The Armchair Generalist has more about what's in the sludge than I've seen elsewhere.
The New York Times confirmed something I suspected: the ponds date from Soviet times. The Soviet disregard for the environment even exceeds that of capitalists, which proves the wisdom of the proverb, "Choose your enemies wisely, for you shall become like them."
The sludge is approaching the Danube River. The good thing is that as it gets into the rivers, dilution makes it less dangerous, although it's going to take a lot of dilution, and the heavy metals will add to the load already in the Danube and its tributaries.