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Former Gasworks, Creiff

Shortlisted for Brownfield Awards Category 5 - Best Application of Remediation Technologies

VertaseFLI designed and implemented an  innovative and complex remediation strategy to  enable a former gasworks site to be gifted to the  Crieff Highland Games Charitable Trust. This was  successfully undertaken whilst mitigating significant  risks to human health and the water environment  associated with historical contamination. 

The site lies in the heart of the small town of Crieff,  Scotland, and is used each year as the location of the  ancient Crieff Highland Games. Upon completion of  the remediation works, the client will gift the land to  Crieff Highland Games Charitable Trust, to ensure  this important tradition is maintained and enjoyed  at this location for years to come.  

The site is 0.2 hectares in size and was heavily  impacted by the gas works, which operated in the  1800s through to the 1980s.  

Gross gas works contamination was identified  and delineated through extensive in-house  site investigation, and a bespoke two-phase  remediation approach was developed; Phase 1 -  remediation of the source of the contamination  within the unsaturated zone, using fully tailored  soil stabilisation/solidification. Phase 2 – in-situ  remediation works within the saturated zone,  utilising a comprehensive borehole network,  numerous technologies and a staged treatment  train approach that was carefully adapted to the  ever-changing ground conditions.  

VertaseFLI were keen to engage with the local  community regarding this exciting remediation  project and provided an informative letter  drop and conversation opportunity prior to the  commencement of the works. Our site manager opened the site office up each Wednesday evening  for interested community members to visit, have a  cup of tea and chat about all things coal tar. The  local school conducted a site visit for the science  class, for which VertaseFLI sponsored new PPE.  The pupils were introduced to the remediation  world in a genuine site setting, engaging them  in environmental legacy issues and highlighting  the real impact humans can have on our local  environment. The students left excited, engaged  and perhaps, inspired to be the next generation of  brownfield experts.  

Bench-scale trials 

VertaseFLI undertook extensive in-house bench scale trials on the grossly contaminated soils  obtained from the site investigations. Monoliths  were formed and subjected to tank immersion  tests for up to 64 days, which confirmed that  material at the site was amenable to treatment via  Stabilisation/ Solidification (S/S). A bespoke design  mix of Pulverised Fuel Ash (PFA) and Ordinary  Portland Cement (OPC / CEM1) was created, which  was sufficient to generate a material that was  strong and durable and with no matrix dissolution.  This methodology effectively allowed 100% of the  contaminated materials to be retained and reused  on site.  

To determine an effective remediation strategy for  the saturated zone, extensive bench-scale trials  were also undertaken on the neat contamination,  in the form of coal tar or DNAPL, extracted during  trials on site in the investigation stage. Extracted  DNAPL was subject to various tests in order to  assess the most effective way of mobilising this  contamination to allow for maximum recovery.  Trials using various pumping methodologies were undertaken on the site during the investigation  phase, utilising VertaseFLI’s bespoke mobile pilot  trial system. The results of these in situ tests were  used alongside the bench-scale trials to produce  a robust and effective remediation strategy for  treating the saturated zone. 

Phase 1 - Unsaturated Zone 

During Phase 1 works, the primary source  of contamination (gas works infrastructure  including tar tanks, extensive pipework network,  purifiers, scrubbers and washers) was excavated.  Approximately 2,300m3 of heavily impacted material  underwent S/S. This related to 100% of impacted  shallow soils on site. 

The excavation removed contaminated material  to the depth of the water table, at approximately  4mbgl. Any deeper structures identified during the  works were removed below this depth as far as  practicable.  

The S/S treatment was implemented on site following a defined sequence of events dictated by  the bench-scale trials to ensure that the monolith  created was comparable to that produced during  the trial works. Monoliths were regularly sent for  verification sampling throughout the works to  ensure compliance with leachability targets and  geotechnical capabilities.  

The validation results from on site treated material  confirmed a high degree of cementitious binding  (stabilising and solidification reactions being  completed), causing reduced permeabilities,  encapsulation and successful chemical binding of  contaminants of concern within the matrix. 

Chemical validation results were obtained from  the base and sides of each excavation prior to the  treated material being reinstated.  

Our on-site engineer provided a watching brief of  the works, ensuring all material movements were  accurately recorded, along with validation data in a  robust soil audit. 

Stabilisation / Solidification and In-Situ Saturated Zone Remediation of a Former Gas Works, Crieff, Scotland.

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Phase 2 - Saturated Zone 

Phase 2 focused on the residual contamination in the saturated zone. The site lies within a Groundwater Protected  Area, and SEPA indicates that the underlying superficial deposits and bedrock are considered to be a highly  productive intergranular aquifer and a highly productive intergranular/fractured bedrock aquifer, respectively.  A comprehensive network of boreholes was installed across the site, which were designed to target the specific  depth of saturated zone contamination. Sonic drilling techniques were used to ensure accuracy of the borehole  installation and to maximise the effectiveness of the following remediation.  

