| Project name: | Contaminated land remediation in UK and Ireland |
| Location: | Ireland |
| Keller company: | Keller Ground Engineering |
| Client: | Various controlled by the Environment Agency |
| Techniques used: | Permeable Reactive Barriers |
| Date: | 1996-present |
Permeable reactive barriers look set to provide a sustainable and very cost effective alternative to dig and dump.
The mantra in contaminated land circles is “source-pathway-target” – remove one of these three components and the problem is generally considered solved. Increasingly though, there is a realisation of the need to consider a fourth aspect – sustainability.
The most common approach to contaminated land is still dig and dump, but this is just transferring the problem and involves large-scale transfer of waste offsite – going right against one of the key measures of sustainability.
Another common and well-established method is to isolate the pollutant source by slurry walls and surface capping. Sealing in the contaminants and preventing groundwater from flowing through the contaminated ground, stops contaminant migration by effectively blocking the pathway.
The problem here is, once this is done, the contaminants just sit there, and remain - as some have termed it - a toxic time bomb.
A major shift in approach appears to be taking place. In September 2002, the UK’s Environment Agency (EA) published new guidelines on the use of permeable reactive barriers or PRBs. The technique falls within the EA’s remit to “promote and encourage the effective use of sustainable remediation technologies,” and for this reason the approach is likely to become much more widespread.
The process involves placing a “reactive” material in the flow path of the contaminated groundwater. These reactive materials either immobilise or transform the pollutants by biological and chemical processes.
Such is the EA’s enthusiasm for the approach that it is even relaxing the licensing requirements for operators wanting to implement a PRB scheme.
A key difference between PRBs and other insitu groundwater remediation techniques is that the contamination moves through the treatment zone. With most other techniques it is the other way round, and it is extremely difficult to quantify or ensure that contaminants come into contact with the treatment reagent.
The technology was pioneered in the US in the 1990s and of the 80 or so projects constructed across the world to date, three-quarters are in the US.
In Europe there have been just a handful of projects, pioneered largely by Keller. It has been working with PRBs in Ireland since 1996 during which time it has installed three systems. In May 2004, a number of new schemes are about to come on stream in the UK.
The process can be achieved by one of two approaches, either funnel and gate or continuous barriers. Funnel and gate systems use impermeable walls, usually slurry walls, which direct contaminated groundwater to gates containing the reactive material. Continuous PRBs are permeable trenches, for example a pea-gravel and reagent filled trench - constructed across the groundwater flow direction.
Generally Keller favours funnel and gate, because it is more versatile. The gates are usually prefabricated steel boxes encapsulated within a slurry wall. This allows for future removal or exchange of the reactive material, which may be necessary if biofouling or excessive precipitation occurs.
Apart from being sustainable, in the sense that it significantly reduces waste transfer offsite and has little or no effect on groundwater hydrogeology, PRBs can also be very cost effective. Keller’s first PRB at Monkstown in Northern Ireland cost £0.75m compared to £1m for a conventional dig and dump. More recently a project at Portadown, also in Northern Ireland, cost £0.5m compared to £2.1m for the dig and dump.
Typical designs are based on 10-30 years of treatment before the source has attenuated, but Keller’s experience is that because it is a passive system, operating costs and maintenance are relatively low. The really good news though is that once the system has been installed and is proven to be working, the EA – at least in the UK – is already taking a pragmatic view and is allowing rapid follow-on development.
