Pollution of the chalk aquifer in the Luton and Dunstable area first came to light in 1988, when the former Lee Valley Water Company announced that it had been forced to install air stripping equipment to remove chlorinated solvents from three of its supply boreholes (ENDS Report 161, pp 4-5). It has now transpired that the first evidence of solvent pollution was obtained in 1982.
The extent and probable sources of the contamination have been under investigation by scientists from Thames Water, Lee Valley Water, the Water Research Centre and the National Rivers Authority. Some of the findings were published in October.1A regional groundwater survey revealed that one or more chlorinated solvents were present in most of the 30 or so boreholes studied. At rural sites, concentrations rarely exceeded 10µg/l. Contamination was commonplace in urban areas, and heaviest beneath premises where solvents have been or are still used. Peak concentrations were 1,020µg/l for 1,1,1-trichloroethane, 631µg/l for trichloroethene, and 206µg/l for tetrachloroethene.
For comparison, the statutory limits for trichloroethene and tetrachloroethene in drinking water are 30 and 10µg/l, respectively. No limit has been set for 1,1,1-trichloroethane.
At five industrial sites where special investigation boreholes were drilled, significant concentrations of solvents were found in both the unsaturated and saturated zones of the aquifer. Levels in the saturated zone were comparable to those detected in public supply boreholes, indicating that contamination of fissure water has occurred throughout the aquifer. Solvent contamination has occurred beneath three sites overlain by concrete, indicating leaks through underfloor drainage systems or through the concrete itself.
Solvent handling practices in local industries were found generally to be "archaic". According to the report, "at numerous establishments solvent storage was frequently in drums in unbunded areas which were moved by forklift trucks and decanted by a tap with only rudimentary spill prevention measures. Degreasing was typically within cold vapour dip tanks where loss by leakage was indistinguishable from evaporation." Waste solvents were commonly stored in unlabelled drums in unbunded areas. Older employees also confirmed that waste solvents were formerly spread on open ground, supposedly to evaporate.
The main potential sources of the aquifer contamination are a number of sites operated by Vauxhall Motors as well as other firms making automotive components. These have been and remain the biggest local consumers of chlorinated solvents. Other solvent users in the area include printing, dry cleaning, paint and adhesives businesses.
The report notes as an aside that a borehole drilled "on a major complex with a long history of automotive manufacture was contaminated by a wide range of organic compounds (including compounds of benzene, phenol, phthalate, toluene, xylene, naphthalene and triazines)."
However, the aquifer contamination is so widespread and involves so many potential sources that tracing the contamination of a borehole to a particular source would generally be impossible, the report says. "Water treatment at the point of abstraction is the only viable long-term clean-up system."
Even if a contamination source could be identified, the NRA lacks the power to force the polluter to carry out clean-up works, the report notes. It can carry out the work itself and seek recovery of its costs from the polluter, but this "can involve large capital outlay with no guarantee that costs will be recovered. The 'polluter pays' principle is therefore rarely achievable." The same legal constraint has discouraged the NRA from instituting remedial action at other sites (ENDS Report 189, pp 7-8). The Government has consistently rejected its appeals for a grant allocation which would enable it to finance clean-up operations at contaminated sites.
The Government also holds the key to action to forestall continuing contamination of aquifers. Section 92 of the Water Resources Act 1991 enabled the NRA to enforce precautionary measures where the use or storage of solvents posed a threat to groundwater, but has yet to be brought into force by regulations.
Likewise, under section 93, the NRA may ban or restrict potentially polluting activities within protection zones around abstraction points, as it has proposed to do under its draft groundwater protection policy (ENDS Report 202, pp 20-23). However, it will initially be for the Secretary of State to make orders designating particular areas as protection zones and enabling the NRA to control specific activities within them.
Similar provisions to those in sections 92 and 93 were made in section 31 of the Control of Pollution Act 1974. Despite pleas by the former water authorities and environmentalists, they were never activated by successive governments.