Anglesey Aluminium Remediation, Treatment of Three Distinct Hydrocarbon Plumes by Geo 2 Remediation Ltd

Shortlisted for Brownfield Awards Category 5 - Best Remediation of a Smaller Site

Decades of heavy industrial activity on the former Anglesey Aluminium site had left a legacy of significant contamination, with diesel leaks from underground tanks, oil spills from industrial compressors and fire damage incidents leading to losses of non-PCB transformer oils

Detailed characterisation by LK Consult Ltd (LKC) and pilot testing by Geo2 Remediation Ltd (Geo2) enabled the design of an integrated in-situ treatment solution aimed at addressing not one contaminant plume, but three separate impacts covering an area of <0.5 hectares.

The remediation strategy ensured a cost-effective solution (totalling £450,000) was adopted, incorporating not just best practice measures, but enabling the best feasible remediation process to be implemented, providing confidence to the client and regulator of the highest quality remediation.

The approach incorporated Multi-Phase Vacuum Extraction (MPVE) to remove Light Non-Aqueous Phase Liquid (LNAPL) from the groundwater; pumping to recover dissolved phase contamination and chemical injection. Treatment targeted saturated soils and residual groundwater within the complex geological setting of made ground, superficial gravels and an underlying fractured schist bedrock.

This combination of techniques successfully removed all detectable LNAPL and achieved a reduction in dissolved hydrocarbon concentrations, in all areas, of greater than 95%. As a result, regulatory approval of the works was secured and the development potential of the site was unlocked.

 

Characterisation
 

The former Anglesey Aluminium site looms large over the island and its iconic stack can be seen from the hills of Snowdonia. Regeneration of which promises to change the site from the UKs largest electricity consumer (until its closure in 2009), into a sustainability focussed Eco-Park, including a biomass power station, bringing employment and investment to the area.

Detailed investigation identified three areas of concern. A plume was encountered around a former Compressor House where up to 750mm of lubricating oil was pooled on the groundwater and a 2,500m2 plume of diesel, associated with a former Above Ground Storage Tank (AST). Vertical delineation exposed contaminant spread within fractures into the underlying schist.

A second impact resulted from a fire in a transformer within an electric substation, known as the Rectifier Yard, converting AC to DC for the smelting process. Migration over years had spread the PCB free LNAPL up to 700mm in maximum thickness, with an associated dissolved plume extending across an area of approximately 1,500m2. Due to health and safety concerns, this area had previously been inaccessible, however recent decommissioning works opened the area for remediation.

A third, smaller impact, approximately 200m2, was associated with a diesel spill at a vehicle fuelling station. Following removal of the underground storage tanks (USTs) a dissolved phase plume remained in the superficial gravels across the former forecourt and had migrated vertically into the bedrock close to the tank farm.

 

The developed conceptual model and Detailed Quantitative Risk Assessment (DQRA) for each of the identified impacts. Proposing a Remediation Strategy to prevent contamination to the underlying aquifer and minimise risk to the nearby controlled waters.

This robust design, focused on best feasible treatment, balancing the needs of the client whilst embracing the SuRF UK approach to sustainable remediation; minimising offsite disposal, avoiding demolition (of buildings with a value to the final development) and ensuring treatment within the schist bedrock, provided sufficient confidence to be fully supported by the regulator, National Resources Wales (NRW).

Implementation

To build confidence in a suitable and resilient remediation, Geo2s first steps were to undertake treatability trials from the core of the plume in key areas, the recoverability of the NAPL and groundwater was tested together with lab scale trials of chemical performance within the substrate. Frozen cores were obtained for dissection to establish detailed contaminant partitioning so that targeted remediation could take place. Destructive testing using six oxidant mixtures was trialled to optimise the chemical injection design to treat all three contaminant types on the site.

The geology of the site comprised a thin made ground cover, overlying coarse schist gravels and sands within a clay matrix and clay horizons, in turn overlying fractured schist bedrock, encountered at varying depths across the site , from 1m bgl in the Garage area, to 6m bgl in the Compressor House. In each instance the bulk of the contaminant plume was associated with key high permeability flow zones typically between 3-5m bgl within the superficial deposits, but also intersecting with fracture networks within the bedrock, in some areas, up to 10m bgl.

The overall similarity in ground conditions across the site meant that a single treatment strategy, varied to suit the differing geochemistry and contaminant loading, could be applied in all three areas.

Deployment of Geo2s Environmental Permit ensured protection of the environment, beyond the mitigation of the known groundwater impacts. Compliance and competence was central to the implementation of this strategy, preventing spills or leaks, minimising emissions and governing discharges to coastal waters, working with NRW throughout the whole process.

Implementation of this combination of techniques allowed a phased treatment utilising modular plant. This enabled rapid redeployment of the plant following successive treatment of each remote impact on this vast site, minimising the cost of the works, whilst prioritising treatment in areas to suit the development programme.

 

 

Treatment
 

Following the high definition pilot works, treatment in the Compressor House and Rectifier Yard comprised LNAPL removal using a Multiphase Vacuum Extraction (MPVE) plant. This technology applies a high vacuum across the surface of the groundwater across the plume, using, large diameter, high-porosity remediation wells to recover the LNAPL by establishing a negative pressure gradient within the targeted flow zones. This approach successfully recovered all measurable NAPL within 6 weeks, with a further 6 weeks of pumped groundwater recovery minimising the risk of rebound and achieving a mass recovery of dissolved phase contamination.

