I would think a volcanic eruption upstream could contaminate rivers and streams with high levels of arsenic.
That would also mean the rivers and streams were probably nearly dead for a long time, but naturally recovered.
A great story about the resilience of life on earth.
Having been all over Northern Chile and the Atacama, I can assure you, there are no rivers and streams there, except when they get a 10” rain once every 500 years.