In‑situ Thermal Remediation and Multi‑phase Extraction at Temple Island, Bristol
Shortlisted for Best Application of Remediation Technologies
At Temple Island, adjacent to the River Avon, lays a fractured bedrock, impacted with light non-aqueous phase liquid (LNAPL). To remediate this conventionally would be slow, inefficient and/or environmentally risky. Using highly targeted, adaptive in‑situ thermal remediation and multi‑phase extraction provided a fast, effective and sustainable solution.
Temple Island has a long legacy of industrial use as a railway diesel maintenance depot. This contaminated the subsurface and throughout various fracture networks of the underlying Mercia Mudstone and Redcliffe Sandstone with a viscous, oily, hydrocarbon.
Several phases of environmental works had been completed at the site since 2020, including removal of contamination in shallow soils and contamination associated with accessible NAPL. However, some remained in the ground prior to the proposed redevelopment.
Sanctus Limited, supported by Remediation Technologies (RemTech), successfully secured the enabling work for the redevelopment by submitting a technical bid to address the remaining source – pathway – receptor (SPR) linkage, as follows:
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Partition residual, largely immobile LNAPL in groundwater within Mercia Mudstone and Redcliffe Sandstone. The presence of LNAPL presented a long-term source of total petroleum hydrocarbon (TPH) impact on the groundwater.
The core remedial objectives associated with the remaining SPR were defined as:
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Recover residual NAPL that remains within the bedrock and associated fractures systems, (following previous remediation phases by others) beneath localised areas of the site to a reasonably practicable extent.
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Reduce potential liabilities associated with hydrocarbon impact in the underlying secondary aquifer and mitigate potential risks associated with the lateral migration of potentially mobile residual NAPL towards the identified water resource receptors through planar and fracture systems associated with the bedrock.
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Achieve conditions that should be supportive of overall betterment in hydrocarbon dissolved phase water quality through reduction of the LNAPL which may represent an ongoing source of hydrocarbons to groundwater via vertical leaching and when in direct contact with groundwater.
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Remedial works will be undertaken to the extent practicable and in line with the sustainable remedial objectives as set out in the CL:AIRE, Sustainable Remediation Forum (SuRF) UK (Ref. 10); and,
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Remedial works will be designed and implemented to achieve regulatory sign-off including demonstratable and evidence-based achievement of remedial targets to discharge town planning conditions related to encountered impacts and remediation of impacts.
The preferred remedial approach was identified as in-situ thermal remediation (ISTR) with dual phase extraction (DPE). An optimisation study was also proposed to enhance the remedial process.
Sanctus Ltd has a long-standing track record of unlocking the developmental potential of complex brownfield sites, transforming them into development platforms for homes or commercial spaces. Sanctus worked in partnership with RemTech, a contractor specialising in complex in-situ remediation solutions. This project demonstrated how specialist experience and the right remedial technology can successfully remediate a persistent contaminant in challenging geological conditions.
Optimising Site Remediation with Data
Several phases of environmental works had been completed at the site, giving a wealth of information about aquifer parameters, contaminant distribution and local groundwater and surface water interaction plus the Mercia Mudstone and Redcliffe Sandstone geology. The effectiveness of the remediation relied not on the volume of data, but on the ability to interpret and apply it intelligently.
1. Review Existing Information
To optimise the remediation method, all of the available site investigation data was thoroughly reviewed. The project team then established the following:
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Contamination was largely associated with very specific fracture networks.
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A geological model could be developed to plan the remediation vertically and laterally across the site.
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The expected changes in contaminant behaviour as the temperature is increased in the ground could be predicted.
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The most appropriate drilling methods to construct the most efficient extraction networks for remediation.
2. A Phased and Targeted Approach
Rather than applying remediation across the entire affected area in one deployment, the work was split into three zones, defined by hydrogeological characteristics and contaminant distribution. The benefits of this phased approach included:
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Continued optimisation and field refinement of the technology over time, using the performance results being collected to improve efficiency in each consecutive zone.
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Reduced power demand for thermal infrastructure when operating smaller, discrete zones. A green energy supplier provided mains electric rather than relying on hydrocarbon powered plant such as boilers or generators.
