BUDAPEST (Reuters) - Pollution levels from a red sludge spill in Hungary have subsided in the Danube and there is no risk of a biological or environmental catastrophe in the river, Hungarian officials said on Friday.
Interior Minister Sandor Pinter told a news conference the spill had not affected the drinking water supply so far and government spokeswoman Anna Nagy said the food chain was safe.
Hungary declared a state of emergency in three counties on Tuesday after a torrent of toxic sludge from an alumina plant tore through three villages 160 km (100 miles) west of Budapest, killing seven people and leaving dozens with vicious burns.
A photograph circulated by environmental group WWF on Friday suggested the reservoir may have been leaking as far back as June. Officials at the Ajkai Timfoldgyar plant had no comment.
"Let's not even consider the pollution that got into the Danube as real pollution now," Pinter said. "It will not be of an extent which would cause biological or environmental damage."
Tibor Dobson, a spokesman for disaster crews, said pH levels were around 8, a "normal" level, in the Danube, down from around 9 when the sludge reached the river on Thursday.
"We have not experienced any damage on the main Danube so far," Dobson told Reuters. Crews have sought to dilute the alkaline content of the spill.
There were still no estimates of the financial damage wrought by the sludge -- waste from bauxite refining that has a strong caustic effect -- over an area of 800-1,000 hectares (1,920-2,400 acres).
Europe's biggest insurer Allianz on Friday said that its Hungarian subsidiary had insured MAL Zrt, owner of the plant and the burst reservoir, against property and liability claims, but declined to detail the size of the insurance cover.
Insurers can reject claims in cases of gross negligence. Reinsurers Munich Re and Hannover Re said they expect their share of any claims to be negligible.
An aerial photograph taken of the reservoir in June and circulated on Friday by environmental group WWF showed streaks of red mud in the emergency water drains outside the facility and apparent damage to surrounding vegetation.
"That is an unambiguous sign of seepage, which should have been stopped and handled," the WWF said.
Gabor Bako, director of aerial imaging specialist Interspect Ltd, confirmed he took the photograph on June 11, and said he had shown the photo to environmental officials before the spill.
MAL Zrt has said there was no advance sign of the disaster.
MAL's chief executive, Zoltan Bakonyi, told national news agency MTI on Friday that MAL's last chemical checks before the disaster showed no irregularities and the sludge in the reservoir was below the permissible level.
The accident's cause remained unknown. But Gusztav Winkler, a professor at the Budapest Technical University who was part of a team which examined the area's soil 30 years ago, said a bad choice of location may have played a role.
"Two entirely different soils meet here, one a sediment type and one of clay," he told Reuters. "When exposed to water, they expand, shift, to a different extent. They move."
He said the collapsed corner of the reservoir was precisely where the clay and the sediment meet in the soil, which may have helped to create tension zones inside the dam's wall.
"It's a rigid structure," he said. "You push it, it breaks."
While there is a good chance the spill's impact on the Danube will be limited, western Hungarian villages that bore the brunt of the sludge torrent could suffer in the longer term, environmental group Greenpeace said.
Test samples from the sludge showed that government health and science agencies had underestimated the ecological dangers unleashed, Greenpeace Hungary Director Zsolt Szegfalvi said.
Arsenic, mercury and chromium levels were especially high at Kolontar, he said, rejecting earlier claims by the National Academy of Sciences in which it said the sludge contained no harmful levels of heavy metals.
"This contamination poses a long term risk to both the water base and the ecosystem," a Greenpeace statement said.
More than 150 people were injured in the disaster, mainly burns and eye ailments from the caustic and corrosive sludge.
All waterlife died in the smaller Marcal River, first struck by the spill. There were also reports of sporadic fish death on Thursday in the Raba and Mosoni-Danube rivers. There were no reports of major damage to the main branch of the Danube.
Downstream from the disaster site, the Danube flows through or skirts Croatian, Serbian, Bulgarian, Romanian, Moldovan and Ukrainian territory en route to the Black Sea.