Southern Water owns four chalk boreholes at River Way in Andover. They supply up to 20 million litres of water per day and are the sole source of water for about 50,000 people. A treatment works on the site is equipped with a back-up diesel generator fed by a 4,000 litre tank.
On 26 April, Basingstoke magistrates heard that the company contacted the Environment Agency in January last year to report that a substantial volume of diesel was missing from the tank.
An initial examination revealed no problem, suggesting that the oil might well have been stolen. But there were traces of oil in puddles near the tank and at a surface water drain outlet in nearby woodland.
The company took the precaution of pressure testing the fuel line between the tank and the generator. It contacted the Agency the following day to say that the test had found a hole in the pipe at a point where it ran underground.
Southern Water immediately began excavating trenches on the site and drilling investigation boreholes to contain and recover the oil. The spill was less than 50 metres from an abstraction borehole which was shut off as a precautionary measure. The area of the spill is classified by the Agency as in source protection zone 1 - at highest risk of pollution.
Oil was discovered in an inspection chamber for a water main and in investigation boreholes at a depth of 2.5 metres floating on the groundwater.
Southern calculated that it had lost about 5,000 litres of oil over two months before the leak was discovered, but the final delivery of 2,000 litres was lost in only three days.
The court heard that a remediation exercise has now recovered 2,000 litres, but work will probably have to continue for a further 12-18 months. Long-term pollution monitoring will also be necessary.
The company told the court that the leak stemmed from a poor quality pipe repair by contractors in 2001. The affected section lacked an anti-corrosion sheath which might have limited the spillage.
It has long been recognised as poor practice to allow oil pipelines to run underground where leaks are difficult to detect - particularly where groundwater is vulnerable to pollution. But Southern Water said in mitigation that it was not illegal to do so at the time of the incident. New oil storage regulations have since prohibited the practice unless strict pollution prevention and detection systems are fitted.
The company said that it was spending £2.75 million to comply with the regulations, and improvements to the site had been planned at the time of the incident. However, the clean-up operation has already cost £1.2 million and further expense is inevitable.
The company told the court that there had been no risk to public health, and water supplies had not been affected. However, an Agency spokesman said that groundwater pollution would have been "a very likely outcome". Serious pollution had only been prevented by a relatively impermeable layer of weather chalk which stopped the oil from reaching the supply boreholes.
Southern Water was charged with causing polluting matter to enter groundwater, contrary to section 85(1) of the Water Resources Act 1991, and was fined £15,000 with £1,530 in costs. It was the company's fourth pollution fine this year (ENDS Report 351, p 59 ).
Agency legal executive Peter Bilbrough reflected that the incident would probably double the company's costs in complying with the oil storage regulations. More prompt investment would have brought huge savings.
Oil pollution threatening water supply boreholes is not uncommon. Only last year, oil leaking from an underground pipe at a Severn Trent depot in Bromsgrove threatened to pollute nearby boreholes (ENDS Report 344, p 60 ). The company escaped with a £2,000 fine even though the pipeline did not comply with good practice.