2.3.5 Conceptual modelling of causal pathways

Summary

In this section, the main causal pathways that describe the interaction between various coal seam gas (CSG) activities and water-dependent assets are described for the Richmond river basin, building on the conceptual understanding of geology, hydrogeology and surface hydrology presented in Section 2.3.2 of this product and the assessment of ecosystems and landscape classes presented in Section 2.3.3 .

The coal resource development pathway (CRDP) includes one existing baseline coal mine (Jeebropilly Mine) and one CSG operation (Metgasco’s West Casino Gas Project).

As the assessment of geology and hydrogeology (Section 2.3.2 of this product) confirmed that there is no hydraulic connection between the Richmond river basin and the Bremer river basin (where the Jeebropilly Mine is located), only hazards and causal pathways associated with CSG operations were thus addressed in this section. The hazard analysis was used to systematically identify activities that occur as part of CSG activities in the Clarence-Moreton bioregion and which may initiate hazards, defined as events, or chains of events, that might result in an effect (a change in the quality and/or quantity of surface water or groundwater). A large number of hazards were identified, some of which were beyond the scope of bioregional assessments (BAs), such as accidents. Others are adequately addressed by site-based risk management processes and regulation. Hazards associated with CSG operations deemed to be within the scope of the Clarence-Moreton Bioregional Assessment were grouped into four main causal pathway groups: (i) ‘Subsurface depressurisation and dewatering’ (ii) ‘Subsurface physical flow paths’, (iii) ‘Surface water drainage’, and (iv) ‘Operational water management’.

Knowledge of the spatial context of the surface water and groundwater systems of the Richmond river basin enabled the development of four major conceptual models of causal pathways.

The ‘Subsurface depressurisation and dewatering’ causal pathway group includes coal mine and CSG operations that intentionally dewater and depressurise subsurface hydrostratigraphic units (such as coal seams and aquifers) to permit coal resource extraction. Dewatering applies to coal mines and, given that no coal mines are being modelled in the baseline or CRDP, it will not be discussed further in relation to the Clarence-Moreton bioregion. Subsurface depressurisation associated with CSG development has the potential to directly affect the regional groundwater system at the point, local or regional scales; in addition, it can indirectly affect surface water – groundwater interactions in aquifer outcrop areas. In the Richmond river basin, there are several aquitards such as the Maclean Sandstone and Bungawalbin Member which can potentially prevent wider impacts of depressurisation on overlying aquifers. However, if these aquitards were compromised by faults, the pressure change can be transmitted much quicker with potential impacts extending to the uppermost aquifers. For the ‘Subsurface physical flow paths’ causal pathway group, subsurface physical flow paths can be affected by hydraulic fracturing and compromised well integrity. The extent of these changes is likely to be minor and limited to the vicinity (<1 km) of the compromised well or the location where hydraulic fracturing occurs. ‘Surface water drainage’ is the causal pathway group which was most commonly identified for CSG operations during the hazards assessment of the Clarence-Moreton bioregion. Subsidence, diverting site drain lines, rainwater and runoff diversion, levee bunds and creek crossings can change, or disrupt, surface water drainage. 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. For the ‘Operational water management’ causal pathway group, operational water management can have a cumulative effect on surface water catchments and stream networks, surface water – groundwater interactions and groundwater conditions.

The linkages between the four potential causal pathway groups and the landscape classifications identified in the Clarence-Moreton bioregion are explained. They are underpinned by knowledge of the geology, hydrogeology and surface water – groundwater interactions in the Richmond river basin and are presented in Section 2.3.5.3.1 . Five examples that demonstrate the rationale of those linkages are provided.

Finally, a synthesis section summarises how all chapters of this product link together, and how the conceptual model framework presented in this product will inform subsequent companion products of the Clarence-Moreton Bioregional Assessment.

Last updated:
11 July 2017
Thumbnail images of the Clarence-Moreton bioregion

Product Finalisation date

19 January 2017