The numerous armadillos at our farm prefer to dine on grubs that they dig up. However, they are omnivorous and will eat nearly anything that contains some calories when they get hungry enough.

Metal poisoning (whether mercury or copper) tends to concentrate as you move UP the food chain.

Are there any small creatures around your ponds (for the two members with dead armadillos) that had a diet that consisted of smaller organisms from the pond, such that your algae treatment copper was concentrated in that food source? I can't think that armadillos would be good at catching frogs or tadpoles. Maybe they are adept at digging up crayfish?

Another concern would be the class of blue green algae that are formed from cyanobacteria. These can create cyanotoxins dangerous to dogs and humans in the pond. Perhaps armadillos are more susceptible and just drank slightly poisonous water?