You know that the acid rain is damaging the lakes, so the second
word would have something to do with damage or destruction, etc.
The fact that this destruction is unnoticed means that the first word would
have to express that the rain is damaging in a way that is not observable.
Insidious means sly, so that and destruction would work the best (nefarious
means wicked, amalgamation means mixture, stagnation means recession) (C).