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.

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.

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.

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.

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.