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Minimised the need for largescale remediation equipment.
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Freed up areas of the site for ground improvement and preparation earlier in the enabling programme.
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Enabled earlier construction of the required utility corridor.
3. Innovative Construction and Extraction Boreholes
Having thoroughly evaluated the geological information for the site, the team identified that open wells provided improved hydraulic connectivity with fracture networks compared to conventional screened boreholes. This would enhance recovery efficiency by allowing better interception of the connected fracture network. The team specified a number of requirements at drilling stage to optimise the remediation:
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Combine open-hole drilling with geophysical testing to gather specific information on the location, continuity, connectivity and orientation of fracture networks.
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Collect core samples to inspect and assess on-site how contamination varied with depth.
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Build a vertical contaminant profile from the core samples to inform the vertical heating and vapour recovery screens.
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Design simple headworks to seal the wells at the surface, prevent ingress of material, and anchor the remediation infrastructure.
Using open boreholes and mapping the fractures and contaminant distribution in each extraction well enhanced recovery efficiency by intercepting fracture controlled groundwater and LNAPL pathways. Leaving the well pipe out avoided treating the entire vertical profile uniformly and potentially only extracting from the most permeable fracture, which may not have been contaminated.
4. Extraction Over Discrete Subsurface Horizons
Once the vertical and lateral extents of the contamination had been established, a plan was made to carry out the remediation in each zone. This involved targeting and extracting contaminants over several discrete horizons by:
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Using specialist equipment to isolate the contaminated fractures within the open boreholes and only applying extraction in those horizons.
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Deploying equipment capable of running either dual phase extraction (DPE) or multi-phase extraction (MPE) to accommodate variable recovery scenarios that could be encountered in a fractured geology.
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Only heating the ground at the depths affected by contamination and therefore reducing energy demand.
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Monitoring induced vacuum and groundwater drawdown in sentinel wells to check that the target fracture networks were connected.
This enhanced both process efficiency and environmental protection, while avoiding unnecessary energy input into less‑impacted strata.
Adopting this approach in the discrete zones from the highest hydraulic gradient progressing towards the River Avon removed the risk of rebound. Performance data showed sustained recovery of LNAPL during active remediation, with declining recovery rates falling to asymptotic conditions. The graphs below (see Figure 1) show the cumulative LNAPL recovered across each zone.
Figure 1: The graphs show the cumulative LNAPL recovered across each zone.
LNAPL was removed from all remediation wells and nearby monitoring wells within the remediation zones with no rebound observed. Laboratory data showed an evolving chemical composition for the recovered LNAPL that further demonstrated effective removal of the more mobile and risk relevant fractions from the wider area. Vapour phase monitoring data also showed corresponding reductions in contaminant concentrations over time, confirming effective mass transfer and capture during treatment. Groundwater and surface water monitoring confirmed that remediation activities did not affect the River Avon.
Given the fractured nature of the geology, any minor amounts of residual contamination trapped in closed fractures or fractures that were isolated from the remediation infrastructure giving rise to residual dissolved contamination were not found to warrant further remedial efforts. Further active remediation was not considered to materially change the risk profile, and all reasonable and practicable measures were therefore considered to have been implemented.
Remediation Success
The remediation at Temple Island successfully addressed the residual LNAPL source to the extent that was reasonable and practicable, in line with the agreed remedial objectives and sustainable remediation principles.
Verification Report approval by Bristol City Council (BCC) and the Environment Agency (EA) (see Figure 2) demonstrates that the residual LNAPL no longer presented a barrier to future development.
