Groundwater Monitoring Well Decommissioning Procedures

Groundwater monitoring well decommissioning is the final stage in the bore lifecycle. It is also one of the most consequential. When a monitoring well has reached the end of its useful life, permanently sealing it protects groundwater resources, removes a potential contamination pathway and satisfies regulatory compliance obligations. This article covers when and why wells should be decommissioned, accepted sealing methods, equipment removal, documentation and the environmental risks of getting it wrong.

Why Monitoring Well Decommissioning Matters

To decommission boreholes or monitoring wells means to permanently seal them so they no longer form a conduit for vertical groundwater movement, cross-aquifer contamination, or surface water ingress. In practical terms, the process involves removing headworks and instrumentation, filling the bore with appropriate sealing materials, and restoring the surface to suit the intended land use.

Poorly abandoned wells can undermine years of hydrogeological work. An unsealed annulus or open casing allows preferential flow paths between aquifers, degrades water quality, and compromises the integrity of surrounding soil and geological structures. What was once a carefully constructed monitoring point becomes a liability.

In Australia, well decommissioning is regulated under state-based legislation and national guidance. In Western Australia, decommissioning is regulated under the Rights in Water and Irrigation Act 1914, and must follow the Minimum Construction Requirements for Water Bores in Australia (4th Edition, 2020). State EPA guidelines and Department of Water and Environmental Regulation requirements also apply. Proper decommissioning protects groundwater resources from contamination, and failing to complete it correctly can result in enforcement action, remediation orders, or ongoing liability.

Monitel approaches decommissioning as part of a complete monitoring lifecycle. Sound bore construction, reliable instrumentation, and disciplined long-term monitoring all influence how effectively a well can be sealed at the end of its service. Planning for decommissioning from the outset is not optional-it is essential.

When Should Groundwater Monitoring Wells Be Decommissioned?

Timing is driven by project risk, regulatory conditions, and the physical condition of the bore. Groundwater monitoring bores provide access to underground aquifers, and that access must be managed responsibly when it is no longer needed.

Project completion is the most common trigger. Examples include closure of a mine pit where post-closure monitoring has confirmed stable water quality over five or more years, completion of a road or rail embankment where settlement monitoring confirms stability, or the end of a contaminated land remediation program where monitoring data demonstrates objectives have been met.

Redevelopment or infrastructure conflicts require decommissioning when land use changes-industrial to residential, for instance-or when new foundations, basements, pipework, or services corridors would conflict with existing wells. Piling and dewatering programmes may also require removal of nearby monitoring bores.

Damaged or non-compliant wells present a different set of concerns. Casing sheared by vehicle strike, corroded steel risers, collapsed screens, or wells that were installed under older standards and no longer provide reliable data all warrant decommissioning. A bore that is no longer functional serves no monitoring purpose and only presents risk.

Temporary works wells-those installed for short-term construction dewatering verification, ground movement assessment, or similar-are typically decommissioned once the permanent structure is commissioned and the bore is longer needed.

Regulatory triggers include expiry of environmental licence conditions, direction from regulatory authorities following a risk review, or conditions in site management plans requiring decommissioning within a set period after the last sample (commonly within six months). When a bore is longer functional or no longer required, the obligation to seal it promptly is critical.

Regulatory Compliance and Standards for Well Decommissioning

Well decommissioning is a regulated activity across all Australian jurisdictions. Non-compliant closure can result in enforcement, fines, remediation orders, or breach of licence conditions.

The primary national reference is the Minimum Construction Requirements for Water Bores in Australia (4th Edition, 2020), which includes explicit decommissioning requirements covering sealing methods, materials, and documentation. Installation must comply with these Minimum Construction Requirements for Water Bores, and so must decommissioning.

At the state level, WA’s Water Quality Protection Guidelines No. 1 (for mining and mineral processing) provide specific direction for mine site monitoring bores. These guidelines require that bores intersecting confined aquifers be backfilled with cement grout, with casing annulus sealed from above the slotted section to approximately 1.5 metres below ground surface. Similar guidance exists in South Australia, where a minimum 5 m depth surface seal is required for all decommissioned bores.

Approvals or notification may be required before decommissioning wells associated with Part IV or Part V EP Act approvals, mining proposals, or contamination management plans. In some states, notification to the relevant regulatory authority is required 60 days prior to decommissioning.

Licensed Water Well Drillers typically perform groundwater well decommissioning, and a bore decommissioning report must be submitted to the relevant regulatory authority after completion. Documentation expectations include method statements, risk assessments, and post-works reports. These records must satisfy auditors, regulators, and future asset owners.

Monitel plans decommissioning in consultation with clients’ hydrogeologists and environmental managers, ensuring that decommissioning aligns with broader regulatory compliance obligations-including groundwater monitoring programs, closure plans, water licences, and groundwater extraction permits.

Environmental and Structural Risks of Improperly Abandoned Wells

Environmental protection is the primary reason for formal decommissioning procedures. The environmental risks of leaving a monitoring well improperly sealed are well documented and can persist for decades.

