There are hundreds of water sources along the Pacific Crest Trail. As you travel, it is practically impossible to know the safety of one source to the next. A source may be perfectly safe one day and contaminated by animal or human waste the next day. You certainly cannot tell the water quality by simply looking at the water: contaminant sizes range from the atomic to the microscopic. The picture below illustrates this point by showing the relative size of a Giardia cyst, one of the most common water parasites, next to a human hair.
The approximate size of a Giardia cyst is illustrated next to a human hair. A Giardia cyst can not be seen with the naked eye.
Photo Credit: H.D.A. Lindquist, U.S. EPA
The same Giardia lamblia (intestinalis) cyst as imaged at different instrument settings by confocal microscopy. (A) is the cyst imaged by transmission (differential interference contrast), only. (B) is the cyst wall selectively imaged through use of fluorescent-labelled (TRITC) antibody that is cyst wall specific. (C) is the cyst imaged through use of carboxy fluorescein diacetate, a viability stain. (D) is a composite image of (B) and (C). (E) is a composite image of (A), (B), and (C). White Scale Bar = 10 microns.
The diagram above illustrates the relative size of the Giardia Cyst, E. Coli
bacteria and the Norovirus.
The Giardia cyst is very large compared to bacteria and viruses (see above). The larger Giardia cyst or Cryptospridiosis oocysts can be removed by most light weight backpacking filters. However, bacteria and viruses are difficult to remove from water due to their small size. The potential health effects of these pathogens vary. For a general summary of different water pathogens click here.
There are a variety of water treatment methods, such as boiling, filtration, iodine or chlorine tablets, chlorine dioxide tablets, or a combination of the above methods. The best method for treating water depends on a variety of factors. For more information to aid your decision making process review A Guide to Drinking Water Treatment and Sanitation for Backcountry & Travel Use by the Center for Disease Control and Prevention (CDC).