One should always treat water in the backcountry as potentially contaminated. Water contamination can come in several forms, pollution including toxic chemicals, bacteria, viruses, protozoa and other organisms that can cause serious problems for the backcountry traveler.
While pollution is a rare problem in the wilderness it's presence cannot be totally discounted. Your best bet in dealing with polluted water is to know the area you will be traveling in. Has there been industrial activity such as forestry or mining? Are there remnants of old equipment decaying along the trail? If so then suspect pollution and do not take water from sources that are likely contaminated.
Bacteria, viruses and protozoa are a more difficult problem. They can
result in a number of diseases including "Giardia lamblia" and
"cryptosporidium". The organisms responsible for these diseases are
impossible to detect by the back country traveller and so one must
assume the water is contaminated and attempt to cleanse it. There are several methods available for treating water in the backcountry. The major ones are: boiling, adding iodine to the water to kill the bacteria/viruses and/or organisms, filtering the water to remove them, chlorine dioxide treatment and ultraviolet light.
This is an effective method for eliminating all microbial threats but has the drawbacks of requiring additional fuel and taking time. To be effective water must be brought to a full boil. Elevation also plays a factor, the Center for Disease Control recommends boiling water for 1 minute at lower elevations and 3 minutes at elevations of 1000 meters and higher. Additional time should be added once above 3000 meters.
At the cost of a little effort filtering will give you instantly drinkable water. Filtering is an effective method of treating water provided a good quality filter is used and you use it according to the manufacturer's instructions. To be effective a filter should have a pore size of 0.2 microns on smaller.
Bleach or iodine can be effective for treating water however they must be used correctly. To be effective water must be at a temperature of 20°C before iodine will work in a short time. If the water is colder longer times are required, often overnight, especially for water at or below 10°C. Also one should be aware that long term exposure to iodine can result in health risks of it's own.
public health concernsregarding the efficacy of UV treatment of potable water. None the less this treatment is available in stores. If you elect to use this method of treatment ensure you read all the instructions and other literature.
The best source of water in the backcountry is probably a spring, however one should not assume that the water coming from a spring is safe to drink. It is possible for it to harbour the same contaminants as ground water, especially if ground water is mixing with the water from the spring.
If springs are not available then water should be taken from a stream, creek or river. Select an area of the stream where the water is flowing as sunlight and oxygen both help purify it. As water tumbles along it becomes reoxygenated. In general look for water that is clear, avoid turbid or cloudy water if possible.
Where no other sources are available lake water can be used. It should be pre-filtered through cloth to remove any large matter. Avoid using water that has a scum over it, there may be toxins in it.
For additional information on this topic see: