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2.3 Conceptual modelling for the Maranoa-Balonne-Condamine subregion

Executive summary

Condamine river weir on Darling Downs in Queensland, 2005 Credit: Arthur Mostead © Commonwealth of Australia (Murray–Darling Basin Authority)

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 and potential impacts on water resources and water-dependent assets in the Maranoa-Balonne-Condamine subregion. A BA considers the difference in results between two potential futures:

  • 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. For the Maranoa-Balonne-Condamine subregion, however, ‘baseline’ includes all CSG developments in the subregion that are reported in the 2014 annual report for the Surat cumulative management area
  • 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 due to the additional coal resource development (ACRD) – all coal mines and CSG fields, including expansions of baseline operations, that are expected to begin commercial production after December 2012.

Methods

This product details the conceptual model of causal pathways of the Maranoa-Balonne-Condamine subregion, following the method described in in companion submethodology M05 on the development of conceptual models of causal pathways. It identifies:

  • the key system components, processes and interactions, which essentially define pathways over and through which water can move
  • the ecosystems in the subregion in terms of landscape classes and their dependence on water
  • the potential hydrological changes that may occur due to coal resource development in the subregion by describing and documenting the baseline and CRDP, including a summary of water management for coal resource developments
  • hazards from coal resource development using the Impact Modes and Effects Analysis (IMEA) hazard analysis approach
  • causal pathways from coal resource development through to hydrological changes, both for baseline and CRDP.

The conceptual model of causal pathways for the Maranoa-Balonne-Condamine subregion leverages existing state-based resources and knowledge of geological, surface water, groundwater and ecological conceptual models. Notably, the geological and groundwater conceptual models are consistent with the Office of Groundwater Impact Assessment (OGIA) regional groundwater model developed in 2012. The OGIA model was revised for BAs to also simulate water-related impacts of coal mine development in the regional groundwater systems. The Assessment team refined the landscape classification and high-level conceptualisation of the causal pathways following discussions at the ‘Conceptual modelling of causal pathways’ workshop held in July 2015.

Summary of key system components, processes and interactions

The spatial extent of the water flows and pathways is limited to the preliminary assessment extent (PAE) of the Maranoa-Balonne-Condamine subregion. The PAE is the geographic area associated with a subregion or bioregion in which the potential water-related impact of coal resource development on assets is assessed. The PAE includes mapped outcrop areas of the Hutton, Clematis and Precipice sandstone aquifers and along the Dawson, Moonie and Weir rivers that are outside the subregion boundary. Coal mine and CSG operations in the subregion target the Walloon Coal Measures of the Surat and Clarence-Moreton geological basins, which has the potential to directly affect the regional groundwater and surface water systems. Hydrological changes to the groundwater system associated with coal resource development can propagate through the alluvium and other aquifers to indirectly affect surface water – groundwater interactions in the aquifer outcrop and subcrop areas.

Important groundwater systems in the subregion are the surficial alluvial, basalt and Great Artesian Basin (GAB) sandstone aquifers. The Condamine Alluvium and Main Range Volcanic aquifers are important water sources for irrigation, stock and domestic and town water supplies. The GAB aquifers of the Surat Basin that underlie most of the subregion are part of one of the largest groundwater systems in the world and are a major water source for stock and domestic and town water supplies. River basins in the PAE are the Border, Condamine-Balonne (including the Maranoa River), Fitzroy and Moonie rivers. Most river systems are temporary, with the exception of parts of the Dawson River, which receive baseflow contributions from rejected GAB recharge in the aquifer outcrop areas. The temporary nature of the surface water systems means that the management, treatment and disposal of water associated with coal resource development can affect surface water quality and surface water – groundwater interactions.

Ecosystems

The ecosystems in the Maranoa-Balonne-Condamine subregion are classified in terms of landscape classes and their dependence on water. Landscape classification is used to characterise the diverse range of water-dependent assets into a smaller number of classes for further analysis. Where appropriate, the approach has built on and integrated with existing classification systems. This classification identified 34 landscape classes, which are aggregated into 5 landscape groups based on broad-scale distinctions in their water dependency and association with GAB or non-GAB GDEs (groundwater-dependent ecosystems), floodplain/non-floodplain or upland/lowland environments and remnant/human-modified habitat types:

  • ‘Floodplain or lowland riverine (including non-GAB GDEs)’
  • ‘GAB GDEs (riverine, springs, floodplain or non-floodplain)’
  • ‘Non-floodplain or upland riverine (including non-GAB GDEs)’
  • ‘Dryland remnant vegetation’
  • ‘Human modified’.

The BA team finalised the landscape classification following discussion with stakeholders at the ‘Conceptual modelling of causal pathways’ workshop held in July 2015.

Most of the PAE (72%) is classed under the ‘Human-modified’ landscape group that includes agricultural, urban and other intensive land uses. In the remainder of the PAE most of the landscape falls into the ‘Dryland remnant vegetation’ landscape class (20%) and the landscape groups ‘Floodplain or lowland riverine (including non-GAB GDEs)’ (5%), ‘Non-floodplain or upland riverine (including non-GAB GDEs)’ (2%) and ‘GAB GDEs (riverine, springs, floodplain or non-floodplain)’ (1%). The ‘Dryland remnant vegetation’ landscape class is not considered to be water dependent. The stream network is classed as ‘Floodplain or lowland riverine (including non-GAB GDEs)’ (48%), ‘Non-floodplain or upland riverine (including non-GAB GDEs)’ (39%) and ‘GAB GDEs (riverine, springs, floodplain or non-floodplain)’ (13%). Most springs and spring complexes in the PAE of the Maranoa-Balonne-Condamine subregion (86%) are associated with GAB aquifers. Aspects of water dependency and the vegetation communities associated with each landscape group are discussed.

