But the Worldwide Fund for Nature (WWF) warned that the dangerous minerals could seep into the ground and eventually poison the drinking water of over one million people in the bustling Seville area in Andalucia.

"The old open pit is gradually filling up with rainwater and faster than this can evaporate, which means that...in 75 years time, this water will come into contact with the Nieblas-Posadas aquifer which supplies the whole Seville area," Guido Schmidt, coordinator of the WWF Donana Project, told Reuters.

"The problem is that if nothing is done, the aquifer will become polluted, which is something that at the time (of the spill) was said to be totally unacceptable," he said.

"Okay, we've still got 75 years left but the more time goes by without anything being done, the worse the situation will get," he added.

GOVERNMENT SAYS MINE SAFE

But the regional government, which played a vital role in the 370 million euro ($332.1 million) clean-up of the Aznalcazar area, argues that the pit is safe and poses no threat.

"The top part of the dam has been completely sealed with a layer of earth, and the water has been removed so that now it's a dry, closed-off storage space," Jose Maria Arenas, director of the technical department of the "Guadiamar Green Corridor" of the Environmental Council of Andalucia, told Reuters.

"There is no risk (of spillage) at the moment because of the maintenance work that the Public Environmental Management Company (EGMASA) is doing."

EGMASA is under contract for six months to pump water out of the pit to keep the tailings below the water table, he said.

Boliden, which closed Los Frailes in December last year, cleaned the mess at the mine site and compensated local farmers for the loss of their crops through the spillage, but the company will have no more part to play at the mine.

"The local government took over the responsibility for closing the mine in the global agreement we reached last autumn," Jan Johansson, president of Boliden said.

While the WWF warns of contamination at the mine, it praises the remediation - clean-up - of the Aznalcazar area.

"We are following the restoration work to see how they are progressing and we believe that it's going extremely well. It's not often that we can say something positive about the action taken by the Ministry of the Environment, but we certainly can in this case," the WWF's Schmidt said.

Tailings are a mixture of water, rock and residual metals that are not sent on for processing, many of which like cadmium, arsenic or lead, can kill off plants and animals.

AREA REVITALISED

But now, four years later the "green corridor" that runs along the banks of the Guadiamar through Donana - one of Europe's largest nature reserves-boasts a thriving range of trees, and fish are gradually returning to the river.

Ninety-five percent of the tailings waste was cleaned from the surface of the land and water, and the remaining traces of metals in the soil are no longer a danger, according to both the WWF and the regional government.

"The whole area looks marvellous. There are metals in the soil...but they don't worry me too much because people can walk through there without any problem. These heavy metals aren't a problem, but we can't forget about them," WWF's Schmidt said.

Los Frailes was one of Europe's largest lead and zinc mines but operations were stopped after the spill, and the regional government thinks it will not be reopened in the near future.

"In the short term I don't think it's very likely that anyone would buy it. That would be like buying a car that was not only very beaten up, but also very ugly," the Environment Council's Arena said.