For the impact assessment in Stage 3 of the GBA Program, a causal network was used to assess the potential impacts of unconventional gas resource development on water and the environment in the Beetaloo GBA region. This assessment was informed by the Stage 2 baseline synthesis and gap analysis that presented knowledge about the geology, and prospectivity for unconventional gas resources, water resources, protected matters and potential impacts to water and the environment in the Beetaloo GBA region.
The impact assessment identifies pathways of ‘very low concern’ where impacts are not physically possible or are extremely unlikely (having an estimated probability of less than 1 in 1,000), ‘low concern’ where impacts can be avoided by current legislation or because the impact does not represent a material change. If impacts can be minimised or mitigated by existing management controls, pathways are evaluated as of ‘potential concern’. Impacts from pathways of ‘potentially high concern’ cannot be avoided or mitigated at the scale of the GBA region ( Table 2 ).
The impact assessment, based on current knowledge, found that there are no pathways of ‘potentially high concern’ in the Beetaloo GBA region. Furthermore, all potential pathways to impacts due to unconventional gas resource development identified in the Beetaloo GBA region can be mitigated when existing regulatory and management controls are implemented.
The pathways of ‘potential concern’ are primarily related to activities that create a disturbance at the surface (transport of materials and equipment, civil construction, decommissioning and rehabilitation, and seismic acquisition) ( Figure 3 , Figure 13 ). The pathways of ‘potential concern’ connect these activities with the protected matters, protected fauna and terrestrial vegetation endpoints, reflecting how surface disturbance has the potential to impact these endpoints. These potential impacts are mitigated through environmental management plans that are intended to ensure that appropriate environmental management practices are identified and implemented during the various stages of an activity. There is high confidence that there are mitigation strategies in place for these potential impacts.
Pathways associated with subsurface activities – such as drilling, hydraulic fracturing, production of hydrocarbons – are of ‘low concern’ or ‘very low concern’ ( Figure 4 , Figure 7 ). The assessment has found stressors of high community concern, such as those involving well integrity or chemical spills, are unlikely to cause material changes to endpoints when existing regulatory and management controls are implemented.
Water extraction
Groundwater is expected to be the source of water for unconventional gas resource development in the Beetaloo GBA region. Conservative groundwater modelling results indicate that it is possible to supply this from the Cambrian Limestone Aquifer without any adverse regional impacts, including to the Roper River or Mataranka Thermal Pools. There is a high degree of confidence for this finding. At a local scale, pathways leading to groundwater drawdown are of ‘low concern’, except where an extraction bore is situated within 1 km of an existing bore. This impact is mitigated through regulations on the extraction of water within 1 km of an existing water bore. Reductions in spring flow, baseflow and impacts upon groundwater- dependent ecosystems due to groundwater extraction are of ‘very low concern’.
Water quality and availability
Surface activities may affect the flow of water by diverting or modifying flow pathways, causing erosion or through sedimentation. Overland flow obstruction and vegetation removal may result in reduced surface water availability or an increase in surface water contamination and is of ‘potential concern’ to small areas where the activities are conducted in the vicinity of waterways. There is high confidence that mitigation strategies related to civil construction will limit any potential impacts on water-related endpoints.
Conservative chemical transport modelling of spills and leaks due to accidental release leading to unconfined aquifer contamination showed ‘potential concern’ only where the depth to groundwater is less than 14 m, which only occurs in very limited parts of the Beetaloo GBA region. Accidental release, which requires a spill to enter the environment beyond containment, is of ‘potential concern’ for water-related endpoints if a spill reaches them. However, evidence suggests that accidental releases of material volumes are very unlikely because of existing avoidance and mitigation strategies.
As the disposal of hydraulic fracturing waste to surface water or groundwater is prohibited in the Northern Territory, it is not possible for controlled release of wastewater from hydraulic fracturing activities to generate impacts on surface water or aquifers. Stringent approval and management requirements, including national guidelines, Northern Territory Government regulations and industry waste management plans, mean that contamination due to waste disposal is of ‘low concern’ in the Beetaloo GBA region.
Aquifer contamination due to creation of new fractures or widening of existing faults or factures (compromised subsurface integrity) is of ‘low concern’ to ‘very low concern’ throughout the Beetaloo GBA region except in the area where the Hayfield sandstone member is prospective. In that case, there is potential for development to impact on confined aquifers, below the Cambrian Limestone Aquifer, that can be mitigated through existing engineering controls.
Environment
Terrestrial vegetation (predominantly rain-fed open woodlands) dominates the Beetaloo GBA region. Activities at the surface such as civil construction, seismic acquisition, transport of materials and equipment and decommissioning and rehabilitation ( Figure 13 ) may cause material changes in vegetation extent and condition. Certain pathways are of ‘potential concern’ and warrant more detailed, local-scale assessment, in particular, those pathways involving invasive plants and insects, vegetation removal and vehicle movement leading to impacts on environment and fauna endpoints. Existing management controls can avoid and mitigate these potential impacts.
The Beetaloo GBA extended region has small areas of wetlands (3%) and riparian vegetation (2%) that have high ecological value and are habitat for many species. The key potential impacts for wetland condition and riparian vegetation extent and condition are from civil construction (for example, roads, well pads or storage tanks) and any activity that requires vehicle movement beyond designated roads, such as seismic acquisition. However, the likelihood of impacts in these ecosystems are mitigated by limiting the amount of activity that occurs within them.