Compliance and Safety
The preparatory works, remediation and validation were in full alignment with the Environment Agency’s land contamination risk management (LCRM), and the BS 10175 investigation of potentially contaminated sites code of practice. Sanctus held the appropriate bespoke mobile plant environmental permit, abstraction licence and trade effluent discharge consent for the duration of the treatment works. Sanctus also obtained flood risk activity permits (FRAP), a marine licence from the Marine Management Organisation (MMO) and Network Rail BAPA. All boreholes were decommissioned in line with EA guidance. The works were programmed to avoid tidal influence or impacting the local river otter population. These works supported the UN Sustainable Development Goals of ‘life below water’ and ‘clean water and sanitation’
Regarding health and safety, we utilised integrated safety management systems (SMS) to foster continuous improvement. The data collected from hazard reporting, safety observations and leading indicators helped us to achieve a zero-accident frequency rate (AFR). Collaborating with all project stakeholders, we prioritised safety in all project meetings with health, safety, security, or well-being talks. All personnel underwent a sector-specific induction prior to entering the site and Sanctus provided toolbox talks identifying trends on site. The only event that occurred was when someone turned their ankle whilst walking on uneven ground. The event was fully investigated and found that the individual had moved physical barriers and entered a restricted zone without authorisation. Actions were taken to ensure the event did not reoccur.
Stakeholder and Public Engagement
Stakeholders were identified and engaged throughout the project design and implementation phases through a series of planned meetings and engagement events. These included:
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A series of local community engagement events including guided tours and coordination meetings with the University of Bristol and neighbouring contractors within Temple Quarter.
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Weekly and monthly project team progress meetings with BCC, SP, Sanctus Limited, NEC Project Manager (Atkins Realis) and Vattenfall UK.
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Coordination with Vattenfall UK, Wessex Water, National Grid, Bristol Water, Avon Fire & Rescue, BT and BCC Street Lighting for district heating, drainage and utilities.
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Regulatory liaison, approvals and project inspections with BCC Planning Authority and Highways, Natural England, Environment Agency, Marine Management Organisation, Network Rail and Wessex Water.
Social Value
Sanctus achieved >£2.2 million of social value measured and verified against Bristol City Council’s Social Value Portal TOM system. This was 274% of our tender stage commitment. This was achieved through Sanctus teams engaging with local charities and community groups, supporting the Bristol Soup Run, raising money for St Peters Hospice, volunteering at the Bristol Cycling Centre, and participating in Bristol City Council-led environmental projects such as invasive species removal and community planting.
Sanctus further demonstrated strong environmental stewardship by diverting 2,900 tonnes of hard-to-recycle waste from landfill through innovative reuse and processing methods, supporting circular economy principles. In addition, Sanctus created three full-time roles for local, previous offenders and under-represented candidates, providing interview coaching, accessible recruitment processes, and industry-recognised certifications NPORS and CSCS. Sanctus delivered outreach through schools, job fairs, and Job Centres to support long-term career development and add social value within the community. In the process, Sanctus spent over £1.8 million with a diverse range of local MSMEs and supported the UN Sustainable Development Goals of ‘reduced inequalities’, ‘no poverty’, ‘quality education’ and ‘decent work and economic growth’.
Innovation and Best Practice
What sets Temple Island apart is not the use of in-situ thermal remediation alone, but how it was applied. Key elements of best practice included:
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Adaptive targeting of fracture networks, with heating and extraction depths refined to focus energy where it delivered greatest benefit and recovery.
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Dynamic system optimisation, adjusting applied vacuum, temperature profiles and extraction strategies in response to recovery trends over time.
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Multiple lines of evidence used throughout delivery, including mass flux/cumulative mass removal/LNAPL thickness trends/vapour recovery data/groundwater chemistry and surface water monitoring.
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Sustainable‑led decisions, with active remediation ceased where diminishing returns were clearly demonstrated.
This approach used the technology intelligently rather than aggressively, aligning with principles of best available technology not entailing excessive cost (BATNEEC) and Sustainable Remediation Forum-UK (SuRF‑UK) guidance.
Conclusion
The Temple Island project demonstrates how an experienced remediation contractor can add significant value by reshaping how remediation technologies are applied. Applying a risk‑based, optimisation‑led, targeted remediation strategy achieved meaningful environmental betterment while avoiding unnecessary energy usage, disruption and environmental impact. Overall, the site was successfully transitioned from a complex brownfield liability to being in a stable condition, suitable to support future development for this strategically important urban site.
The project highlights the critical role of specialist contractor expertise in bridging the gap between remediation design intent and effective, sustainable outcomes on challenging brownfield sites. Temple Island sets a clear benchmark for best practice in applying data‑led remediation intelligently.