Following NAPL removal, an intensive chemical injection programme, comprising two successive applications of Fentons based chemical oxidation, activated by naturally elevated FeII within groundwater, was undertaken in all three areas. The oxidant was mixed into a solution at the desired concentration by in-line dosing equipment and injected under pressure using high pressure pumps (up to 180psi) driving the liquid through the superficial matrix and into the fracture zone, maximising direct contact oxidation. By focussing the bulk of the treatment-agent into the most impacted areas, the injection effectively targeted the residual source. Contamination in the schists demonstrated the presence of vulnerable fracture zones, high-pressure injection of oxidant exploited the same flow pathways treating the dissolved phase impact within the bedrock.

Oxidant injection was repeated in each area within two weeks of the first application to capitalise on the rapid oxidising kinetics and the partitioning of soil-sorped contaminant into the groundwater. This extended the period of active treatment, b promoting a sustained reaction and allowed for oxidation of contaminants desorbed and mobilised by the first injection, to achieve optimal contaminant destruction. Each injection initially targeted the downgradient and peripheral plume extents, establishing a curtain of oxidants encircling the impact, before spiralling inwards towards the source, eliminating the potential for uncontrolled escape.

A total of 720m3 of oxidant was applied in the Compressor House, with a further 600m3 in the Rectifier Yard and 80m3 in the Garage areas. These volumes were determined based on the results of the detailed treatability trials, considering the impacts of the matrix oxidant demand as well as the contaminant concentration and distribution on oxidant consumption.

Application of oxidants in this manner could be a hazardous pursuit and as such Geo2 designed an automated dosing plant, mixing an appropriate concentration solution of hydrogen peroxide within a closed system. These engineered systems eliminated many of the most serious potential risks to staff, enabling the entire process to be managed, controlled and implemented using only a small team of trained WAMITAB accredited operatives.

Optimisation

Oxidant application causes a widespread desorption effect, where soil sorbed contamination from both the saturated and unsaturated zone is mobilised, generating an initial surge in dissolved phase concentrations and a potential rebound of LNAPL contamination in monitoring wells. Rather than this undermining the effectiveness of the initial phases of treatment a targeted, intensive application of the MPVE plant following oxidant reaction, was used to optimise the approach allowing rapid recovery of any un-oxidised mobilised contamination, a technique developed by Geo2 over recent years. This achieved a rapid and substantial mass recovery from areas which may not have been effectively treated by pumping alone.

The MPVE approach recovered 1,711m3 of impacted groundwater in the Compressor House and 2,146m3 in the Rectifier Yard.

The Garage area displayed the least pre-treatment impact, however the presence of shallow competent schists (<1m bgl) proved challenging for oxidant application. To ensure remedial goals were achieved within a tight timescale in this scheduled development area, Geo2 amended the remediation design and supplemented the initial oxidant application with a targeted dose of an Oxygen Release Compound building upon the enhanced aerobic conditions within the groundwater (resulting from the degrading oxidant), this slow-release chemical acts to enhance the rate of continued biodegradation of hydrocarbons over a period of up to 12 months. This approach ensured renewed and ongoing reductions in the post treatment sampling, combating the post-oxidation rebound and achieving remedial objectives.

Results
 

Application of the high vacuum recovery system achieved removal of all measurable LNAPL in each area. The subsequent chemical injection totalled an application of 1,400m3 of oxidant over a six-week period across the three areas. Oxidant application was tailored in each location to the differing contaminant type, concentration and distribution, and achieved reductions in TPH concentration in excess of 95% in diesel, transformer oil and compressor oil impacts, within the superficial deposits.
 

  • Reductions in each area were achieved from peaks concentrations of 710,000µg/l in the diesel and lubricating oil impacted Compressor House to a worst case 806µg/l;

  • 4,400,000µg/l in the transformer oil impacted Rectifier Yard to worst case 2,012µg/l; and,

  • 120,000µg/l to 1,787µg/l in the diesel impacted Garage area.
     

Maximum concentrations in the deeper aquifer were comparatively low (peaking at 4,600µg/l), with improvements in the schists in excess of 75% in both the Compressor House and Garage areas.

Comparing the results of the treatment against the 80-90% improvements anticipated in the chemical oxidant pilot trials, demonstrates that the treatment has been highly successful. The addition of the post chemical oxidation groundwater mass recovery serving to ensure a higher degree of treatment than would have anticipated by the LNAPL removal and oxidant application alone.

 

Outcome
 

Validation sampling undertaken by Geo2 and LKC, over a minimum period of three months following completion of treatment, exhibited sustained improvements and steady state conditions in each area of the site. The strength of the data and quality of the groundwater monitoring was sufficient to demonstrate to the regulator, NRW, that the objectives of the remedial works had been achieved.

The characterisation of these plumes and site-specific pilot test work, coupled with close consultation between Geo2 Remediation Ltd and LK Consult Ltd ensured that a robust, but cost-efficient remediation design was implemented. By building upon sound science, robust site investigation and using site-specific modelling to confidently characterise the plumes, the approach was optimised by Geo2 using innovative application techniques to maximise the benefit of an integrated treatment approach, delivering best practice and exceeding expectation.

These remediation works represent one of the first key steps in regenerating this high-profile industrial site. Sustainable remediation sets the agenda for an eco-friendly development which promises to transform a legacy of contamination and divert towards this site’s future following redevelopment as a green energy producer.

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