Background
The ‘Meadow View’ development site in Hertfordshire has a brownfield legacy mainly from ‘The Murphy Chemical Company Factory’, which was responsible for the manufacturing of agricultural and horticultural pesticides, from the 1920’s until the mid-1980’s.
Since decommissioning, the site has been subject to numerous site investigations, identifying soil and groundwater to be grossly contaminated with petroleum hydrocarbons, polyaromatic hydrocarbons and pesticides from the Organochlorine, Organophosphate and Triazine groups (over 400 individual constituents of concern) presenting an incredible remediation challenge.
Determinants were identified as potential risks to human health and controlled water receptors, including the River Lea which forms the southern boundary. The site had been partially remediated with a reed bed filter system with abstraction wells utilising a long-term ground water pump and treat strategy. However, in 2014 this system ceased operations with no demonstrable improvement.
With the land granted permission for a 37 Unit residential development, Sanctus began the process of regulatory liaison in 2018 to achieve the necessary permits for remediation.
The site overlies made ground and alluvial superficial deposits up to 9m thick, under which a weathered chalk band of 5m separates the structured chalk. The site has two groundwater units to consider, both the superficial deposits in partial continuity with the river and deeper groundwater in the underlying chalk principal aquifer.
Technical Challenges & Solutions
The technical challenge of the site was clear – remediation of 8,000m3 of contaminated soil and the re-engineering of 20,000m3 site won material.
Sanctus completed a thorough remediation options appraisal, following the Tier 2 Sustainable Remediation Framework guidance (Surf UK) to select the most suitable remediation technology with careful consideration of sustainability and the impact to the surrounding community and environment. Sanctus ultimately concluded that it was not possible to achieve remediation targets using any conventional approach.
Sanctus’ approach sought mass reduction of onsite contaminant sources, treatment of impacted soil and removal of grossly impacted water from excavations. A combination of remedial technologies would be utilised, including soil source treatment using bioremediation, with dewatering of excavations combined with pump and treat to aid with groundwater improvement.
This approach will achieve in significantly reducing the mobile contaminant fractions, thereby reducing the impact on surface and groundwater quality.
The soil source treatment was developed to excavate and treat the materials so that soils will be safe and suitable to be re-used on site using a bespoke series of bioremediation processes. Sanctus used trials to create a culture of microbes that preferentially remediated pesticides before the degradation of hydrocarbons to maximise source reduction, without the creation of toxic daughter products.
This approach minimised landfill material and removed the need for imported primary aggregate avoiding almost 2,000 lorry movements through a beautiful residential area. This avoided~200 tons of CO2 that would have been released from vehicle movements alone.
In deeper areas near the chalk aquifer, Sanctus applied an additional proprietary treatment product to stabilise the contaminant mass and actively aid in-situ biodegradation of contaminants. This minimised the need for deep excavations and maintained the structural integrity of the chalk protecting the principal aquifer by reducing plume development.
Innovation – Pilot Trial
Sanctus undertook a treatment trial of 1,000m3 of contaminated material following the ‘Land contamination pilot trial and small-scale remediation schemes: Regulatory Position Statement (RPS) 215’ to provide proof of concept for the bioremediation technologies prior to full-scale commencement of works.
Sanctus selected areas with the highest concentrations of the contaminants of concern to achieve proof of concept in the most difficult areas. The trials also confirmed the effectiveness of products in reaching the remedial targets and confirmed they were not producing toxic daughter compounds. This also satisfied regulators, clients and the highly engaged local community.
A dedicated soil treatment area was constructed to allow screening, processing, and engineering of contaminated soils into windrows ahead of biological treatment.
An onsite water treatment plant was also constructed to aid in the betterment of controlled waters following dewatering of excavations. This 3-stage treatment comprised of a particulate clarifier, oil water separator and pressured granular activated carbon (GAC) pod. Sanctus carried out rigorous commissioning tests of the system prior to discharge to the River Lea. The system was efficient in removing contaminant loading, with concentrations of TPH reduced from120,000 μg/l to<5.0 μg/l.
Sanctus selected 4 discrete bioremediation products to pilot during the trial to determine the best combination of microbes. A series of windrows were formed, with 4 applied with products and 1 windrow providing a control. Following homogenisation of the windrows, representative baseline samples were obtained to provide a basis to track contaminant loss over the course of the trial.
Soils undergoing treatment were grossly impacted, with visible black product, heavy staining and odour.
Engineered covers were used to create and maintain optimum moisture content and stimulate aerobic bioremediation, while ensuring emissions from the process were contained.
An independent consultant and MCERTS accredited laboratory was used to prove the effectiveness of the treatment.
The initial trial period was scheduled for 8-12 weeks to achieve the remediation targets.
By week 6 concentrations of all contaminants reduced below their remedial targets. Significant mass reductions compared to as-dug concentrations were observed including; reduction of 93% TPH, 75% Total Pesticides and 77% Total PAH. These results were achieved with no toxic daughter products being produced.
This trial clearly demonstrated the effectiveness of treatment and allowed the safe re-use of materials.
Demonstrating Best Practice
Planning and Data Analysis
Following review of 15 previous site investigations/reports ranging from 1984 to 2019, and assessment of over 7,800 data points from recent investigations Sanctus carried out a detailed gap analysis to develop a GIS system of contaminant concentrations and distributions.
