The contractor, Fugro Environmental, part of ENSR International, was working on behalf of the Environment Agency to investigate groundwater pollution in Bontddu, Gwynedd, and prepare a remediation plan. In September 1996, around 60,000 litres of petrol had leaked from underground tanks at a service station in the village, causing groundwater and river pollution (ENDS Report 274, pp 37-38 ).
Fugro began drilling on the morning of 3 April 1997. It intended to drill a series of monitoring boreholes in a car park about 25 metres from the garage forecourt.
However, at around 10.30am, residents in a cottage ten metres from the drilling site reported a smell of vapours. The contractors investigated and identified vapours inside the house at around 8% of the lower explosive limit - but decided to continue drilling since their procedures stated that they should stop only if levels reached 20% of the limit.
As drilling continued into the afternoon, the odours grew worse. Two environmental health officers arrived on the scene just after 3pm and entered the house with a Fugro employee.
Once inside, the group was greeted with rumbling noises coming from underground, before a fireball blew out of an under-stair cupboard. Two of the residents suffered minor burns to hair and clothing, and a carpet was set alight. Several other houses were immediately evacuated, and some remain uninhabited today.
Appearing before Dolgellau magistrates on 28 September, ENSR pleaded guilty to failing to ensure the safety of people not in its employment, contrary to section 3(1) of the Health and Safety at Work Act 1974, and failing to carry out a suitable and sufficient risk assessment, an offence under regulation 3(1)(b) of the Management of Health and Safety at Work Regulations 1992.
It was fined £12,000 for the first offence and £3,000 for the second. It was also ordered to pay £23,130 costs - most of which related to fees charged by AEA Technology which had investigated the incident on behalf of the HSE.
The choice of drilling technique appears to have been a significant factor. "We didn't like the drilling technique because you're pushing air into the ground at high pressure," HSE officer Brian Neale told ENDS.
It appears that some of the air was being forced into the unsaturated zone above the groundwater, forcing petrol fumes to escape to the surface through channels between boulders in the ground. The explosion was underground - as witnessed by Fugro employees who saw a boulder fly out of one of the boreholes they had just drilled.
According to ENSR Managing Director Peter Witherington, the technique was chosen to dilute vapours at the point of drilling and avoid explosion risks. The drill had a shroud which lines the hole as it is drilled and through which air is channelled back to the surface.
"There is only a very small amount of air that is in contact with the ground," Mr Witherington said - but he conceded that if drilling at the site again ENSR would probably use a mud-flushing technique instead.
ENSR International bought Fugro in January 1997. It is linked to the big US environmental business of the same name and its ultimate parent is the giant German utility business RWE.
A lengthy remediation programme, using vapour extraction techniques, eventually kicked off in Bontddu last January. The project is being funded by the garage owner's insurers and conducted by Aspinwall & Co.
However, residents continue to complain of bad petrol odours, and the work came to a temporary halt in October because heavy rainfall has pushed the groundwater too high.
Meanwhile, an Environment Agency prosecution of the garage owner Gwilym Evans has been held up for several months but is due to be heard in December.