The police investigated the case with officers from the Malta Environment and Planning Authority and said it was an “accident” caused when oil leaked from an old 45-gallon tank.

Mepa said the operator of the yard was cooperating fully with the authorities and will be covering the costs of the entire clean-up operation.

“Mepa will consider what appropriate action to take against the operator after the clean-up operation within the valley is completed,” it said.

Most of the oil was removed over two days by Mepa-contracted pollution response company Alpha Briggs, whose director described the “fuss” over the incident as “much ado about nothing”.

But biodiversity expert Alfred Baldacchino and hydrologist Marco Cremona have warned about possible long-term impacts of the spill which tainted a 400-metre stretch of the valley.

Mr Baldacchino, who accompanied The Times on site yesterday, said the spent oil leaked from the quarry and streamed into the nature-rich valley with the help of rainwater. “Some impacts are immediately evident,” he said, pointing out plants covered in the tar-like substance and “microhabitats” destroyed by pockets of concentrated oil.

The clean-up operation saw several men using shovels, white absorption pads and a suction machine to remove the oil.

Mr Baldacchino said this would have further disrupted the biodiversity, with any flora and fauna being vacuumed away with the oil and water. The long-term impact would only be measured once the rainy season was over and experts assessed the damage.

When The Times was on site, the only people involved in the operation seemed to be Ballut Blocks and Alpha Briggs.

Mr Baldacchino said this operation should be managed carefully and supervised at all times, particularly by Mepa, which is responsible for the protection of biodiversity.

Meanwhile, Mr Cremona warned that certain components of spent oil were “toxic” and some may have dissolved in the water.

“It is likely that some of the contaminated water seeped into the ground, which explains the black stains on rocks which show the original level of the contaminated water.”

The valley lies over the mean sea level aquifer, whose water is pumped up for public supply by the Water Services Corporation from a station adjacent tothe valley.

“If the oil-contaminated water seeped through, it could take anything between a few hours to decades to reach the aquifer and be pumped up. However, since the water is at the bottom of a valley, its journey will probably be shorter than the average for the country, which is estimated at 40 years,” Mr Cremona said.

It takes only a small amount of oil to contaminate large tracts of water and make it unfit for drinking, according to the limits set by the EU Drinking Water Directive. For instance, one litre of Benzo(a)pyrene, a compound found in spent oil, will make 100,000 litres of water unfit for drinking.

In the past, where reservoirs were contaminated with spent oil, WSC took action by ceasing supply, testing the pumped water regularly and discarding the remaining supply.

“I presume WSC and the health authorities will act cautiously and temporarily discontinue production from the station until samples are collected and lab tests abroad give the all clear,” Mr Cremona said.

This could take weeks or months and could prompt WSC to take legal action against the polluter to recover costs. But according to Alpha Briggs’s director Paul Pisani, the incident was blown out of proportion. “The problem is that we are making a fuss about nothing ... This was just a 45-gallon tank.”

He added that if full this would have been equivalent to 205 litres.

Asked if this could have affected biodiversity, he admitted: “I’m not a biologist.” But when asked if contaminated water could have seeped into the ground, he said: “No... Oil stays on the surface. And we cleaned it. There is no problem.”

Mr Pisani also denied chemicals were used to disperse the oil in the clean-up process.

Meanwhile, Mepa said it ensured the clean-up was done “sensitively and in the shortest period of time” by calling in a private company with the expertise and equipment to deal with these situations.

Mepa was alerted by the police department on Monday afternoon, the same time passer-by Marcus Camilleri alerted the police and The Times to the case.

Meanwhile, questions sent in the afternoon to Ballut Blocks, the Environment Ministry, and the health authorities have all remained unanswered.

The Ministry of Resources and Rural Affairs simply said the clean-up was coordinated by Mepa officials and the director of the cleansing department.