It depends what you need to do with your water. I am on a well. My well is in a state with a lot of agriculture. You could drink that water and not have a clue what you are drinking. When we bought the place, a water test was done, and the nitrate level was two and a half times the upper safe level. That water would kill a baby under 6 months of age if boiled and made any more concentrated. As you BOIL water, whatever is in it gets more concentrated. What you want when you boil water is to collect the steam, which is FREE of those contaminants. You want to distill water. If you are on the run, it's a bit tough to carry around a 'still'. If you are stationary, it's not hard to set up a distiller using a pressure canner.
Nitrate levels alone are an indicator of other chemicals and often microbes in your water. If you plan on taking water from a fresh water source and hope to run it thru some kind of purifier, you will have to settle for some chemicals in your water, and the fact that this process takes a long time.
Collecting rainwater off a steel roof into a barrel may be your safer option, then run that thru some of the better filters. That will work OK, unless we have infested birds that could cause us to fall ill. Even then, chlorinating it or putting iodine in it, may mitigate the risk.
I live in an area rich in water resources. No one would EVER think my region has a 'water' issue, until they try to drink it, untreated.
Also, keep in mind even municipal water, doesn't have to test for most contaminants, only a few. Many on city water having been drinking Hexavalent Chromium for decades without knowing. (Erin Brokovich fame) We have arsenic, radon, pesticides and herbicides in our water around this region. Which is why most of us have RO systems. Those systems go off when the electricity goes off. Short of getting a generator and hoping to find gas, we won't have those systems. Often people get coliform bacteria too. I don't want to rag on and on about water quality, I will just say, it isn't easy to get pure water any more.
I have a well. I have 2 large pressure canners. I bought an Amish bucket (I would have bought a Bison pump if I could afford it). I bought a Vortex coil system for distillation. I live between 2 major rivers, less than 1200 meters from one, but I wouldn't drink that water unless I was dying anyway. I am up North in the land of fresh water. I live near a spring fed fresh water lake, and I would not drink that either, altho it is cleaner than the river. That would be my backup. I would still distill that water. I also live near some Springs, those could possibly be cleaner. But so far, my well, appears to have the safest water at this point. I probably won't die from drinking what is in there because I am older. I would not want a kid growing up on that water. I have been on some type of rural residential well now for 30 years of my life and until 2006, it was untreated water with a nitrate level below 10mg/L. Now that factory farming has taken off, no rural well should be considered safe in the US. I doubt Europe is any better.
In a survival situation, we won't be drinking crystal clear water. We will all have to make do.