VertaseFLI built a complex, bespoke DNAPL/ water treatment system in our inhouse engineering facility. The  system was designed to be fully automatic, allowing constant remote monitoring and operation of the system.  This capability ensured that the treatment system could be safely and effectively run 24 hours a day, 7 days a  week in order to maximise recovery. 

Throughout the works, the conditions within the boreholes were continually monitored and recorded, allowing  the methodology to be frequently adapted and optimised to ensure the greatest contamination recovery. Prior  to the saturated phase recovery programme, DNAPL thickness’ of up to 2.8m were recorded in the boreholes.

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Stage one of the saturated zone remediation involved removing the readily available free DNAPL from the plume. A  combination of total fluid pumping and path flow management was implemented, utilising the treated groundwater  for reinjection. After recovery rates of DNAPL decreased to <10 litres per day, thermal enhancement (stage 2) was  implemented. Stage 1 ran for 16 weeks and removed 3,300 liters of DNAPL. 

The aim of Stage two was to raise the temperature of the saturated zone in order to further mobilse the DNAPL  and allow for enhanced recovery, as the successful bench scale trial had demonstrated. The plume was heated  in-situ, using 6 specially designed stainless steel injection boreholes, and injecting recycled treated groundwater  at 40-50 degrees Celsius. Groundwater temperatures were continually monitored throughout this stage, and the  injection temperature and volumes were adjusted on site for the individual injection boreholes in order to optimise  DNAPL mobilization across the plume. Ideal average ground water temperature across the plume was between  20-22 degrees Celsius. It was identified that higher temperatures negatively altered the state of the DNAPL causing  removal issues, and lower temperatures gave impaired viscosity and hence poor recovery. 

Successfully optimising the plume temperatures resulted in DNAPL being extremely effectively mobilised, and  allowed for further enhanced extraction. Once recovery rates began to drop to an unstainable level (30 litres  of DNAPL per day); the final stage of the treatment train was implemented - stage 3. The thermal enhancement  phase ran for 20 weeks, recovering 18,900 litres of DNAPL. 

Stage three continued with passive total fluid pumping and path flow management whilst allowing the ground  temperatures to return to baseline conditions. Any DNAPL recharge was carefully monitored on a weekly basis to  ensure that the pump location and speed was optimised to allow for maximum DNAPL removal. This final stage  ran for 5 weeks until the recovery rate was below 5 litres of DNAPL per day. During stage 3 a total of 3,900 litres  of DNAPL was removed. 

A total of 22,807 litres of DNAPL was efficiently recovered from the site during the 10.5 months of operation.  Graph 1 (below), details the DNAPL recovery rates and ground water temperature vs time. 

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MNA Schedule 

Post-remediation, a schedule of 12 months MNA monitoring was agreed with SEPA and implemented at the site. The  monitoring is currently ongoing and is due to be completed in August 2021.  

Six monitoring boreholes were installed on the site's perimeter to mirror historic borehole locations to give  comparable data. The boreholes give representative conditions for up and downgradient of the source area and of  the plume itself. Groundwater samples are obtained using a low flow methodology, in-field data is recorded, and the  samples are analysed for numerous chemical constituents consistent with baseline data.  

Interpretation of the results thus far shows a positive trend in decreasing contaminants of concern. For example,  VertaseFLI are currently seeing an average percentage decrease of 85% for benzene and 83% for naphthalene when  compared with pre-remediation values. The data trend suggests that values will continue to decrease with time. 

Post Remediation Modeling 

The steady-state Domenico solution to the analytical advection-dispersion equation (which accounts for vertical  dispersion in two directions) has been used to model the fate and transport of a risk-driving contaminant  (benzene) during migration from the on-site source area in the direction of groundwater flow (towards the River  Earn).  

This modelling, undertaken in house, demonstrates that the dissolved phase contaminant plume is unlikely to  extend beyond 200m down-hydraulic gradient of the source area at concentrations above 10µg/L. This distance  is significantly shorter than the distance to the River Earn, which is the closest receptor, located approximately  360m down-hydraulic gradient of the site boundary.

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Summary 

VertaseFLI used a combination of innovative techniques, trialled during extensive bench-scale investigations, to  effectively and demonstrably remediate a heavily impacted former gasworks site in a town centre location.  

In summary, the contamination treated throughout the project consists of: 

  • 2,300m3 contaminated unsaturated material successfully stabilised, 

  • 22,800 litres of DNAPL removed, 

  • 7.5 million litres of water treated, 

  • 0m3 DNAPL contaminated soils disposed of offsite. 

The complex nature of DNAPL is well known to be challenging to remediate, however with the combination of soil  stabilisation and solidification, in-situ pumping, path flow management and heat enhancement technologies, Vertase  FLI successfully mitigated the potential risks to human and environmental health, whilst minimising offsite disposal,  ensuring sustainability of the project. 

Importantly, the remediation works are intended to negate the requirement for the local authority (Perth and Kinross  Council) to seek to enforce statutory remediation of the site in the future under the Part IIA regime. As such, the scheme  has not only achieved significant betterment of the soil and groundwater quality, but shall also help to reduce any  future burden on the public purse with regard to remediation costs to address the historic legacy of the site.