Potential impacts on water due to unconventional gas resource development are considered based on 2 sets of impact pathways. The first set are those impact pathways related to water extraction for resource development. The second set of impact pathways relate to potential impacts on surface water and groundwater systems, including springs, that result in changes to water availability or quality. Findings from targeted investigations based on user panel concerns and priority knowledge gaps identified in the baseline synthesis and gap analysis ( Section 7.2 (Huddlestone-Holmes et al., 2020)) are featured in Box 1 , Box 2 , Box 3 , Box 4 , Box 5 , Box 6 ).
Potential impacts related to water extraction for resource development
Groundwater is expected to be the source of water for unconventional gas resource development in the Beetaloo GBA region. The resource development scenario adopted in this assessment – up to 1,150 wells drilled over 25 years – requires an average of between 0.8 and 1.8 gigalitres per year. Groundwater modelling results indicate that it is possible to supply this water from the Cambrian Limestone Aquifer without adverse regional impacts, including to the Roper River or Mataranka Thermal Pools.
At a local scale, conservative numerical modelling (Geological and Bioregional Assessment Program, 2020b, 2021b) was used to estimate groundwater drawdown. Groundwater drawdown is of ‘low concern’, except within 1 km of existing bores sourced from the Cambrian Limestone Aquifer where there is ‘potential concern’ for unconfined aquifer drawdown heading to impacts on those bores. This impact is mitigated through regulations on the extraction of water within 1 km of an existing water bore. Reductions in spring flow and baseflow, and impacts on groundwater-dependent ecosystems are of ‘very low concern’ due to groundwater extraction.
A Northern Territory Government groundwater modelling study assessed the impacts of groundwater extraction from the Cambrian Limestone Aquifer on flow in the Roper River. Results show it is possible to extract an additional 40 gigalitres per year over the current extraction from an area between the Beetaloo GBA region and Mataranka (Bruwer and Tickell, 2015). This suggests that the 1.8 gigalitres per year required under the resource development scenario may be supplied from the Cambrian Limestone Aquifer without significant impact. The water allocation plan currently being developed for the Daly Roper Water Control District and the requirement for licensing of groundwater extraction for petroleum activities will also assist in mitigating these pathways.
Estimating water demand is difficult without knowing the design of hydraulic fracturing operations, the amount of flowback that may be reused or recycled from hydraulic fracturing operations, or the rate of development. The 1.8 gigalitres per year estimate is an average over the 25-year life of the development scenario and annual rates may be higher during the construction phase of a development. The estimate is conservative in that it assumes 40 megalitres of water required for drilling and hydraulic fracturing for each well with no reuse or recycling.
Groundwater extraction from aquifers causes a decrease in pressure and groundwater levels in the pumped aquifer in the vicinity of the production bores and may cause a decrease in groundwater pressure or groundwater levels in over or underlying aquifers. To protect sensitive ecosystems, such as groundwater-dependent ecosystems, prevention or mitigation options are required where predicted drawdown is greater than 0.2 m (DENR, 2020). To ensure groundwater resources are used sustainably, a separate 1 m drawdown threshold was used for aquifer interference with existing water users based on the recommendation in the Pepper inquiry (Pepper et al., 2018).
FIGURE 8 Level of concern for causal pathways from stressors to endpoints for the Beetaloo GBA region
Endpoints are on the x axis and stressors are on the y axis. The colour of the squares shows the level of concern for the pathway, and the size of the squares shows the percentage of the endpoint area potentially impacted. Where a square has multiple colours, it indicates that there are different levels of concern spatially for the endpoint. Stressors are ordered from surface to subsurface and endpoints are ordered from subsurface to surface.
