The Remediation of Chemical Groundwater Contamination at Harwell

The Remediation of Chemical Groundwater Contamination at HarwellGLOSSARYAODAquiferBoreholeChlorinatedsolventsGACGroundwaterLevelHGCPRemediationSSASVETEVEUnsaturatedzoneVacuumextractionVOCsWater TableWSAZone ofwater tablefluctuationAbove Ordnance Datum. The height of a point (eg the water table or groundlevel) above mean sea level (the Newlyn datum, since 1921), usually expressedin metres (eg 120 m AOD).An aquifer is a permeable rock that stores and allows groundwater flow readilythrough it.A deep, narrow vertical shaft used for the extraction or monitoring of groundwateror soil vapour. Typically, boreholes drilled at Harwell in relation to the chemicalgroundwater contamination programme are between 50 and 200 mm in diameterand up to 55 metres deep.Liquids used to remove grease from metal components in engineering workshopsand also as laboratory reagents. These were disposed of in large quantities to thewaste pits at the SSA and WSA. The principal groundwater contaminants are allchlorinated solvents:• carbon tetrachloride;• chloroform;• 1,1,1-trichloroethane;• trichloroethene; and• tetrachloroethene.Chlorinated solvents form part of a larger grouping of chemicals called VOCs(see below).Granular Activated Carbon. A product often derived from coal or coconut husks,which readily adsorbs organic chemicals. Widely used in many industries for thetreatment of contaminated liquids and gases.See Water Table.Harwell Groundwater Containment Plant – the replacement water treatment plantat the WSA, which became fully operational in June 2007.In the context of pollution, used to describe works carried out to reduce theimpact of the pollution.Southern Storage Area. The former RAF bomb dump, used for wastemanagement and disposal operations by UKAEA from the 1940s to the 1970s.Soil Vapour Extraction. A technique used for removing VOC contamination fromsoils and rocks, usually above the water table.Thermally enhanced vacuum extraction. Collective term for a group of techniqueswhich use heat energy to improve the rate of contaminant removal using SVE. Inthe case of the WSA project, conductive heating elements are used to heat thechalk above the water table.The part of the aquifer above the water table.See SVE.Volatile Organic Compounds. A broad grouping for a range of different organiccompounds characterised by their readiness to evaporate under normalconditions. Many of the chlorinated solvents fall within this category.That level beneath the land at which the pores of rock or soil become completelyfull of groundwater. Also referred to as the groundwater level.Western Storage Area. Licensed landfill, used for the disposal of chemical wastesfrom the 1970s to 1980s.The part of the chalk that is between the lowest and highest observed water tablelevels. At Harwell, the Chalk water table fluctuates by up to 18 metres, from thelowest to highest levels. Depending on the elevation of the water table, this partof the aquifer may be available for SVE or TEVE.LSG Groundwater Report vi March 2009