceptual model for the Namoi subregion.
Summary of key system components, processes and interactions
The geology of the Namoi subregion is central in explaining the location of coal mining and coal seam gas (CSG) operations. The coal resources under development in the Namoi subregion are primarily in the geological Gunnedah Basin with the main economic coal seams in the Black Jack Group and the Maules Creek Formation. 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.
Coal mining and coal seam gas (CSG) operations can induce changes in groundwater level in the vicinity of operations. This may change the magnitude or direction of flow between groundwater and surface water systems.
The main near-surface aquifers in the Namoi subregion are associated with the alluvial sediments along the Namoi River and its larger tributaries (Mooki and Peel rivers, Coxs and Pian creeks). Other aquifers include porous sedimentary strata in the Great Artesian Basin (GAB) and the Gunnedah Basin.
The main surface water resource of the Namoi subregion is the Namoi River. It drains an area of approximately 42,000 km2 flowing from east to west from its headwaters in the Great Dividing Range. Except for the Namoi River all Namoi river basin waterways are either ephemeral or intermittent. The Peel River is now permanent because of releases from Chaffey Dam. Prior to regulation it was also temporary.
In the Namoi subregion the potential impact of hydrological changes due to coal resource development is investigated in ecosystems at the land surface. Dividing the Namoi subregion into landscape classes enables a structured approach for assessing these potential impacts. The landscape classification describes the main ecological and human systems (including agricultural production systems, and industrial and urban uses), and provides a high-level conceptualisation of the subregion at the surface.
In the Namoi subregion, 29 landscape classes were derived and allocated to one of six landscape groups based on broad-scale distinctions in their water dependency and association with floodplain or non-floodplain environments, groundwater-dependent ecosystems (GDEs) and remnant or human-modified habitat types.
The landscape groups are as follows:
- ‘Floodplain or lowland riverine’
- ‘Non-floodplain or upland riverine’
- ‘Dryland remnant vegetation’
These landscape groups are expressed as a percentage of the preliminary assessment extent (PAE). The PAE is the geographic area where potential impacts on water-dependent assets due to coal resource development are assessed. Landscape classification shows the following:
- The ‘Human-modified’ landscape group, which includes agricultural, urban and other intensive land uses, comprises 59% of the PAE.
- The ‘Dryland remnant vegetation’ landscape group, which is not considered to be water dependent, comprises 24% of the PAE.
- Approximately 6% of the PAE is covered by the ‘Floodplain or lowland riverine’ landscape group and includes the lowland riverine systems and the adjacent landscape classes associated with the riparian and backplain environments.
- Approximately 10% of the PAE and almost 72% of the watercourses are included in the ‘Non-floodplain or upland riverine’ landscape group, which includes a large extent of vegetation classified as groundwater dependent.
Coal resource development
Potential hydrological changes due to coal resource development are quantified in the Namoi subregion for two potential futures:
- baseline coal resource development (baseline): a future that includes all coal mines and CSG fields that are commercially producing as at 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 results between CRDP and baseline is the change that is primarily reported in a BA. This change is due to the additional coal resource development – all coal mines and CSG fields in the Namoi subregion, including expansions of baseline operations, that are expected to begin commercial production after December 2012.
The CRDP for the Namoi subregion consists of six baseline coal mines and ten additional coal resource developments. Under the baseline, there are five open-cut coal mines: Boggabri Coal Mine, Rocglen Mine, Sunnyside Mine, Tarrawonga Mine and Werris Creek Mine; and one longwall mine: Narrabri North. The ten additional coal resource developments are Boggabri Coal Expansion Project, Caroona Coal Project, Gunnedah Precinct, Maules Creek Project, Narrabri South, Tarrawonga Coal Expansion Project, Vickery Coal Project, Vickery South Coal Project, Watermark Coal Project and Narrabri Gas Project. Eight of these additional coal resource developments are modelled for the Namoi subregion, with the remaining two mines, Vickery South Coal Project (open-cut coal mine) and the Gunnedah Precinct (open-cut and underground), not being modelled due to insufficient information. Analysis of the impacts of these two developments will be restricted to commentary in product 3-4 (impact and risk analysis).
