Solution features such as swallow holes are commonplace in chalk areas, but until recently they were not thought to be a pathway for pollutants where the chalk is overlain by thick deposits of clay. However, in 1996 an incident in Tangley, Hampshire, showed how pollutants can pass rapidly into groundwater. The case culminated recently in the conviction of an MP, Michael Colvin, for contaminating groundwater with cattle slurry (ENDS Report 278, p 50 ).
In April, similar geological conditions at Horndean, also in Hampshire, led to the dismissal of a planning appeal by Hughes Waste Management. The company had wanted to develop a landfill at Hazleton Farm, an area with natural clay deposits. But the Environment Secretary agreed with the inquiry inspector's conclusion that the clay might not prevent contamination of the chalk aquifer.
"It had always been assumed that when you had a good clay cover the chalk was less permeable than if you didn't," said the Environment Agency's groundwater protection officer for the area, Bob Barnes. "We're now looking into a project to identify [other] high-hazard areas in Hampshire."
The potential problems are not confined to Hampshire. Chalk aquifers supply more than half the groundwater used in the UK. The chalk extends for more than 20,000 km2 over much of southern and eastern England, and much is covered with clayey deposits. As well as landfills, a wide range of industrial and farming activities pose risks if solution features are present.
Solution features in chalk underneath clay deposits can cause subsidence - a hazard documented in the insurance industry. However, they also raise the possibility - as in the Tangley case - of there being hidden channels in the clay through which pollutants can pass straight into the aquifer.
Clive Edmonds, a specialist on solution features with consulting engineers Peter Brett Associates, was an expert witness for the Agency in both the Tangley case and the landfill inquiry. "At Tangley, my investigations proved the existence of a solution feature at the farm where clay-with-flints overlie the chalk," he says.
"There was also evidence that solution features may be present at Horndean," according to Dr Edmonds. "There are a number of large depressions in the area close to the proposed landfill."
In the landfill inquiry, Hughes and its consultants Aspinwall & Co argued that it would be possible to survey the site using microgravity techniques to detect any small voids in the chalk. If any were detected, they proposed to excavate the clay and inject cementitious grouts into the solution features.
But the inspector concluded that interpreting the findings of any site survey would lead to disputes between the Agency and the operators. There would be an "inherent conflict between the commercial interest of avoiding cost.and the public interest in ensuring the stability of the landfill," his report says.
The inspector added that it was "plainly possible" that there could be solution cavities beneath the site which would remain undetected. The risk of subsidence - causing a failure of a proposed polyethylene liner - would persist "indefinitely".
The Secretary of State dismissed the appeal and agreed with the inspector that there was "a significant risk of severe water pollution" were the landfill to proceed.
The Agency said the decision was "crucially important" for safeguarding Portsmouth's water supply. It is now planning a programme to study other high-risk areas, initially confined to Hampshire, to inform mapping of its groundwater protection zones.
However, a spokesman for the Agency's southern region argued that the conditions at Tangley and Horndean were "fairly unusual" and that it would be too expensive to survey the whole region for possible solution features.
A recent review by the British Geological Survey, part-funded by the Agency, concluded that rapid groundwater flow in the chalk in southern England is "generally associated" with overlying clay deposits and may be widespread.1 Rapid flow means that pollutants can pass from the ground surface into an aquifer in a matter of days, rather than having to percolate slowly through the unsaturated zone.
One explanation for this phenomenon mooted in the paper is that clay deposits can be quite acidic, thus promoting the formation of solution features in chalk. Another contributory factor may be that run-off from clayey chalk soils tends to concentrate at discrete points rather than being spread across the chalk.
Evidence as to the prevalence of solution features in chalk includes the presence of E. coli bacteria in private water supplies in Kent and elsewhere - thought to have passed from farmland into solution features. Similarly, sand particles originating from deposits which overlie the chalk have been found in fractures deep within aquifers.