I was just reading this question What is the safest way to purify water?
Now I walk a lot in Snowdonia in North Wales and on occasion I drink from mountain streams. I've even camped next to mountain streams and used the water as my main water source.
I haven't used any purification tablets or boiling (generally) and suffered no ill effects. I always ensure the water is fast running and 90% of the time it's fed from mountain springs, so I've always assumed it's safe, in fact I'm sure I've read it's generally cleaner than most tap water!
I'm not sure where I picked up this information, I'm pretty sure my Dad used to do this.
There will be very limited chemical pollutants (from fertilizers, etc.) in the water as the area has virtually no arable farming.
Mining is probably the worst potential pollutant but I always check the source of the water(I've been walking in the are since I was very young and know the sources of most of the streams)
Am I mad or risking my health?