[daveslash]

Purify when you can, absolutely - but if you have to choose between a stream and a spring, take the spring. Not guaranteed to be safe, but if it's popping up out of the ground, it's more likely to be safe than flowing surface water. Also, always consider what's up-stream.

[atatatat]

This is counterintuitive to what I've learned about sediment filtration — any insight for a layperson?