AMC Outdoors, June 2001

By Paul G. Gill Jr., M.D.

Alexander the Great was marching across the desert with a thirsty army. A soldier came up to him, knelt down, and offered him a helmet full of water. The story goes that Alexander asked, "Is there enough for 10,000 men?" When the soldier said no, the leader poured the water out on the ground.

I don't know about you, but there have been times out there on the trail when I have been so thirsty that if someone knelt at my feet and offered me a helmet full of water, I would have chugged it down in one huge gulp. Even if there were 10,000 other parched-mouth hikers looking on.

But you can't always count on a loyal underling to supply you with potable water. On a hot day, you can easily lose a quart or more of water an hour through your lungs and pores. You'd have to carry a gallon-sized canteen to keep up with your fluid losses and stave off dehydration and heat exhaustion. If you know how to find and disinfect water, however, you'll never have to suffer from backcountry thirst.

Finding Water
You'll rarely have difficulty locating a water source in the wilderness if you remember that water flows downhill, that it carves a channel of some sort in the earth, and that areas with a rich water supply have the lushest vegetation. Here's where you should focus your search:

• The bases of hills or the foot of a cliff. If you don't find surface water, dig where the greenery is densest.
• If you're in the mountains, look for a ribbon of green meandering down a canyon slope.
• Old campsites: Trails leading away from the site will lead you to water.
• Ground water: You can often find springs in low, forested areas and at the bottoms of ravines. Sedge, arrowhead, cattails, willow, greasewood, rushes, alder, and elderberry plants grow where ground water is near the surface.
• Dry streambeds: If you dig a little, you're sure to find water.
• Rain runoff: Look for water in large leaves, in natural depressions in rocks, and in hollow logs.
• Snowbanks: You can melt hard snow or ice on a fire or stove, or pile it on a dark sheet to melt in the sun. Don't use pink or stale snow, and make sure you filter the water through fine-mesh cloth.

If you carry a clear plastic bag with you, you can always make your own water with a vegetation still. Fill the bag with leaves and grass, inflate it and tie off the top, and leave it in the sun for a few hours. Water will condense out of the vegetation and collect in the bottom of the bag.

Spring water is generally cleaner than surface water, and lake water is often cleaner than stream water because particles and microorganisms tend to settle out of still water.

Clarifying and Disinfecting
Sometimes you can find water as pure as Perrier in the wild. But just because it's pure doesn't mean it's potable. Pure water is water that contains no chemicals or particulate matter. Potable water is water that has few or no disease-causing microorganisms and is safe to drink. The sad fact is that Giardia lamblia, the one-celled parasite that causes Beaver Fever (giardiasis), and a long list of other microbes that cause diarrheal illnesses have been found in some of the most remote and pristine lakes and streams in the American wilderness. And you can't judge the potability of water by its appearance, smell, or taste. Nor does the fact that your dog drinks it mean that it is safe. There are lots of things my dog eats and drinks that I wouldn't dream of putting to my lips.

The goal in treating water is not to sterilize it. That would require the removal or destruction of all microorganisms. All you really want to do is to disinfect it; that is, remove or destroy the microbes that can make you ill.

The first step in the disinfection process is to clarify the water if it is turbid (cloudy). Silt and clay particles and organic debris suspended in turbid water may be contaminated with bacteria and parasitic cysts. Clarifying the water before you disinfect it will allow you to use a lower dose of chemical disinfectant, and it will do wonders for the smell and taste of the water. There are two techniques you can use:

Sedimentation: Let the water stand in a container for about an hour. The large particles will settle to the bottom and you can pour the clear water into another container.

Coagulation-flocculation: Small particles and chemicals will remain suspended in the decanted water. But if you add a coagulant, such as alum (aluminum sulfate), they will stick together and either settle to the bottom or float to the surface. Add a pinch of alum to a gallon of water, stir or shake for five minutes, and allow the suspension to settle for an hour before you pour it through a coffee filter or fine-weave cloth. Then disinfect the water using heat or chemical means. You can buy alum, or pickling powder, at your grocery store. Or you can use lime (one pinch per gallon of water), baking powder (three ounces per five gallons of water), baking soda (one ounce per five gallons of water), charcoal from a wood fire (two pounds of charcoal per five gallons of water), or fine white ash from a wood fire (two ounces per five gallons of water).

After you have clarified the water, the next step is to disinfect it. Here you have a choice of three methods:

Heat: If you don't need too much water and you have plenty of time and fuel, the simplest way to disinfect water is to boil it. Giardia cysts, bacteria, and viruses are all destroyed by bringing water to a boil, even at high altitudes. To add a margin of safety, boil the water for one minute, or leave it covered for five minutes after it boils. Boiled water is deoxygenated and tastes flat, so pour it back and forth between two containers a few times or briskly shake it in a partly filled container to restore the oxygen and the taste to it.

Snow or ice is likely to be just as contaminated as surface water, so bring it to a boil just as you would any other water.

Filtration: Filters are simple to use, they improve the taste of the water, and they are quick. On the other hand, they clog frequently when used to purify or disinfect turbid water, and you need to pump them to force water through the microscopic pores of the filter. And, while filters remove bacteria and Giardia cysts, they don't remove viruses. If you are in a remote wilderness area, filtration might be sufficient to disinfect the water. Otherwise, the water will have to be either boiled or chemically treated before it is used.

Chemical disinfection: Halogens (iodine or chlorine) destroy Giardia spores, bacteria, and viruses when they are in contact with them in high enough concentration for a sufficient period of time.

There are several factors to consider when using halogens to disinfect water, the most important of which are contact time (the amount of time the halogen is in contact with the water) and the concentration of the halogen. The greater the concentration of halogen, the less contact time is necessary. Conversely, if you increase the contact time, you can use a lower concentration of halogen. For example, if you double the concentration of halogen, the water will be disinfected in half the time. Or, if you double the contact time, you can halve the concentration of halogen.

When disinfecting cold water, either the contact time or the halogen dose has to be doubled to insure disinfection. If the water is very cold, triple or quadruple the contact time or the halogen concentration.

The final factor you'll need to consider is halogen demand. This is the amount of halogen that is neutralized by nitrogen compounds, vegetable matter, and other impurities in the water.

In calculating the dose of halogen you'll need to use to disinfect water, you'll have to estimate the halogen demand. If you are disinfecting clear, mountain water, assume the halogen demand is 1 part per million (ppm)/liter. If cloudy, the halogen demand may be as high as 4 ppm. If you don't want to take the time to clarify it, double the dose of halogen.

One of the drawbacks to chemical disinfection is that halogens can impart a bad taste to water. Here's how you can improve it:

• Use less halogen and increase contact time.
• Clarify the water before disinfecting.
• Filter the water with granular activated charcoal after contact time.
• Add flavoring or ascorbic acid to the water after you disinfect it.

Iodine and chlorine have approximately equal killing power. Many people prefer iodine because it doesn't taste as bad as chlorine, it's more stable and persistent, and it reacts less with nitrogen compounds in water. However, iodine can have unwanted health consequences, especially in people with thyroid disease and pregnant women, and should not be used for longer than a few months.

—Paul G. Gill Jr., M.D., has written many articles and several books, including The Pocket Guide to Wilderness Medicine and First Aid (Ragged Mountain Press).