BHP’s proposed development of the Caroona Coal Mine was discontinued in August 2016. As per companion submethodology M04 the CRDP was not revisited and the timeline described for the project will continue to form the basis for modelling hydrological changes and include Caroona Coal Mine. The CRDP needs to be viewed as an indicative future that highlights potential changes for water resources and water-dependent assets and areas where further investigation may be warranted.
Hazard analysis and causal pathways
A hazard analysis identified the potential hydrological changes due to coal resource development. In the Namoi subregion a hazard analysis workshop was held in May 2015, where participants identified and scored detailed hazards using the Impact Modes and Effects Analysis (IMEA) process to systematically identify coal resource development hazards with the potential to change the hydrology. Individual hazards constitute causal pathways and can be aggregated by common impact cause and impact mode into four causal pathway groups:
- ‘Subsurface depressurisation and dewatering’
- ‘Subsurface physical flow paths’
- ‘Surface water drainage’
- ‘Operational water management’.
The causal pathway groups under the CRDP are the same for open-cut and underground coal mines and the CSG operations.
The Narrabri Gas Project is targeting the main coal seams in the lower Permian Maules Creek Formation within the Gunnedah Basin. All four causal pathway groups are relevant. Specifically, there is the potential for connectivity between the target coal seams and the surface and near-surface aquifers through the stratigraphic formations. Agricultural bores accessing the alluvial aquifers and the Pilliga Sandstone aquifer in the GAB may have reduced flow if the hydrostatic pressure change from CSG depressurisation propagates vertically through the overlying sequence. Propagation of the depressurisation cone may be impeded or enhanced by faults and the presence or absence of aquitards. If an aquitard is thin or absent, then this may enhance propagation of a depressurisation cone laterally or vertically. To the east of the Narrabri Gas Project area, the Gunnedah Basin strata, including the target coal seams, are in direct contact with the overlying alluvium and associated aquifers. If propagation of the depressurisation cone were to extend this far to the east, then there is the potential for overlying aquifers to have reduced pressure.
The mines in the CRDP located in the Mullaley sub-basin (Narrabri North and South, Watermark, Sunnyside and Gunnedah Precinct) primarily target the Hoskissons Coal in the upper Permian Black Jack Group. These mines are overlain by Surat Basin strata and alluvium. In the south of the Mullaley sub-basin around the Watermark development, the Surat Basin strata are not present.
The Maules Creek, Boggabri (including expansion), Tarrawonga (including expansion), Vickery Project (including Vickery South) and Rocglen mines located in the Maules Creek sub-basin are targeting the coal seams in the Maules Creek Formation. There is the potential for interaction of mine dewatering with the Upper Namoi alluvium and the Namoi River.
The Werris Creek Mine is located in the Werrie Basin targeting the coal seams in the Willow Tree Formation. The Werrie Basin is hydrologically isolated from the Gunnedah Basin as the Hunter-Mooki Thrust Fault System is considered to be an impermeable boundary to regional groundwater flow. For this reason Werris Creek groundwater is not incorporated in the groundwater modelling for the Namoi subregion. The surface waters around the Werris Creek Mine flow into the Namoi River, and so form part of the surface water modelling.
In summary, causal pathways in all four causal pathway groups have the potential to cause hydrological changes due to additional coal resource developments in the Namoi subregion. All four causal pathway groups are relevant for all six major landscape groups.
The following knowledge gaps were identified in the development of the conceptual model:
- There are limited long-term, consistent surface water quality and quantity data, which are required for developing models that can predict water quality into the future.
- There is a lack of detailed understanding of the interaction between the surface water and groundwater systems, particularly at the local level.
- The CRDP for the Namoi subregion was confirmed and ‘locked in’ for this BA as of December 2015; therefore, any project-related changes since then have not been reflected in the CRDP presented in this product. For example, BHP’s proposed Caroona Coal Mine is discontinued but still part of the CRDP in this BA. Given the current early stage of the mine developments, the provided mining scheduling and production rates are estimates only.
- 2.3.1 Methods
- 2.3.2 Summary of key system components, processes and interactions
- 2.3.3 Ecosystems
- 2.3.4 Coal resource development pathway
- 2.3.5 Conceptual modelling of causal pathways
- Currency of scientific results
- Contributors to the Technical Programme
- About this technical product