Remediation of brownfield sites is often required to return land to a condition from which it can be developed and regenerated. Undertaking this remediation work brings a unique array of challenges.
All remediation work must meet the agreed requirements of regulatory bodies, such as the Environment Agency (EA) or local authorities, to either reduce the level of contamination below screening target levels or achieve a betterment level to enable the land to be developed.
But what are the challenges of remediating contaminated land for commercial development? There are a number of key issues to be aware of which I have explored here, using Chancerygate’s four-acre Tonbridge Trade Park development in Kent as a working example.
The site was last utilised as a tar distillery and creosote works with operations incorporating numerous above and below ground tanks. The site had been extensively remediated over the preceding 20 years, but none of the previous remediation had improved the site to a degree which would allow it to be bought back into meaningful use.
Meeting the demands of the regulatory bodies
Remediation of contaminated sites enables land to come back into use for commercial development, which can only benefit a local area and support economic growth.
Agreement was reached with the EA to remediate the site on a “betterment” basis for gross contamination within the impacted soils, and an improvement in contaminant concentrations within the groundwater on site.
This agreement is normally mirrored in the environmental planning condition imposed by the local authorities to enable the site to be developed for commercial use.
At Tonbridge, due to the complex nature of the contamination present on site, remediation works involved a combination of techniques and methods, including the pumping and treating of ground water from excavations, ex-situ bioremediation of soils, and the provision to carry out stabilisation/solidification where required. Groundwater pumping and treatment was identified as the most efficient method of contaminant mass removal.
Decontamination of soil
The primary objective of the remediation measures at Tonbridge was to achieve ‘betterment’ by treating heavily contaminated soils. The site was primarily impacted by hydrocarbons due to its historical use as a tar distillery. By improving the environmental condition of soils beneath the site, the risks to human health and environmental receptors were to be reduced to levels approved by the EA as suitable for redevelopment.
Soil remediation works included excavation and segregation of grossly contaminated soils and the ex-situ treatment (bioremediation) of this material to allow its re-use on site where possible. We then used lime stabilisation and solidification techniques on the highly impacted residual contamination within the soils which effectively de-mobilised any remaining contaminants.
During this excavation process previously unknown buried asbestos stockpiles and tar ingots were discovered necessitating their removal from site as “special waste”.
Decontamination of ground water
Contamination from impacted soils had been leeching into the groundwater on site for decades. We therefore implemented water treatment measures comprising a scheme of dewatering of excavations via pumping, with groundwater passing through a Water Treatment Plant on site which separated oil-based product from the water and treated the remainder with active carbon before discharging it under licence into the local foul water sewer.
The EA’s main concern at Tonbridge was on-site contamination migrating off-site towards the adjacent watercourse situated 500m from the northern boundary.
To monitor the risk of the water borne contamination spreading beyond our site boundary, we installed a ring of monitoring wells around the site perimeter which enabled us to prove the groundwater treatment undertaken had been effective and will also enable this to be checked at regular intervals during development of the site and beyond. If required, these wells will allow further treatment or pumping to be undertaken.
Working with live services on site
Whether it’s gas, water or electric cables, retaining existing services on a site can create potential issues during a remediation project.
At Tonbridge, a low-pressure gas main, feeding off-site users ran across the north-east corner of the site and a main foul water sewer ran south to north through the middle of the site. Exclusion zones were therefore created during remediation works to ensure they weren’t compromised by the excavation works
The site had an existing substation, which still fed off-site users and remained in-situ during the site remediation, with live low voltage cables running to the northern boundary necessitating a further exclusion zone. Fortunately, these services were located away from contamination hotspots and the agreed remediation strategy was based on an overall site wide improvement, so avoiding working in these areas had no impact on achieving the target improvement and issuing a final validation report to the EA.
Engaging early to facilitate remediation for commercial development
Each contaminated site will require a bespoke remediation strategy and to ensure this can be achieved from both an economic and technical perspective, early engagement with the local authorities and environmental regulators is key.
By overcoming these hurdles with regulatory buy-in, the facilitated commercial development will help support the economy, increase local employment and become an asset to the wider local community.
Chancerygate has experience of remediating contaminated sites to achieve high levels of betterment and enable speculative commercial development. Find out more about our existing development portfolio on our Current Projects page.