Vertical migration is perhaps the most significant risk. An unsealed annulus allows saline or contaminated water from one aquifer to migrate into another. In coastal WA, where salinity intrusion is a genuine concern, unsealed bores can deepen flow paths for seawater to penetrate inland or into deeper freshwater aquifers. Annular seals prevent vertical leakage and contamination between aquifers-and when those seals are absent or degraded, the bore becomes a direct conduit.

Contaminant pathways are created when legacy wells bypass low-permeability clay layers or confining units. Hydrocarbon plumes, industrial contaminants, or agricultural chemicals can migrate through an unsealed bore to impact sensitive receptors such as wetlands, groundwater dependent ecosystems, rivers, or production bores. Decommissioning prevents contamination and groundwater migration risks.

Surface contamination and safety hazards arise when open or poorly capped wells allow stormwater, debris, fauna, or accidental spills to enter the ground. There are also physical trip and fall hazards on active or redeveloped sites.

Structural and geotechnical issues can develop over time. Piping or erosion along the casing, subsidence around a poorly backfilled hole, and interaction with nearby foundations or buried services all present risk. Surrounding soil can be destabilised where voids form around deteriorating casing.

Long-term liability is often underestimated. Poorly decommissioned wells can trigger future claims, costly site investigations, and remediation obligations decades after the original project has closed. New landowners discovering unsealed bores face expense and delay that could have been avoided through proper closure.

Pre-Decommissioning Assessment and Planning

Thorough assessment underpins safe, compliant decommissioning. The method selected for sealing must be based on accurate knowledge of the bore’s construction and the site conditions surrounding it.

Key data to review includes original bore construction logs, the drilling method used, casing and screen details (casing and screens are typically made from uPVC or stainless steel), filter pack and annular seals materials, total depth, known stratigraphy, and the hydrogeological conceptual model. If the bore was properly developed using techniques such as surging and low-flow pumping, this should be recorded. It is also worth confirming whether monitoring bores achieved parameter stability before sampling, as this speaks to the reliability of the data collected.

Current condition must be confirmed through a site survey: location and level (top of casing and reduced level), measured total depth, inspection for blockages, deformation, or collapsed zones, and identification of any in-hole devices or instrumentation.

Risk assessment should consider whether the site is contaminated, the proximity of receptors (production bores, rivers, wetlands, residential areas), and the presence of nearby buildings or critical infrastructure.

Method selection depends on bore diameter, depth (a shallow 20 m environmental well requires a different approach to a 150 m mine dewatering observation bore), lithology (unconsolidated sands versus fractured rock), groundwater pressures, and whether artesian conditions are present. Each of these factors will determine the sealing materials and placement method used.

Monitel coordinates this assessment with clients’ hydrogeologists to ensure decommissioning integrates with the site’s conceptual model and future land use plans.

Accepted Sealing and Grouting Methods

The objective of sealing is to create a continuous, low-permeability barrier that removes the well as a hydraulic and contaminant pathway. Boreholes must be sealed with grout or bentonite during decommissioning, and the method must suit both the geological conditions and the regulatory framework.

Pressure grouting via tremie pipe is the standard method for deeper or submerged sections. Neat cement grout, cement–bentonite mixtures, or high-solids bentonite slurry is placed from the bottom of the bore upward, displacing water and minimising the risk of voids. Cement-based sealing materials must meet ASTM C150 standards. Sealing mixtures should use drinking-water quality water for hydration to avoid introducing contaminants.

Bentonite-based methods use sodium montmorillonite pellets or chips for shorter sections. Bentonite expands significantly when hydrated, enhancing sealing performance. However, bentonite seals should not be used where roots may invade or where highly saline or chemically aggressive groundwater may compromise swelling. Annular seals usually consist of bentonite or cement-bentonite, and sealing materials for boreholes include bentonite pellets or chips.

Material selection depends on site conditions. Neat cement grout provides greater strength in fractured rock or where structural integrity matters. Cement–bentonite mixes reduce shrinkage and lower permeability. Bentonite-only seals may be acceptable for shallow environmental wells where strength is less critical and chemical conditions are favourable.

Staged sealing may be required in multi-aquifer wells, with horizon plugs placed across confining layers to maintain aquifer isolation. This is designed in line with the original bore construction and hydrostratigraphy, using bridging materials or cement plugs at specific depth intervals.

Curing and verification involves allowing adequate setting time, monitoring grout returns at the surface, and comparing the volume of grout placed with the theoretical annulus volume. Simple volume checks can identify whether voids or incomplete filling have occurred.

Step-by-Step Decommissioning Procedure for Monitoring Wells

The following procedure provides a logical overview of groundwater monitoring well decommissioning procedures, guiding engineers and site managers through each stage.

Site preparation and safety comes first. The work area must be isolated, traffic managed (particularly in heavy-vehicle corridors on mine sites or transport corridors), utilities located, and spill control and waste management measures established. Safety planning should account for the specific hazards present at each site.