Baseline and coal resource development pathway

The CRDP for the Maranoa-Balonne-Condamine subregion includes five baseline open-cut coal mines, five baseline CSG fields and two additional open-cut coal mines. The CRDP is based on information available as of July 2015 and was finalised after feedback provided at the CRDP workshop held in December 2014. The five baseline coal mines are: Cameby Downs Mine, Commodore Mine, Kogan Creek Mine, New Acland Coal Mine Stage 2 and Wilkie Creek Mine (which ceased operations in December 2013). The two proposed coal mines are: New Acland Coal Mine Stage 3 and The Range. To ensure consistency with OGIA reporting, the baseline includes all CSG projects in the Maranoa-Balonne-Condamine subregion that are reported in the 2014 annual report for the Surat cumulative management area. The five baseline CSG projects are: Australia Pacific LNG Project, Queensland Curtis LNG Project, Santos Gladstone LNG and Santos Gladstone LNG Gas Field Development projects, Surat Gas Project and Ironbark Project.

Water affected by open-cut coal mines in the subregion is typically treated for discharge off site and for use in mine-related activities such as dust suppression and coal washing. Water management for CSG operations is consistent with the Queensland Government’s Coal Seam Gas Water Management Policy. This policy aims to strategically manage co-produced water by prioritising its beneficial use by new and existing users as well as water-dependent industries; it proposes treatment and disposal of co-produced water in a way that firstly avoids, and then minimises and mitigates, impacts on environmental values. Reverse osmosis is the preferred water treatment technology of the four major CSG operators in the subregion (Arrow Energy Pty Ltd, Origin Energy Limited, QGC Pty Limited and Santos Ltd).

Hazard analysis

A dedicated hazard analysis, IMEA, is used to systematically identify activities that may initiate hazards, defined as events, or chains of events that might result in an effect (change in the quality or quantity of surface water or groundwater). A large number of hazards are identified; some of these are beyond the scope of BA and others are adequately addressed by site-based risk management processes and regulation. The main hazards considered for BA in the Maranoa-Balonne-Condamine subregion associated with CSG operations are potential impacts on aquifers, impacts associated with storage, processing and disposal of treated water: raised groundwater levels, soil salt mobilisation and leaching from storage ponds. Hazards associated with open-cut coal mines include the potential to link, or cause leakage between, aquifers, deliberate pit wall dewatering, subsidence and enhanced aquifer interconnectivity caused by post-closure water filling the pit.

Causal pathways

Hazards associated with CSG operations and open-cut coal mines that are considered to be in scope for BA in the Maranoa-Balonne-Condamine subregion are grouped into four main causal pathway groups:

  • ‘Subsurface depressurisation and dewatering’
  • ‘Subsurface physical flow paths’
  • ‘Surface water drainage’
  • ‘Operational water management’.

The ‘Subsurface depressurisation and dewatering’ causal pathway group associated with the CRDP has the potential to directly affect the regional groundwater system, and indirectly affect surface water – groundwater interactions in aquifer outcrop areas.

The ‘Subsurface physical flow paths’ causal pathway group involves physical modification of the rock mass or geological architecture by creating new physical paths that water may potentially infiltrate and flow along. This hazard may occur due to hydraulic fracturing of coal seams or when the integrity of wells drilled for groundwater or gas extraction is compromised. The cracking that occurs in the rock mass above underground longwall panels may also subsequently cause enhanced hydraulic connection. The extent of these changes is likely to be restricted to aquifer or aquifer outcrop areas within tenements, but can also affect connected watercourses within and downstream of tenements. Incomplete knowledge of the location of subterranean faults and fractures of the geological layers means that uncertainty exists in the precise spatial extent of groundwater level decline due to subsurface depressurisation and dewatering. However, the uncertainty analysis reported in companion product 2.6.2 (groundwater numerical modelling) for the Maranoa-Balonne-Condamine subregion allows a probabilistic estimate of maximum groundwater level decline.

The ‘Surface water drainage’ causal pathway group contains the most frequent hazards associated with CSG operations and open-cut coal mines. Intercepting surface water runoff, altering surface water system and subsidence of land surface can change, or disrupt, surface water drainage in the PAE. Effects on surface water direction, volume and quality can have medium-term (5 to 10 years) to long-term (10 to 100 years) cumulative effects on watercourses within and downstream of tenements.

The ‘Operational water management’ causal pathway group involves the modification of water management systems and may have cumulative effects on surface water catchments and stream networks, surface water – groundwater interactions and groundwater conditions. Effects are likely to be in the medium to long term and include watercourses in aquifer outcrop areas that are within and downstream of tenements.

Gaps

The combination of limited water quality and quantity data availability and use of a groundwater model that is focused on the deeper regional aquifer, limits the value of developing a coupled surface water – groundwater numerical model in the Maranoa-Balonne-Condamine subregion at this time.

Further work

The causal pathways described in this product guide how the modelling (product 2.6.2 (groundwater numerical modelling)) is conducted and how product 3-4 (impact and risk analysis) is framed for the Maranoa-Balonne-Condamine subregion.

Last updated:
11 July 2017
Thumbnail of the Maranoa-Baloone-Condamine subregion
PRODUCT FINALISATION DATE
26 May 2016
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