Data: Geological and Bioregional Assessment Program (2021g)
Among the water-related endpoints, increased drawdown due to groundwater extraction that exceeds these thresholds is only of ‘potential concern’ for agricultural productivity and unconfined aquifer condition endpoints within 1 km of existing bores sourced from the Cambrian Limestone Aquifer where drawdown may exceed the 1 m threshold (Methods snapshot, Figure 8 ). This impact is mitigated through regulations ( Water Act 1992) that stipulate authorisation for water extraction for hydraulic fracturing purposes cannot be given within 1 km of an existing bore without landholder agreement or scientific investigation. Groundwater drawdown due to groundwater extraction is of ‘low concern’ for confined aquifer condition and riparian vegetation extent and condition endpoints ( Figure 8 ). Reductions in spring flow and baseflow are of ‘very low concern’ due to groundwater extraction ( Figure 8 ).
Overall, there is high confidence in the assessment of impacts on water. Potential impacts due to groundwater extraction are gleaned from conservative groundwater modelling in the unconfined and confined aquifers based on local information; therefore, there is high confidence in the assessment ( Table 3 ). A water allocation plan currently being developed for the Beetaloo region will regulate groundwater extraction; therefore, there is high confidence in the ability to mitigate potential impacts. Studies on groundwater recharge will inform the development of the water allocation plan ( Box 1 , Box 2 , Box 3 ).
Licensing of the extraction of surface water for any unconventional gas extraction activities in the Northern Territory is prohibited under the Water Act 1992.Furthermore, surface water is an unreliable source of water for hydraulic fracturing in the Beetaloo GBA region as there are no permanent streams within the Beetaloo GBA region.
Methods snapshot: groundwater modelling
The assessment of groundwater extraction and groundwater drawdown ( unconfined aquifer drawdown and confined aquifer drawdown ) is based on analysis of local datasets and analytical modelling (Geological and Bioregional Assessment Program, 2020b, 2021b). These are models that directly estimate groundwater drawdown by considering 3 mechanisms: (i) drawdown from groundwater extraction in the same aquifer, (ii) drawdown from groundwater extraction in an over or underlying aquifer, and (iii) drawdown from depressurisation of an underlying gas reservoir. As detailed scenarios of groundwater extraction locations and volumes are not available, the models simulate groundwater extraction everywhere in the Beetaloo GBA region for 200 megalitres extraction in one year for a single groundwater extraction bore. The predicted quantity of water required to hydraulically fracture 5 wells without any recycling or reuse is 200 ML. The models use local information on layer thicknesses and the presence of faults. They use conservative estimates of hydraulic properties based on regional measurements as local information is not available at the scale required.
Surface activities may affect surface water flows 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 (via erosion and sedimentation) and is of ‘potential concern’ to small areas of the agricultural productivity endpoint (only near existing surface water features), and riparian vegetation extent and condition , wetland condition , surface water condition , and unconfined aquifer condition endpoints. There is high confidence that regulations and mitigation strategies can mitigate potential impacts on water-related endpoints.
Overland flow obstruction includes complete, partial or minor impediment to the movement of water over the landscape. Obstructions can act as a dam or diversion that may partially or completely change the flow path of water. Altered flow paths may lead to impacts on water-related endpoints through a reduction in flow to streams, wetlands, waterholes and sinkholes (a recharge mechanism).
Vegetation removal involves the destruction of above-ground native vegetation. Overland flow obstruction and vegetation removal stressors may both impact on water-related endpoints through increased soil erosion and sedimentation, leading to surface water contamination. These stressors may lead to impacts on endpoints that rely on surface water availability and water quality.These include agricultural productivity (only in the vicinity of existing surface water features provide water stock), riparian vegetation extent and condition , wetland condition , surface water condition where surface water availability and quality are important forthe endpoint function, and unconfined aquifer condition , which may be impacted by decreased recharge through existing surface waterways. There is high confidence that impacts due to overland flow obstruction and vegetation removal can be mitigated by adhering to regulations and mandatory requirements such as site selection for surface activities listed in the Code of practice: Onshore petroleum activities in the Northern Territory (the Code) (Northern Territory Government, 2019c).
