The Remediation of Chemical Groundwater Contamination at Harwell

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The Remediation of Chemical Groundwater Contamination at Harwell4 UNSATURATED ZONE REMEDIATION, WSAAn important component of the strategy for managing the groundwater contamination atthe WSA is the unsaturated zone remediation project. The unsaturated zone is the part ofthe chalk aquifer that lies above the water table (Figure 6). Several tonnes ofcontamination are known to be trapped in this zone, and this contamination will continue toact as a long-term source of groundwater contamination without remediation.The removal of the wastes from the WSA disposal pits was necessary before remediationof the unsaturated zone could begin. Drilling boreholes through the wastes would havebeen hazardous and could have spread the contamination. The pit contents were removedduring 2004.A very encouraging pilot scale study of various remediation techniques was carried out atthe WSA by AIG Engineering Ltd (“AIG”) in late 2005. The pilot scale study establishedthat a technique called soil vapour extraction (SVE), originally developed in the USA butnow widely applied in the UK, could be successfully used to remove contamination fromthe unsaturated zone.Figure 9 illustrates how SVE works. Using vacuum pumps, air is drawn through cracks andfissures in the chalk via boreholes drilled into the unsaturated zone. Volatile contaminantsare drawn out with the air, and are adsorbed onto GAC filters.In addition to the basic “cold” SVE technique, a trial was carried out of thermally enhancedvacuum extraction (TEVE). In TEVE, electrodes (shown during installation in Figure 10)are used to heat the rock, which can increase the clean up rate and can also accesscontamination that would not be removed without the thermal treatment.As the pilot scale study was very succesful, full scale unsaturated zone remediation wasimplemented in 2006. Three operational phases have taken place to date.Remediation of the unsaturated zone at the WSA has removed nearly 1,800 kg ofchemical contaminants. This material has been recovered from the rock beneath four ofthe twenty-five former disposal pits. Following a review of historical data, and testingcarried out during the past year, it has been established that unsaturated zone remediationat five further pit locations has the potential to recover significant contaminant mass. Up tothree further phases of work are planned to undertake remediation in these areas.It had been planed to install an impermeable cap over the whole of the WSA oncompletion of the remediation works. This requirement is under review with theEnvironment Agency, because the benefits of the cap are not clear if the much of theremaining unsaturated zone contamination has been removed.During 2007, the WSA Unsaturated Zone Remediation project was approved as aTechnology Demonstration Project by CL:AIRE 2 . CL:AIRE is a respected, independentnot-for-profit organisation established to stimulate the regeneration of contaminated land inthe UK by raising awareness of, and confidence in, practical and sustainable remediationtechnologies. A Technology Demonstration Report is currently being finalised on theunsaturated zone remediation work at the WSA and will be published by CL:AIRE later thisyear.2 CL:AIRE - Contaminated Land: Applications in Real EnvironmentsLSG Groundwater Report 10 March 2009
The Remediation of Chemical Groundwater Contamination at HarwellFigure 9Illustration of remediation techniques for the unsaturated zone and zoneof water table fluctuation at the Western Storage AreaFigure 10 Installation of conductive heating elements in boreholes for the TEVEsystemLSG Groundwater Report 11 March 2009
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The Remediation of Chemical Groundwater Contamination at Harwell

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