The Silverton district has 400 closed or abandoned mines. Almost all have some water where dissolved chemicals native to the strata can corrosively migrate.
This mine was plugged in accordance with an EPA plan.
In many places, plugging a mine is not a bad idea. It’s mostly done to limit collapses and to keep idiots from venturing into abandoned mines. Arizona has a program to locate and do this for all its abandoned mines.
https://www.earthworksaction.org/files/publications/AZ_AML_303d.pdf
https://www.earthworksaction.org/files/publications/CO_AML_303d.pdf
However, this was an exception. I suspect this was too much water to be trickle down from precipitation.