After years of quietude in an isolated reservoir, a river of iron-red sludge broke free and raged though several Hungarian villages Monday, leaving in its wake questions about its long-term environmental impacts.

The initial onslaught of what the mining industry calls red mud—the remainders of refining bauxite into alumina (which is not aluminum)—left four people dead, three missing, and scores of others suffering from chemical burns, according to multiple news reports. It’s still unclear what caused the undoing of the reservoir holding the sludge (see it pre-collapse here).

In the days that followed, workers raced to keep the toxic torrent from reaching the Danube for fear it would travel to other European locales. Although that battle was lost Thursday, environmentalists still have hopes that the impacts will be minimal and remain in Hungry, according to a Reuters article.

“Based on our current estimates, it (pollution) will remain contained in Hungary, and we also trust that it will reach Budapest with acceptable pH values,” Gabor Figeczky, Hungarian branch director of the environmental group WWF, told Reuters.

The red mud, which has a high alkalinity that makes it caustic like lye or bleach, entered the Marcal river first, killing all wildlife, according to the article. Although its pH had lowered by the time it hit the larger Raba and Mosoni-Danube, it was still a threat. Response workers added tons of plaster and acetic acid to the water in an attempt to further neutralize the pH—with fair results. According to the Christian Science Monitor, the polluted water that entered the Danube had a pH of 9, as opposed to the Marcal’s contaminated pH of 13. The Marcal's normal pH is near 7.

The alkalinity of the mud, however, may be just the beginning of the environmental woes caused by the spill. According to the industry publication MetalMiner, the red mud slurry could contain a variety of heavy metals depending on the source of the bauxite being refined.

Testing of red mud from a Turkish site demonstrates that “the waste can contain thorium and uranium and confirms it can be highly caustic,” the article states. “The majority of the constituents are relatively harmless, iron oxide, aluminum oxide, silica and sodium, titanium and calcium oxides. It’s the minor constituents and caustic pH of this sludge that could prove to be the most dangerous contaminants.”

The tragedy is reminiscent of the December coal ash spill in Kingston, Tennessee, but regulatory officials say a red mud spill couldn’t happen in the United States because the waste storage method differs. According to an Associated Press article, alumina manufacturers in the United States dry their mud before storing it. Although they have to take precautions against the material becoming airborne, it allows them to recycle caustic soda, which is a key ingredient in extracting the ore.

The difference in mud storage methods might be why the Hungarian sludge spill is thought to be the first of its kind. That leaves responders and environmentalists in the lurch when it comes to approaching cleanup and recovery, Marton Vau of Greenpeace Hungary told the Christian Science Monitor.

"The most serious part of this disaster is that there is no experience of a catastrophe like this," he said.