Protected fauna
From north to south in the Beetaloo GBA region the climate changes from tropical to semi-arid. Tropical savanna species are found in the north (for example, the Gouldian finch) and typical Australian desert species in the south (for example, the greater bilby). As the region becomes more widely surveyed, more species are expected to be added to biodiversity records.Any decline in persistence of the 6 species assessed is assumed to be material.
Water controls the ecology and movements of the 6 protected fauna quite differently. Pathways of ‘potential concern’ between activities and the protected fauna species are primarily related to activities that create a disturbance at the surface (civil construction, decommissioning and rehabilitation, transport of materials and equipment, and seismic acquisition, Figure 13 ).The pathways of ‘potential concern’ connect these activities with key stressors, namely invasive plants and animals and vegetation removal, resulting in mortality of native species and habitat degradation, fragmentation and loss. Common processes within these pathways that are of ‘potential concern’ are competition and predation, and ecosystem burning.
Causal network for the Beetaloo GBA region
The causal network illustrates the complex and interconnected nature of the natural environment and unconventional gas resource development activities in the region to be assessed ( Figure 5 , Figure 6 ). The network allows evaluation of how undertaking these activities may cause potential stress on the environment. The evaluation considers how the existing regulatory controls and operational practices can mitigate potential impacts. Mitigation and management practices are most effective at the activity–to–stressor step of a causal pathway, whereas links from stressors to natural processes and from natural processes to endpoints are often difficult, if not impossible, to mitigate.
The causal network assessment is supported by the systematic and transparent evaluation of confidence. Confidence in the assessment is generally high where the available knowledge base supports the (i) cause-and-effect relationship, (ii) threshold of material change and (iii) availability or effectiveness of mitigation strategies along a pathway. Where there is insufficient knowledge to support a robust and meaningful evaluation of the cause-and-effect relationship or threshold of material change, the precautionary principle is applied so that uncertainty about potentially serious hazards does not lead to underestimation of impacts. Potential impacts to water and the environment from unconventional gas resource development are identified but only adverse impacts are assessed.
A key assumption in the causal network assessment is that the complex ecological and hydrological systems in the region are adequately represented by the causal network. An advantage of the causal network is that it allows experts in unconventional gas resource development activities, environmental risk assessment, ecology, geology and hydrology to interact in a common framework. This reduces the chance of omission of important parts of the assessed system. The causal network for the Beetaloo GBA region has 62 nodes, connected by 197 links, with 2,078 causal pathways.
The causal network for the Beetaloo GBA region uses a development scenario to provide activity- or scale-dependent inputs to the network. The scenario assumes peak production of 365 petajoules per year (or 1,000 terajoules per day), over a 25-year time period, with a maximum of 1,150 wells (at 4 to 10 wells per pad). The estimated maximum area disturbed in this scenario by access tracks and well pads is between 8 and 35 km 2, contained within an area between 430 and 7,700 km 2. This scenario would require 21 to 46 gigaliters of water over the 25- year time period, based on an estimate of 40 megalitres for drilling and hydraulic fracturing per well. The impact assessment for the Beetaloo GBA region considers activities associated with the development and production of unconventional gas resources in a generic way and at a regional scale for the duration of the development scenario.
Using causal networks for impact assessment
The causal network approach to environmental impact assessment provides a comprehensive and clearly identified set of deduced direct and indirect pathways where unconventional gas resource development activities may impact on environmental values (endpoints in the network). Consistent and systematic evaluation of likelihood, consequence and mitigation strategies allows integration of qualitative and quantitative information, even when the available knowledge base is limited. The evaluation of the causal network is based on the understanding of the individual nodes and the links between them. All nodes and links are evaluated independently and individually before being combined to assess the pathways between development activities and the endpoints. Detailed context, evaluation and assessment accessed in GBA Explorer produce a robust assessment and improve objectivity. Changes in knowledge can be incorporated in the causal network. The assessment is a spatial evaluation, allowing areas where impacts on water and the environment are unlikely to be identified, as well as where potential impacts cannot be ruled out.
GBA Explorer
The GBA Program has developed an innovative way to allow stakeholders to visualise and interact with the assessments’ evidence base and evaluations; through the GBA Explorer . This unique tool allows the possibility for the causal network and its assessment to be readily updated as new information becomes available.
Mitigation
Mitigation of potential impacts identified in the causal network assessment occurs through environmental management practices (for example, activity-specific Environmental Management Plans required under the Petroleum Act 1984 (NT)). Environmental Management Plans are intended to ensure that appropriate environmental management practices are identified and implemented during the various stages of an activity. Once approved, an Environmental Management Plan becomes an enforceable or statutory document. The Environmental Management Plan must also incorporate the requirements prescribed in the Code (Northern Territory Government, 2019c). Additional regulation of petroleum activities, particularly related to drilling and hydraulic fracturing operations, are contained in the Schedule of Onshore Petroleum Exploration and Production Requirements (Northern Territory Government, 2019a). For example, Well Operations Management Plans that include requirements to identify and manage well integrity risks throughout the life of a well. Well Operations Management Plans must also incorporate the requirements prescribed in the Code (Northern Territory Government, 2019c).
In the impact assessment these environmental management requirements for mitigation, along with those required through other Northern Territory Government and Australian Government regulatory instruments, and industry practices, were used when evaluating the cause-and-effect relationships. There is a high degree of confidence in the overall assessment that there are no causal pathways in the Beetaloo GBA region that cannot be mitigated.
