A recent Backpacker magazine article about research by Wilderness and Environmental Medicine in many national parks was enlightening. It studied outdoor water sources and back country illnesses.
Drinking untreated or unfiltered open groundwater – from springs, creeks and small lakes – is rarely a problem. Of all back country travelers, less than 3% of users got sick from something attributable to their water; and none died. The many signs in state and national parks saying you must treat the water are mostly for the legal protection of the park – so if you get sick, you can’t sue them. Rarely does it matter if you really treat the water.
You still must use good judgment: obtain your water as close as possible to the source, like springs; better to be flowing than stagnant; better to be exposed to sun than in constant shade; not near or downstream from livestock or agriculture or human activity; and not from beaver ponds.
Of those who got sick drinking from reasonably safe sources, four groups had equal results: those using mechanical filters, iodine tablets, chlorine tablets and no treatment. Those who got sick were about the same percentage from each group. Thus, the type of water treatment – or no treatment – was insignificant.
Those who got sick were believed to be regardless of water source. Most illnesses were from lack of personal hygiene – not washing hands after peeing or pooping, not washing hands before preparing food and not washing dishes after preparing food.
The bottom line advice is you should treat water as a precaution. But if that care is not possible then don’t have a cow; most of the time it is irrelevant. Far more important is personal hygiene.