Removal of headworks involves cutting and removing surface monuments, concrete pads, protective casings, and covers. Accurate survey references should be maintained where needed for future records. Casing and obstructions must be removed to properly seal a borehole during decommissioning.

Internal equipment removal follows. Pressure transducers, vibrating wire piezometers, data logger cables, tubing, and any other devices are extracted. The bore is then flushed or bailed to remove sediments and debris before sealing.

Bore conditioning involves measuring current total depth, clearing any obstructions using appropriate methods, and confirming whether the bore is open to its original drilled depth. If collapse has occurred, the reduced depth must be documented and the sealing plan adjusted.

Seal placement uses a tremie pipe from the base upward. Grout or bentonite is placed continuously, with minimal interruption, to avoid segregation or bridging. The process should follow the staged approach described earlier where multiple aquifers are intersected.

Surface reinstatement completes the procedure. A shallow surface plug is constructed where required, followed by replacement of topsoil, pavement, or hardstand. Surface restoration is a requirement after well decommissioning, and the restored surface must suit the future land use or project handover requirements. For a straightforward single borehole, decommissioning can take one day.

Removal and Management of Monitoring Equipment

Groundwater monitoring wells often contain high-value and sensitive equipment that must be recovered before decommissioning.

Submersible level loggers, pressure transducers, and multi-parameter water quality sondes should be retrieved with care. Calibration records and serial number traceability must be maintained for each instrument. Automated monitoring reduces manual site visits and improves data reliability, so instruments in good condition can often be redeployed on other projects.

Some instruments, such as vibrating wire piezometers grouted within the bore, may not be fully retrievable. Where extraction is not possible, the presence of the instrument must be documented and the sealing design adjusted accordingly.

Final data downloads from loggers are critical. Last readings must be archived, and decommissioning dates linked to the project’s monitoring database for future audits and groundwater modelling.

Waste classification and disposal must follow relevant state guidelines. Electronic waste, contaminated equipment, and inert materials should be segregated. Any equipment that has been exposed to contaminants on an impacted site requires handling as classified waste.

Monitel’s focus is on preserving data integrity through to the last measurement and ensuring instruments are either redeployed or disposed of responsibly.

Documentation, Record-Keeping and Reporting

Robust documentation is essential for audit-ready regulatory compliance and future site understanding. Decommissioning records are required for regulatory compliance in every Australian jurisdiction.

Key records include:

• Original and final bore depth
• Casing and screen details (materials, diameters, screen intervals)
• Decommissioning date, personnel, and contractor details
• Type and composition of grout or bentonite used
• Volumes placed and depth intervals for different backfill or seal materials
• Any deviations from the planned method

Survey data should capture final surface levels, coordinates, and their relation to the site grid or Australian Height Datum where relevant to hydrogeological models.

Photographic evidence-before and after images of headworks removal, sealing activities, and final reinstated surface condition-adds verification and supports future audits.

Monitel’s deliverables typically include a formal decommissioning report or addendum suitable for submission to regulators, environmental auditors, and asset owners.

Integration with Broader Groundwater Monitoring and Site Closure

Decommissioning should be planned as part of the broader groundwater monitoring strategy, not treated as an isolated task at the end of a project. It connects directly to closure planning and long-term environmental protection objectives.

Well decommissioning must be sequenced with the end of monitoring programs, final sampling rounds, and verification of groundwater models-for example, confirming post-dewatering recovery at a mine or tunnel project before removing the monitoring network.

Coordination with other ground and structural monitoring systems is important. Inclinometers, settlement arrays, and structural health sensors may still be collecting data at the time bore decommissioning is being considered. Data continuity must be preserved until risk criteria are met.

Future data requirements should also be a consideration. Replacement bores may be needed, sentinel wells may need to remain active, and key datasets should be preserved for long-term performance assessment rather than lost during the decommissioning process.

Monitel’s Role Across the Full Lifecycle of Groundwater Monitoring Wells

Effective well decommissioning begins with good bore construction, appropriate instrumentation, and reliable long-term monitoring. Each stage of the lifecycle influences the next.

Monitel supports clients from initial groundwater monitoring network design, through drilling oversight, installation of sensors and telemetry, automated data acquisition, and reporting for regulatory compliance. When monitoring objectives are met or infrastructure changes demand it, Monitel assists with planning and executing compliant decommissioning procedures-including method selection, site supervision, and preparation of audit-ready documentation.

By engaging Monitel across the full monitoring lifecycle, project teams can be confident that their monitoring wells are designed, operated, and ultimately decommissioned in a way that safeguards groundwater resources, protects water quality, and reduces long-term environmental liability. Monitel’s broader geotechnical instrumentation capabilities ensure that decommissioning decisions are made within the context of the complete monitoring programme, not in isolation.

To discuss upcoming bore and well decommissioning monitoring or other requirements, speak with Monitel’s engineering team.