A while ago there were a bunch of treads regarding water quality. Some had statements of, "I called the water company and they say my water only only X PPM!!" I'm sure that very true when it leaves the water plant, however it then goes into the grid of doom before being served to your home. Pipes that run in the street, to your home, etc can very very very old, a lot older than you want to know. I felt that these two pictures might help show what kind of things can make your 50 ppm water jump to 350ppm.
This is what I call a water mushroom, it's a water cut off valve that you might find out in the yard. Basically the thing you hit with the lawn mower.
All good and dandy, but this is what the other part of that looks like. This is taken from an abandoned steam tunnel in one of my buildings. The part probably is anywhere from 1920 to 1960's in age. It's city property and their responsibility to replace and maintain. All the water serving the building flows through this valve, and the rust can be flaked off by touch. From my experience, removing a pipe in this condition usually shows the similar deterioration on the inside as well. Granted not to the same degree, but it's there changing your 50ppm water.
I just kinda felt this might help illustrate water quality a bit.