It seems the geologist even underestimated the disaster He was advising residents to check their water at the time so they could compare it later for contamination. Instead of a hidden change in water parameters, we got a river that looks like carrot juice.
The water could have become dangerous before the river changed color?
I just assumed that from the geologist advising people to test their water so they would have a baseline - so they would be able to make claims later. Obviously there's no need for such proof now.