Lake water is untreated surface water and should never be consumed directly without proper purification. Even if a lake appears clean and clear, it is impossible to visually determine the presence of microscopic threats that cause serious illness. The inherent risk from biological contamination is significant because this water has not passed through the multiple-barrier treatment process used by municipal water systems.
Primary Hazards: Pathogenic Contaminants
The primary danger in untreated lake water comes from biological contaminants originating from human and animal fecal matter. These microorganisms include bacteria such as Escherichia coli and Salmonella, often introduced through runoff or sewage discharge, which cause acute gastrointestinal illness. Viruses like Norovirus and Hepatitis A can also survive in water for extended periods.
The most challenging pathogens are protozoa, specifically Giardia lamblia and Cryptosporidium parvum. These parasites are protected by a tough outer shell (cyst or oocyst), allowing them to survive long times and making them highly resistant to standard chemical treatments like chlorine. Ingesting these cysts can lead to debilitating diseases.
Indicators of High-Risk Water Sources
Environmental conditions and physical observations indicate an increased likelihood of concentrated contamination. Water sources with low movement, such as stagnant coves, are higher risk because they promote pathogen survival and growth. Warmer water temperatures, especially during summer months, accelerate bacterial reproduction and can lead to harmful algal blooms.
Proximity to human or animal activity significantly elevates the contamination risk, as runoff introduces fecal matter and excess nutrients into the lake. Water that is cloudy or turbid with suspended particles is dangerous, as the sediment can shield microorganisms from chemical disinfectants and ultraviolet light. A green or blue-green film or foul odors may signal an active algal bloom, which can produce potent toxins harmful to the nervous system.
Making Lake Water Safe for Consumption
The first step in purifying lake water is pre-filtering to remove large debris and reduce turbidity. This process, which can be accomplished using a clean cloth, coffee filter, or bandana, is necessary because suspended particles can harbor pathogens and clog fine-pore filters. Reducing the cloudiness allows chemical and ultraviolet treatments to function more effectively.
Boiling is the most reliable method because high heat successfully inactivates all waterborne pathogens, including bacteria, viruses, and the tough cysts of protozoa. At sea level, a rolling boil for at least one minute is sufficient to make the water safe. At elevations above 6,500 feet, the boiling time should be extended to three minutes to compensate for the lower boiling temperature.
If boiling is not feasible, chemical treatments using iodine or chlorine dioxide tablets can be used, but their effectiveness is limited against Cryptosporidium. For physical removal of pathogens, a specialized water filter is required. These devices use a pore size of 1 micron or smaller, which is fine enough to physically block protozoa like Giardia and Cryptosporidium. For the removal of smaller bacteria such as E. coli and Salmonella, a filter with an absolute pore size of 0.2 to 0.5 micron is necessary.