The project was carried out by environmental consultants Dames & Moore. It pre-dated a clean-up of Texaco's Aberdeen fuel terminal in which similar technology was employed (ENDS Report 201, p 6). Dames & Moore's project is of particular interest as, unlike the Texaco clean-up, validation data on residual contamination levels have been disclosed. However, the identity of the site owner is confidential.
Soil from the first of the three contaminated areas on the site contained xylene at concentrations of up to 970ppm and toluene at up to 400ppm.
Sixteen boreholes between 10 and 20 metres deep were connected to a high vacuum unit and extraction was carried out for 18 months from April 1989. The volatile and semi-volatile compounds adsorbed onto the soil were vaporised under the low pressure conditions and pumped to the surface.
Verification testing of soil samples after the extraction was complete showed that levels of both toluene and xylene had been reduced to below 1ppm. The total recovery of xylene and toluene was roughly 1,400kg. The chemicals were adsorbed onto activated carbon for disposal by incineration.
In the second area, the main contaminant was trichloroethylene, which was present both in the soils of the unsaturated zone and in a localised area of shallow perched groundwater. Typical initial concentrations were 120ppm in the soil and 1,000ppm in the groundwater.
Dames & Moore used a dual phase extraction system comprising a vacuum unit, six soil vapour boreholes and a large diameter dewatering well. This allowed the groundwater to be stripped of contamination by suspending it as small drops in the upflow of air.
Over 40 kilograms of trichloroethylene was recovered. Dames & Moore says that levels in the soil are now below 0.3ppm and in the groundwater below 10ppm.
The third contaminated area contained several chlorinated solvents, including trichloroethane at up to 200ppm. Thirteen boreholes were sunk and the trichloroethane level is now below 0.01ppm.
Dames & Moore is using its vacuum extraction technology at several other sites in the UK, but no final validation data are yet available.