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Shipping out contamination

Recycling of contaminated material from the site of a former Dublin gasworks is not being carried out on site.

Instead it is being shipped to the Netherlands and Belgium Remediation of the former site of the Sir John Rogerson's Quay gasworks in the heart of Dublin's docklands area is not only the largest remediation scheme ever undertaken in Ireland but it also has a slight twist.

While clean-up involves excavation and removal of contaminated soils for treatment and possible re-use, this is happening in the Netherlands and Belgium rather than on site.

This slightly unusual step to minimise waste was taken after careful consideration of alternative methods to simplistic dig and dump, explains Parkman Environment regional director John Crowther.A number of methods were assessed and jet washing of contaminated gravels was chosen.And the best option, it was decided, was to ship the gravel to mainland Europe.

For nearly 200 years, almost without exception, towns and even villages in Ireland satisfied their energy needs through the manufacture of gas from coal (and later from oil) at the local gasworks.

At the peak it is estimated there were 114 gasworks in Ireland, of which the Sir John Rogerson's Quay gasworks in Dublin was the biggest. It occupied 9ha between the River Liffey and the Grand Canal Dock, where coal was offloaded.

Historically, gas was produced from the heating of coal in an oxygen free atmosphere within a retort with various purification processes.

It is the disposal of the unwanted by-products including spent oxides and tars that has caused residual contamination of the ground. At Dublin a chemical works was established to convert these substances into marketable materials including creosote and even fruit sprays.

After acquiring the site from Bord Gais Eireann (the Irish gas board) in 1998, the Dublin Docklands Development Authority (DDDA) intends to transform it into a mixed development of residential, commercial, amenity and retail uses.

Parkman was commissioned to advise on remediation and appointed Peirse/Soils joint venture as remediation contractor.

Crowther explains that the first step was to carry out a detailed study of the site's historical development including the evolution and layout of the various production processes. This was followed by a focused site investigation using trial pits, boreholes, groundwater and gas monitoring and the collection and analysis of hundreds of soil and water samples.

Data was processed in a risk assessment that examined the potential pollution linkages the degree of hazard represented by each of the contaminants; possible harm to ground and surface water, construction workers and eventual occupiers; and the potential contact pathways to the hazards.

The risk assessment had to meet recognised international standards, says Crowther, because it forms a fundamental element of the application to the Environmental Protection Agency for a waste management licence, along with the contract specification and drawings and the results of monitoring of various ambient conditions.

Parkman's strategy at Dublin was to remove the contamination source rather than intercept the potential pathways. This involves large scale excavation typically to depths of 3m but in places down to 5m. In all, 195,000t of contaminated material will be removed.

The move away from traditional dig and dump methods and the absence of any suitable licensed landfills in Ireland meant that alternative clean up solutions were sought.

Parkman project manager Tony Brown carried out a detailed study of on-site treatment methods, including assessment of time-scales and capacities of mobile plant processes. Jet washing of gravels was proposed to allow potential engineering materials to be reclaimed, to take place in mainland Europe.

Material with organic contamination mostly goes to the Netherlands for thermal desorption.This heat treatment produces a residue which can be used locally for construction purposes. Inorganically contaminated material goes to Belgium for soil washing.

Contaminated material is transferred directly by covered conveyor to the ship berthed at the quay, avoiding the use of road transport vehicles. Each ship removes up to 3,500t of contaminated material in 24 hours, equivalent to 175 conventional lorry loads.

However, the remediation process itself still forms a potential pollution linkage. Crowther explains that the safety of site operatives working close to materials being excavated and handled was assured by carrying out safety, health and welfare risk assessments to determine an appropriate control strategy.

Workers have to pass through decontamination units when entering and leaving 'dirty'areas.Personal protective equipment includes air quality monitors, which are also installed around the site perimeter.

Significant efforts are also being made to monitor and control odours, especially from exposure of tars in excavations. This includes the use of sprays, and taking account of wind direction during excavation of the worst contaminated areas.

A key part of the strategy involved construction of a 2,000m long cut-off wall around the site perimeter, keying into cohesive glacial till at depths between 8m and 16m.

This was installed by foundation contractor Bachy Soletanche and has three main functions: forming a retaining wall for an underground car park; limiting the volume of groundwater entering the site (pumping is also being used to aid excavation); and finally to prevent re-contamination of the site after remediation from known sources outside the site boundaries.

The wall comprises a 600mm wide cement/bentonite slurry trench with 750mm diameter concrete piles spaced at up to 2250mm centres. The slurry provides a continuous low permeability barrier while the piles provide structural capacity.

Crowther says rigorous record keeping will allow a detailed validation report demonstrating site conditions after remediation to be prepared for the client, regulators and prospective buyers.This document will also act as the application to the Environmental Protection Agency for surrender of the waste licence.

The main goal of remediation is to generate confidence, he says.'Our role is to ascertain the truth as far as the process is concerned, at a level of detail that thoroughly satisfies all these stakeholders.'

'Gasworks remediation is a complex business, 'he adds, 'requiring a diligent approach to health and safety as well as a clear understanding of the treatment options. A welldefined strategy is a necessity to ensure that all linkages have been considered.'

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