If you have this question, the answer: You can’t, for certain, unless you are a microbiologist with lab gear in tow. Otherwise, the only way to assure safety is to disinfect the water before you drink it.
This goes for all naturally occurring water, no matter how remote and pure-seeming the source.
Maybe you’re thinking: “But last fall we hunted caribou out of a drop camp in northern Alaska, drank water straight from the stream for a week and none of us got sick. Best water I ever tasted, too.” I understand. I’ve been there and done that, many times; drank the wild water and got away with it, bowels intact. But I no longer consider this a safe way to proceed.
To be clear, I’m not saying all water everywhere is contaminated with micro-beasties; I am saying there’s no way to know if a given bit of surface water is safe or contaminated. Even in the allegedly pristine wilderness, drinking untreated water is tantamount to a game of gastric roulette. In my opinion–and based on some less than pleasant experiences–it simply isn’t worth the risk.
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What we’re concerned about here are “waterborne pathogens,” which are invisible to the eye but tough on the human gut and immune system. These microorganisms come in three broad types. Largest are the protozoa, single-celled parasites such as the infamous Giardia lamblia and Cryptosporidium parvo (“crypto”). Next, moving down the size scale, are the bacteria. Well-known and dreaded examples include E. coli, Salmonella, and Vibrio, among many others. Smallest are the viruses, such as hepatitis A, norovirus, and enterovirus, to name but a few. All of these organisms are measured in microns. Viruses–importantly, as we will see–are the smallest, at 0.005 to 0.1 micron. To give these numbers a context, examine the period at the end of this sentence; it measures approximately 500 microns. Apparently, one speck of water can hold a lot of pathogens.
There are several ways to clean these nasties out of drinking water. One is the old standard: boiling. This can still be a feasible approach in some camp settings, and in an actual survival situation–assuming a fire or heat source is available–boiling may be the only option available. Only bringing the water to a rolling boil is enough to kill all the bad bugs. (In some cases you might want to pour the water through some filter first to remove debris and sediment.) Obviously, boiling is time-consuming and impractical for regular hunting and traveling situations.
Another, more efficient, way to treat water is with a modern filtering device. This uses microfilters (with a minimum pore size of 0.2 to 0.4 microns) to separate protozoa and bacteria from the water. However, with one exception to be noted, filtering devices cannot screen out viruses. This level of treatment is considered safe for most American and Canadian wild-area waters (where viruses are uncommon), but not for international environments, most particularly not for “developing” countries and regions.
This raises the important distinction between water filters and water purifiers. A filter merely screens out contaminating particles of a certain size. A purifier eliminates or destroys all of the dangerous organisms, including viruses. (Some devices employ both filtering and purifying functions, offering the best of both worlds.) For international travel to developing regions, a purifier, such as one or more of the following products, is a must.
This ingenious gadget uses ultraviolet light rays to destroy all protozoa, bacteria, and viruses. Just immerse the unit’s probe into the water, press a button, and give a stir. When the treatment is completed (taking less than a minute to purify up to 16 ounces of water), the SteriPEN shuts itself off. The “pen” is light and easy to carry. It requires four lithium AA batteries, which according to specs should power the purification of 200 16-ounce containers of water.
On the downside, SteriPEN works only in clear water; it cannot function properly if the water is cloudy or dark. A companion pre-filter can help improve clarity, but might not prove adequate in all circumstances. The unit is battery-dependent, which means if the batteries wear out, the purifier is useless. (Always carry spare batteries.) The basic Journey Purifier retails for $90 to $100. The Adventurer model costs $50 more and uses a rechargeable battery with solar-powered charger case–an added security factor well worth the extra investment.
Note: If you dip a cup or bottle into a water source and then purify the contents with a SteriPEN, any water above the treatment line has not been purified. Droplets or film on the cup or bottle rim can still carry pathogens.
MSR Sweetwater Purifier System
This device uses both filtration and chemical treatment to provide a rigorous two-step purification of the water. On the positive side, the unit guarantees a near fail-safe level of protection with all kinds of water, cloudy or clear. The device is solidly constructed and is much less likely (than the SteriPENfor instance) to break or malfunction if dropped. There are no batteries to wear out or replace. Hand pumping is required, and treated water can have a slight chlorine taste. Full purification needs about five minutes of wait time to eliminate all viruses. This system needs Sweetwater solution, available in 2-ounce bottles.
This is the only pump filtration device that eliminates viruses without using chemicals. The company’s carbon-treated filtering matrix screens out even the tiniest pathogens. Pump operated, the XL and Base Camp models work at a rate of nearly two liters per minute, which should be quick enough for the most impatient user, The Trav-L-Pure has a slower flow, but is a self-contained unit, meaning there are no dangling hoses to mess with.
All three of these models are robust and sturdy, able to survive rough handling and accidents. Since no chemicals are used, water taste is not affected. Nor is there any waiting period; water pumped through a First Need is immediately drinkable.
Downside: First Need filter canisters cannot be cleaned in the field. (Carry an extra.) With very turbid water, use a pre-filter first to prevent canister-clog.
No matter which type of filter or purifier you use, I suggest chlorine dioxide for backup, just in case anything happens to your higher-tech device. Sold under the brand names of Aquamira, Micropur MP1, and Pristine, this chemical will kill all protozoa, including crypto, and all bacteria and viruses. It is available in tablet or liquid form is easy to pack and carry and works in all types of water–warm, cold, clear, or turbid (although initial sediment filtering may be advisable). You can also use it to treat large amounts of water at one time.
The major downside is the waiting period: thirty minutes in most situations, but a full four hours if crypto is a suspected risk. When feasible, treat next-day water the night before, letting the purifier work while you sleep.