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Safe drinking water isn't an issue on a day hike, because you can carry what you need. However, backpackers will most likely be using chemical water purification of filters to resupply, using natural sources of water. These can fail at times. Filters break and get clogged. Pills get lost, damaged, or are just not effective if the water you start with is too dirty.
What you need then, if you want to be fully prepared when entering the wilderness, is a little knowledge, just in case. You need to know how to find safe drinking water, or make it. Here are some tips to remember.
1. Look for glacier water. Collect water that is running out from under a glacier or is still near the glacier that it melted from, and it is more likely to be safe. It is still possible it is contaminated, but in an emergency, it is probably better than the water from a cow pasture (or even an elk meadow). Even if you have chemical purification tablets or a filter, glacial water is more easily purified than muddy water (but allow lots of time for chemical purification of water that is cold).
2. Use a map. Use a map to see if there are farms, grazing lands or campgrounds upstream from where you are collecting. If there are, be sure to boil the water or use a heavier-than-normal dose of whatever chemical disinfectant you are using. Or collect water somewhere else.
3. Examine the surface of the water. A rainbow-colored sheen to the water, may mean it is toxic. But if the color is from natural bacteria (which are generally harmless), it will usually break apart when you poke a stick into it. If it doesn't break apart, it may be petroleum-based, and should be avoided, because it is more likely to be toxic.
4. Start with clear water. When water is murky, your filter may clog or your chemical treatment may not be effective. Let the water settle for a while in any container you have, and then pour off the clearer water after the dirt settles to the bottom. Strain water through a bandana or t-shirt to get the big stuff out.
5. Look around the area. When near trail crossings or places where people camp, try to collect your water upstream of these areas. For the trouble of a short walk, you may also want to get above any meadow where animals graze.
Always look for the clearest water that is furthest from sources of contamination. Then filter, treat with chemicals, boil, or take your chances if it is an emergency. Most of the things you get from water will not give you symptoms immediately (7 days or more in the case of giardia), so you might have to drink to hydrate yourself and worry about the consequences when you are safely home. Of course, since safe drinking water is a necessity, a better idea is to plan ahead.