Potential impacts related to accidental spills
Spills and leaks due to accidental release could lead to unconfined aquifer contamination This is of ‘potential concern’ in very limited parts of the Beetaloo GBA region where the depth to groundwater is less than 14 m (Geological and Bioregional Assessment Program, 2021f, 2021p). Direct pathways for contamination of unconfined aquifers from the land surface, for example via sinkholes or open karsts, are mitigated by the chemical handling requirements stipulated in the Code (Northern Territory Government, 2019c).
Any accidental release of contaminants beyond an engineered bunding or control into surface waters is conservatively assumed to be material and is of ‘potential concern’ for wetland condition and riparian vegetation extent and condition endpoints. However, compliance reporting of spills and leaks from the Northern Territory and other jurisdictions indicates that existing avoidance and mitigation strategies prescribed in the Code (Northern Territory Government, 2019c) and other Northern Territory regulations are effective in mitigating this pathway.These pathways have been evaluated as ‘low concern’ to ‘very low concern’ for springs.
Community and government consultation during the initial stages of the GBA Program identified potential impacts on water quality ― particularly groundwater ― from the use, handling, and storage of chemicals and flowback waters during extraction of unconventional gas in the Northern Territory as an area of community concern. Chemicals or compounds used or produced in unconventional gas resource development may be unintentionally released to the environment through spills and leaks if they escape any engineered bunding or control ( accidental release ). Hydraulic fracturing , drilling , waste and wastewater management , production of hydrocarbons and processing of hydrocarbons all involve material volumes of chemicals and compounds.
The pathway from accidental release to surface waters is of ‘potential concern’ for wetland condition and riparian vegetation extent and condition endpoints. This is based on the conservative assumption that any spill into surface waters, should it occur, is material. There are limited options to avoid or remediate surface water contamination once it has occurred due to rapid spreading of chemicals through surface water and partitioning to, and accumulation in, sediments (National Research Council, 2000; Eggleton and Thomas, 2004; Jaffé, 1991). Existing controls prescribed in the Code (Northern Territory Government, 2019c) and other Northern Territory regulations can effectively mitigate this pathway.
Unconfined aquifer contamination from accidental release at the land surface is of ‘very low concern’. Where unconfined aquifers are greater than 14 m deep, conservative chemical transport modelling shows that concentrations of contaminants will have dispersed and diluted to levels that are not material (Geological and Bioregional Assessment Program, 2021f, 2021p). Areas where the depth to unconfined aquifers is less than 14 m are limited to the vicinity of Western Creek, where the watertable is shallow within the Antrim Volcanics (the Cambrian Limestone Aquifer is absent or unsaturated), and along Lagoon Creek in the north-east of the Beetaloo GBA region where the watertable sits in the Bukulara Sandstone (and the Cambrian Limestone Aquifer is absent). The vast majority of the area of ‘potential concern’ is outside the Beetaloo GBA region and within the Beetaloo GBA extended region where unconventional gas resource development activities will be limited to major transport corridors and accidental release incidents are less likely.
The Beetaloo GBA region includes karstic formations such as caves, cavities and sinkholes, which may allow for increased vertical flow rates. It is considered unlikely that well pads, storage tanks or other infrastructure will be constructed over undetected shallow karst features due to site characterisation and precautionary testing requirements outlined in the Code (Northern Territory Government, 2019c). Similarly, the likelihood of a truck roll-over into a sinkhole leading to unconfined aquifer contamination is considered very unlikely. Pathways from accidental release to springs ( spring condition) have also been evaluated as ‘low concern’ as activities will not occur close enough to the springs to have an impact.
Potential impacts due to accidental release are primarily managed through existing avoidance and mitigation strategies prescribed in the Code (Northern Territory Government, 2019c) that prevent accidental release occurring in the first place. This includes requirements to store chemicals and compounds within secondary containment (engineered controls), operators to have spill management plans and regular monitoring of chemical storage, compounds and wastewater. Confidence in existing mitigation strategies that reduce the chance of accidental release is high, whereas confidence in pathways leading to surface water contamination is low due to the limited knowledge base available to establish thresholds of material change for each endpoint.
