I mean, things can change subtly. Water and soil can be contaminated even if it's not always something that you physically see happening. During my ecotoxicology course in college, we went to a seminar about a town I believe here in Indiana where one of the factories running nearby was contaminating a huge area with microscopic particles it was putting out through its smokestacks and the wind was spreading. This was years ago and I don't remember all the details, but they were testing trees in the area and could clearly see when the factory started up by how deep in the rings of the trees that contamination could be found. And of course water is contaminated all the time. Even if you're on a personal well, something could have seeped into it. I don't know, I would test those things just for my own peace of mind, personally, but my chickens drink from the same well that we draw into the house for drinking and bathing with.