The contamination at Harwell Laboratory came to light two years ago after Thames Water found carbon tetrachloride in a borehole at the nearby village of Blewbury (ENDS Report 182, p 6). Although the source of this contamination was never proved, investigations turned up two old landfills at Harwell which had both caused extensive groundwater pollution (ENDS Reports 186, p 6 and 196, p 7).
The western site is the more highly contaminated. Water drawn from investigation boreholes contained up to 20,000µg/l of chloroform and a cocktail of other solvents. The south site was an older landfill with a more diffuse plume containing trichloroethylene and carbon tetrachloride at concentrations of up to 1,000µg/l and 500µg/l, respectively.
Pump and strip plants have now been ordered for both sites. Each will have a capacity to treat 1,200m3 of water per day. Water extracted from the vicinity of this site will contain about 30-40mg/l of each of the solvents trichloroethane, trichloroethylene and chloroform, with traces of other chemicals such as benzene, toluene and xylene. Solvent levels at the southern site are much lower, at up to 160µg/l.
The air strippers, which will be supplied by R J Parsons, will reduce solvent levels by a factor of up to 1,600. The vapours extracted will be released to the air.
The National Rivers Authority (NRA) has told Harwell to ensure that discharges of cleaned water to the aquifer comply with statutory drinking water standards. At the western site this has posed problems because the water also contains 800µg/l of chlorobenzenes, 29µg/l of PCBs and traces of aldrin, lindane and DDT. An activated carbon filter will be used in an attempt to remove these.
Running costs of the plants are likely to be about £120,000 per year. Both are expected to operate for ten years or more.
Further costs will be incurred in cleaning up the pollution sources around the landfills. At the western site, steam stripping, biological remediation and excavation are being considered. Excavation is likely to be expensive because the contamination has reached a depth of 20 metres. Vacuum extraction is under consideration for the southern site.