“Drinking water's just as risky as the so-called deadly whiskey and it's often a mistake to breathe the air.”          Benjamin Hapgood Burt

“How many cases of giardia or crypto have you seen in the ER”, I asked an emergency room physician when I met him some years ago near Vogalsang High Sierra Camp. “None” was the answer. This was a surprise because at that time it was generally thought that backcountry water was highly contaminated with Giardia and Cryptosporidium and that the consequences of infection with either were severe. Much has been learned since those days. 


Most remote wilderness water is very unlikely to cause any illness. Spring water is very safe. Even though the risk of drinking backcountry ground water is very low, treatment of the water is a reasonable precaution.


Only those who drink markedly contaminated water or those with immunodeficiency are at risk of getting a severe disease from drinking untreated remote wilderness water if good judgment and experience is used in selecting the water source. If a healthy person is affected, the ailment may be unpleasant but will be of short duration and easily diagnosed and treated.

Park authorities over stress the need to treat wilderness surface water. Coliforms and other fecal contamination cause most water borne backcountry diarrhea not the infamous Giardia or Cryptosporidium. However, the most frequent vector is not the water but hand to mouth transfer. 

Washing hands with soap and water after every urination or defecation, treating ground water and not sharing or eating any food that has been touched by the hand of another person will prevent nearly all backcountry gastrointestinal illness.  To wash hands a small dab of liquid soap with a few splashes of water will do the job. A scrub with toothbrush can do a better job. Alcohol gel in place of soap and water is a reasonable alternative.

I prefer chlorine dioxide to treat water. Iodine is acceptable and faster but not as effective. Boiling is effective but not convenient. Filter pumps are heavy and inconvenient.


Wilderness acquired diarrheas is not an emergency diseases, they are easily diagnosed and treated and are not life threatening except  for those who have severe immune deficiency like AIDS or taking chronic steroid treatment.

The Yosemite Hiking Guide says, “Be sure to treat river, stream, lake, or spring water,” while a Yosemite National Park source says, “Water quality throughout Yosemite National Park is considered to be good and is generally above state and federal standards. An inventory of water quality performed by the National Park Service indicated pristine conditions in many parts of the park, with some water quality degradation in areas of high visitor use.”

Rangers and land managers recommend that wilderness visitors treat all water they drink in order to diminish liability risks and because of a small risk to a few vulnerable people. Their major audience are going to be in the densely inhabited areas of the park where contaminate water is likely. Public health researchers who have studied the topic believe that the risks of wilderness-acquired diarrhea have been over-stated and are poorly understood by the public.

1970s article had a big impact on public concern over wilderness-acquired diarrhea. More than half the people in a group of college students became ill after hiking in Utah's Uinta Mountains. The report concluded their illness was considered water-generated giardiasis. Yet no cysts were identified in the suspect water and the authors discounted food or fecal-oral spread with little supporting evidence. All of the findings were typical of a hand-to-food contamination epidemic. This vector was not understood at the time. 

A survey of fifty U.S. state health departments found eighty outbreaks of giardiasis in 1991. Nineteen outbreaks were linked to contaminated water. Just two were associated with campers or backpackers. Two of these departments considered water-associated giardiasis to be a problem for backpackers in their jurisdiction, though the study found they had no data to support the claim. Apparently there are no well-documented reports in the medical literature supporting the concept that North American wilderness waters are inherently unsafe for consumption.

High-profile episodes of gastrointestinal illness spreading throughout cruise ships demonstrates how such diseases can be spread rapidly from hand-to-mouth contact by people living in close quarters. The drinking water on cruise ships was not the source. It's personal hygiene that is the issue.

In 1993 a Cryptosporidium outbreak in Milwaukee caused illness in more than 400,000 consumers due to a malfunctioning municipal water system. More than 100 deaths were linked to the event, mostly individuals with weakened immune systems. 

Most surveys of backcountry water in the United States have found very low or no Giardia cysts. Persisting high level of cysts are unlikely in most wilderness water. It is likely that transient high cyst levels are caused by recent local fecal contamination.  Fecal coliform bacteria represent a much higher risk of gastrointestinal disease than giardia or cryptosporidium. Risks for all water borne diseases are highest in surface water near campsites, heavily visited locations, trails used by pack animals and cattle pastures. A continuing source is required to maintain a high level of contamination. 

The appeal of drinking from a clear cold bubbling mountain stream is misleading. Calm clear water is safer that moving water. Deeper water is safer than shallow water. There is less silt, cysts and bacteria in still water. Hard-shelled Giardia and Cryptosporidium cysts and coliform bacteria sink in still water. Water near the surface of a still body of water undergoes prolonged exposure to UV radiation that kills these organisms. 

Water from springs with no inflow of surface water and deep well water are generally safe because passing through soil filters the water. This process consists of extensive filtration and the actions of beneficial soil bacteria that eliminate pathogens and many other toxins. Composting converts vegetable scraps and garden debris into potting soil by the same bacterial process. Avoid drinking untreated surface water shortly after heavy rain because surface water runoff can cause bacterial loads to increase in concentration by several orders of magnitude.

Camp at least 200 feet away from a water source and properly dispose of human waste at least as far or farther. Snow or ice? Choose clean snow. Bacteria can live for months in ice. Treat any water that shows an algae bloom or harbors significant floating algae. Avoid it if possible.

The National Outdoor Leadership School (NOLS), which emphasizes strict hand-washing techniques, water disinfection and washing common utensils, reports that gastrointestinal illnesses to be almost none in their programs. By contrast, studies of hiking trips on the Appalachian Trail that averaged almost 5 months, reported that more than half of the hikers experienced at least one episode of diarrhea that lasted an average of two days. The longer the trip the more likely the illness.  On these long trips each of the following independently reduces the incidence of diarrhea: treating water, routinely washing hands with soap and water after urination or defecation, cleaning cooking utensils with soap and warm water, and, surprisingly, taking multi-vitamins. 

Practical water treatment in the wilderness includes filtering, chemical disinfectants, a portable ultraviolet light device, or boiling. Of these boiling is the most secure. Bringing to a boil is sufficient for purification of clear water for drinking. For irrigation and washing of major wounds, boiling for fifteen minutes will decontaminate water as much as is practical in a wilderness setting.

Filters are convenient though usually do not remove viruses and are expensive and heavy. UV electronic devises are sufficient but expensive and heavy and less reliable than chemical treatment. Iodine is cheap, easy, light, small and reasonably fast. It does not treat Cryptosporidium and is partially effective in treating Giardia. Iodine gives a unpleasant taste to the water. Vitamin C, ascorbic acid, will neutralize most of this taste. This accounts for the effectiveness of some powdered drink mixes that have Vitamin C in neutralizing this taste. A bottle of Iodine pills should be replaced a year after the bottle is opened. Chlorine dioxide releases a strong oxidizing agent that kills bacteria, viruses, Giardia and Cryptosporidium. It is very light, easy to use and relatively inexpensive. In order to neutralize the cysts four hours is required. It comes in a foil wrap that extends shelf life to four years.

Another approach used in poor countries is to purify clear water by putting it in a clear polyethylene (PET) bottle and leaving it in direct sunlight for six hours, and best placed on a reflector like corrugated steel or a space blanket and angled to maximize sun exposure. When partly sunny or overcast two days is recommended.


Work less on treating water and more on washing hands.

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