Investigations related to accidental release
Two GBA Programinvestigations have added to the knowledge base forincreasing understanding of the chemicals in flowback water and natural attenuation (natural processes that reduce contaminants in soil or groundwater) of hydraulic fracturing chemicals ( Box 4 , Box 5 ).
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 impact surface water or aquifers in the Beetaloo GBA region.
Stringent approval and management requirements, including national guidelines, Northern Territory regulations and industry waste management plans, give confidence that contamination due to waste disposal is of ‘low concern’ in the Beetaloo GBA region.
Controlled release of wastewater is the intentional release of treated or untreated water into the environment. The release of hydraulic fracturing wastewaterto surface waterwaysor groundwater is not permitted in the Northern Territory so this stressor is not possible. Wastewater is disposed of through evaporation to reduce volumes, with the residual material (including brines and pond liners) disposed of as regulated waste at a licensed waste facility according to the Waste Management and Pollution Control Act 1998 (NT).The Code (Northern Territory Government, 2019c) requires all wastewater to be accounted for along with the methods for monitoring of storages, any reuse or recycling and ultimate disposal.
Waste disposal is the handling, storage, transport and disposal of solid waste materials – excluding wastewater – that result from unconventional gas resource development. Any waste that leaves the site of a petroleum activity must be managed in accordance with the Waste Management and Pollution Control Act 1998 (NT). Waste that cannot be reused, recycled or treated is disposed of in designated waste management facilities. Solid waste includes drill cuttings, which can only be disposed of onsite if strict conditions set out in the Code (Northern Territory Government, 2019c) are followed, including independent certification that on site disposal plans meet required standards.
The stringent approval and management requirements for waste disposal are based on established guidelines (Commonwealth of Australia, 2013b) and the Code (Northern Territory Government, 2019c), which ensure there is high confidence that any contamination can be effectively managed and/or mitigated; therefore, waste disposal is of ‘low concern’.
Potential impacts related to subsurface activities
The 1,204 km2 where the Hayfield sandstone member is prospective for unconventional resources is also the area where there is potential connection with the Bukalara Sandstone, a confined aquifer. In this 4% of the Beetaloo GBA region, aquifer contamination due to creation of new fractures or widening of existing faults or factures (compromised subsurface integrity) is of ‘potential concern’. The Bukalara Sandstone aquifer is not currently used for water in this area as the shallower Cambrian Limestone Aquifer provides a better resource. Outside this small area, the distance separating unconfined aquifers and unconventional gas resources and avoidance and mitigation measures in the Code (Northern Territory Government, 2019c) reduce the level of concern for potential unconfined and confined aquifer contamination to ‘low concern’ and ‘very low concern’. Subsurface activities are of ‘low concern’ for springs.
Creation of new fractures or the widening of existing faults or fractures may lead to compromised subsurface integrity due to hydraulic fracturing or through changes in subsurface stress from gas extraction. There is potential that this could result in confined aquifer contamination and unconfined aquifer contamination .
For an aquifer to be contaminated, there must be a flow path between it and the underlying unconventional gas resource that is the source of contaminants. This flow path may be a fault or fracture. The distance between the aquifer and the reservoir will determine the time it takes for contaminants to reach the aquifer and whether they will be of a concentration to have an impact. Conservative modelling shows that a separation distance of less than 139 m between an aquifer and reservoir may allow contamination of the aquifer by fluid flow up a fault or fracture (see compromised subsurface integrity node description). In the Beetaloo GBA region, the area where the Hayfield sandstone member is prospective for unconventional resources, is also an area where there is less than 139 m separation from the Bukalara Sandstone, a confined aquifer.This aquifer is not currently accessed in this area as the shallower Cambrian Limestone Aquifer provides a better resource. This pathway can be mitigated through industry practices in the design and operation of hydraulic fracturing activities, including geotechnical modelling and monitoring (Kear and Kasperczyk, 2020). There is high confidence that the mitigation strategies required through regulations in the Code (Northern Territory Government, 2019c), including requirements for detailed geotechnical modelling where the interval between the formation being hydraulically fractured and overlying aquifers is less than 600 m, is adequate to avoid, mitigate and manage this risk. Outside this small area, the distance separating unconfined aquifers and unconventional gas resources, and the avoidance and mitigation measures in the Code (Northern Territory Government, 2019c), reduce the level of concern for potential unconfined and confined aquifer contamination to ‘low concern’ and ‘very low concern.’
