Having worked extensivly with negatives and prints made by the Kolb Brothers-The famed photographers of the Grand Canyon, I can say that the water they used to wash both negatives and prints has had little effect. They washed their negs in a stream and the prints were washed there as well or in water carried from the stream. The stream was dirty. Think about what people used in the past and the lack of clean water in their processes and think about the number of prints and negs that are still around in perfect condition.
I am not saying don't try to remove the junk. I am saying don't fret over it too much. If you put water out over night and let it sit the chlorine will evaporate and the smell should go away unless you water folks really dump the stuff in.
I can't imagine using rain water for much of anything because we get so little of it. Living in a drought sucks.