This was fundamental in the execution of targeted sampling, identifying contaminant plumes, refining the Conceptual Site Model (CSM), and creating a high quality DQRA to derive achievable remedial targets for the full-scale remediation.
Environmental Monitoring
Sanctus deployed its bespoke environmental permit and specialised operating procedures to protect the sensitive residential setting.
The location and specialised equipment for environmental monitoring stations was agreed with regulators and were subsequently monitored daily.
Odour and Vapour – Extensive vapour protection measures were utilised during the works including; use of an odour suppression system (with specific variants agreed with the local community), point source and passive air monitoring using qualitative and quantitative methods, with a PID and passive Tenax tubes.
Windrow Monitoring – The windrows were closely monitored for soil characteristics including Volatile Organic Compounds (VOCs), temperature and moisture content to ensure that optimum conditions were consistent.
Significant reductions in VOCs were noted from all windrows across the trial period. The largest loss of VOCs was noted from a baseline of 76ppm to 9ppm by week 6.
Controlled Waters – A network of 8no ground water sentinel boreholes, in up stream and downstream locations from the river were routinely sampled and tested for an extensive suite of contaminants comprising over 400 determinants.
Additional rapid monitoring rounds focused on monitoring in-situ physio-chemical parameters throughout the programme which were frequently compared to baseline levels and derived trigger levels ensuring a betterment of site conditions.
Remote Data Acquisition
Telemetered time lapse video footage was captured 24/7 throughout the trial. This allowed senior engineering specialists hundreds of miles away to remotely receive and monitor live progress on-site, whilst actively reducing the carbon footprint of operations.
Public / Stakeholder Engagement
The key stakeholders included; local residents, parish councillors, community groups, St Albans City and District Council, the local Member of Parliament and the Environment Agency.
Sanctus take pride in our commitment to public engagement believing that our projects are for the benefit of people, wildlife and the environment. This aspect of our approach is especially important in sensitive residential areas with engaged communities.
The challenge was to effectively explain our methodology simply, gain the trust of the community, and demonstrate the long-term positive benefits our work would have on the local environment.
Several public consultation events were provided allowing Sanctus to present the scheme in detail, with drop-in sessions to support members of the community over a variety of dates and times.
Sanctus considered feedback and implemented systems with the community which included site specific changes to: working hours, detailed traffic management plan, restricted and minimal deliveries at peak times, and removal of HGV traffic through the village.
The most effective approach has been a genuine open-door policy, where members of the public are able to visit the site office by simply ringing a designated bell at the site entrance, and are then talked through our works, operating procedures and environmental protection measures by our on-site expert.
This level of engagement was well received by all and helped establish an informative relationship between the project team and all stake holders. It was rewarding to hear members of the local community thank Sanctus for their engagement, effort and involvement when declaring in open forum that we were the people that were here to help solve the contamination problems of the past.
In Conclusion
Cost Effective and Highly Effective Sustainable Treatment Methodology
The trial demonstrated to the regulators that the onsite treatment methodology was deliverable, successful in achieving the remedial targets and would result in a betterment to the environment.
The only alternative solution would have involved in excess of 1,900 HGV movements, ~8,000m3 of waste deposited in landfill, ~8,000m3 of primary aggregate quarried and imported at an additional cost of £3.4m to the client which would have rendered the project logistically and financially unviable.
The site would therefore have remained contaminated, whilst denying safe development for much needed new homes.
Economic, Environmental and Social Benefit
The site will be transformed from a highly contaminated industrial memory, to provide essential homes for 37 families bringing a bright future in the place of a literal dark past.
The remediation of the site has severed pollutant linkages which previously impacted the water quality of the River Lea and Principal Aquifer, as well as protecting the health of the community, future residents and wildlife. The works used local labourers, apprentices, students and suppliers to deliver the project. As we complete these works, we will leave an up skilled and more experienced labour force ready to face a new remediation challenge.
There has been significant cost and social benefits relating to the investment made in public engagement. It has increased public buy-in, meaning that the works were able to run smoothly and on programme. When working within a community it is always the right thing to do to create an engaged and inclusive dialogue and where possible achieving a fantastic outcome for people, wildlife and the environment – this is very much the Sanctus way.
Compliance with Legislation, Codes and Guidance
Sanctus engaged Regulators, regularly and patiently for over a year to obtain the relevant permits ahead of offsite operations including; abstraction and treatment of contaminated groundwater, discharge following treatment to surface water and a flood risk activities permit. This liaison supported the targeted investigation, DQRA (establishing practical and robust remediation criteria in line with CLR11/LCRM) and Sustainable Remediation Framework guidance (SurfUK).
Application to Full Scale Remediation
The trial developed a bespoke solution for petroleum hydrocarbons and pesticides within the soil, which was accepted by the Environment Agency and St Albans and City District Council and alleviated any apprehension and proved the effectiveness of the treatment.
This resulted in a formerly unviable project bringing 37 much needed homes to the local community.
We very much hope the detailed planning, preparation and testing of this innovative approach and close working with the regulator and local planning authority will help introduce a new suite of tools available to others to help restore, recover and remediate brownfield sites to provide much needed homes across the country, protecting the green belt and reducing the overall environmental impact of ground engineering.
Full scale remediation is ongoing and due to be completed in the autumn.