Causal pathways between subsurface activities and the spring condition endpoint are of ‘low concern’. The springs are too far from the Beetaloo GBA region to be affected by these activities.
Due to government and community concerns about potential impacts of well integrity on water,well integrity throughout resource extraction and once wells are decommissioned was considered in the impact assessment. Well integrity is a fundamental component of petroleum well design and is the main focus of the Schedule (Northern TerritoryGovernment, 2019a) and the Code (Northern TerritoryGovernment, 2019c). Pathwaysrelated to compromised well integrity and compromised decommissioned well integrity are evaluated as having ‘low concern’ to ‘very low concern’. These evaluations are supported by the findings of multiple domestic and international inquiries (for example, Pepper et al. (2018) as summarised in Section 2 of Stage 2 hydraulic fracturing technical appendix (Kear and Kasperczyk, 2020)).
Compromised well integrity refers to the possibility of breaches of a well system that allow the unintended movement of fluids (gas, liquid hydrocarbons and water) into or out of the well. Well integrity is a fundamental component of petroleum well design, and is the main focus of regulations (Northern Territory Government, 2019a, 2019c). Confidence in regulatory process is increased through inclusive participation of stakeholders, implementation of transparent processes and open communication (Huddlestone-Holmes et al., 2017). One of the ways the Northern Territory Government is addressing transparency and open communication is through the publication of documents such as Well Barrier Integrity Verification reports for wells through the POINT website .
Following international best practice, a minimum of 2 independent well barriers – are required by regulation (International Organization for Standardization, 2017; Department of Natural Resources‚ Mines and Energy (Qld), 2019; Northern Territory Government, 2019c) to provide redundancy; such that a failure in one well barrier does not lead to unintended fluid infiltration into geological layers or to the surface. Compromised decommissioned well integrity refers to breaches of a well system after it has been decommissioned. After a well has been decommissioned the requirements for 2 independent well barriers and isolation of the surface and aquifers from each other and hydrocarbon-bearing formations continue to apply. The well barriers must be designed to maintain integrity of the well in perpetuity (Northern Territory Government, 2019c).
Investigations related to subsurface stressors
The amount of available hydrogeological data and understanding varies greatly between different groundwater systems in the Beetaloo GBA region. Investigations undertaken for the GBA Program fill some of these knowledge gaps ( Box 2 , Box 6 ).
FIND MORE INFORMATION
GBA Stage 2 hydraulic fracturing technical appendix (Kear and Kasperczyk, 2020)
Fact sheets are available on the Geological and Bioregional Assessment website
- Fact sheet 3: Assessment of groundwater quality from a possible leak of a flowback storage tank from shale gas operations (Geological and Bioregional Assessment Program, 2021f)
- Fact sheet 19: Recharge processes in the Beetaloo Sub-basin (Geological and Bioregional Assessment Program, 2021n)
- Fact sheet 23: Structural flow implications for unconventional resources exploration, Beetaloo Sub-basin case study (Geological and Bioregional Assessment Program, 2021o)
- Fact sheet 24: Water quality risk assessment from use, handling, and storage of chemicals and flowback at onshore gas operations in the Beetaloo Sub-basin (Geological and Bioregional Assessment Program, 2021s)
- Fact sheet 25: Groundwater sources to the Mataranka Springs Complex (Geological and Bioregional Assessment Program, 2021l)
Product Finalisation date
- At a glance
- Explore this assessment
- Executive summary
- 1 About the assessment
- 2 About the region
- 3 Causal network for the Beetaloo GBA region
- 4 Assessment results
- 5 Additional studies
- 6 Monitoring
- 7 Conclusion
- Contributors to the Program