The storage pond filled with caustic alumina-processing waste that burst and inundated several Hungarian villages this week was on a 2006 watch list of more than 150 industrial sites that were at risk for accidents that could contaminate the Danube River, according to an environmental group. The group, the International Commission for the Protection of the Danube River, coordinates conservation activities among the 14 nations that border the river or its tributaries and drew up the list. Even as the government was concluding Thursday that effects of the spill on the Danube would probably not be as serious or widespread as initially feared, environmental experts said it was a reminder of the Danube's vulnerability to industrial mishaps and mistakes like the one that occurred this week near the city of Ajka. The Danube region is dotted with factories and mines left over from the Communist era, many of which are shuttered. But older waste-storage facilities still harbor dangerous material and, as building materials age, are ever more fragile and accident-prone. 'Countries and companies have been progressively trying to deal with this problem, but clearly whatever actions had taken place here were not sufficient,' said Philip Weller, executive secretary of the Danube commission, based in Vienna. 'This underlines for us the need to take precautionary measures.' Anna Nagy, a spokeswoman for the Hungarian government, said that as the remnants of the sludge spill approached Budapest on Thursday night, the pH in the water was 8.1, within normal range and safe for fish and plants. 'It has come down to a great extent,' from its high of 12 to 13 at the accident site, she said, adding, 'so it seems the techniques and measures have been successful.' The wave of dangerous red sludge that bored through the Ajka area on Monday, killing at least four people and sending more than 100 to the hospital with chemical burns, was still being diluted Thursday by a mix of natural forces and government cleanup efforts. As the leading edge of the industrial waste, known as red mud, was carried to progressively larger rivers, its effects were diminished, although all life died immediately in the streams and river near the spill. There, Hungarian scientists had sought to neutralize the effect of the caustic alkaline substance by adding acidic material. The U.S. Environmental Protection Agency normally defines corrosive chemicals with a pH of 12.5 or greater as hazardous waste. When the leading edge of the spill reached the main Danube around noon on Thursday, said officials from WWF-Hungary, a conservation group, it was still highly alkaline, with a pH of 9.8, far higher than is normal in the river. It was apparently further neutralized as the remnants of the sludge mixed with the waters of the mighty Danube. 'I would still expect the effects to remain within Hungary,' said Gabor Figeczky, the acting chief executive officer of WWF-Hungary. It is unclear whether the company had tried to enhance safety at the reservoir after it was identified to be 'at risk' by the International Commission for the Protection of the Danube. Attempts to reach the company on Thursday were unsuccessful. The government has opened an investigation into the leak to determine financial and criminal responsibility, Nagy said. 'This is not a natural catastrophe,' she said. 'It was somebody's fault.' She added that even though the sludge pond had passed a recent inspection, 'there were still a lot of questions about whether the pond was being used appropriately.' 