Lake water, even if it appears pristine, is never safe to drink without treatment. It contains a complex mix of biological, chemical, and physical contaminants that pose significant health risks. These threats include bacteria, viruses, and parasitic protozoa, which can cause severe gastrointestinal illness. Purification requires a sequence of treatments designed to eliminate these hazards, involving both the physical removal of debris and the inactivation of microscopic pathogens.
Essential First Steps: Clarification and Sediment Removal
Purification begins with water collection and physical clarification, as suspended solids make later disinfection steps ineffective. Source water away from the immediate shoreline, where debris and animal activity are concentrated. Draw from the surface layer to avoid stirring up sediment. Turbidity, the cloudiness caused by small particles, reduces the effectiveness of chemical treatments like chlorine and can block ultraviolet (UV) light purifiers.
The first physical step is straining the collected water to remove large debris, such as leaves, twigs, and coarse silt. Use a clean cloth, such as a bandanna, or specialized items like a coffee filter. Following straining, use sedimentation to remove finer suspended matter by allowing the water to sit undisturbed in a container for several hours.
Gravity causes heavier particles to settle at the bottom, clarifying the water above. Carefully pour or siphon the clear water into a new, clean container, leaving the sediment behind. This preparatory step ensures that subsequent disinfection methods act directly on the water and are not blocked by organic material.
Primary Disinfection: Killing Pathogens
With the water clarified, the next stage is primary disinfection to eliminate biological threats like bacteria, viruses, and protozoa. Three common methods exist: boiling, chemical treatment, and specialized filtration. Boiling is the most reliable method, as high heat effectively kills all waterborne pathogens.
Boiling
At sea level, bringing the water to a vigorous boil for at least one minute is sufficient. At higher altitudes (above 6,500 feet or 2,000 meters), the boiling temperature is lower, requiring an extended boiling time of three minutes to ensure complete pathogen inactivation. While boiling is highly effective against microbes, it does not remove physical particles and requires a heat source.
Chemical Treatment
Chemical disinfection offers a lightweight alternative, typically using unscented household liquid bleach (6% to 8.25% sodium hypochlorite). For clear water, add eight drops of 6% bleach (or six drops of 8.25% bleach) per gallon of water. If the water is cloudy or very cold, double the dosage. The treated water must be mixed thoroughly and allowed to stand for a minimum of 30 minutes before drinking.
Iodine (2% tincture) is another chemical option, requiring five drops per quart or liter of clear water and a 30-minute contact time. Specialized purification tablets, often containing chlorine dioxide, are also effective; follow the manufacturer’s instructions for dosage and waiting time. These methods are most effective when the water has been thoroughly clarified.
Specialized Filtration
Specialized filtration systems provide a non-chemical means of purification by physically removing pathogens. Microfilters designed for backcountry use typically have a pore size of 0.2 microns (0.0002 millimeters) or smaller. This size blocks bacteria and larger protozoa, such as Giardia and Cryptosporidium. Since viruses are smaller than 0.2 microns, chemical disinfection or boiling is needed as a secondary measure if viruses are a concern.
Identifying and Avoiding Chemical Contaminants
The purification methods discussed previously primarily eliminate biological contaminants, not chemical ones. Boiling, chemical disinfection, and microfiltration do not remove dissolved chemical pollutants, such as heavy metals, pesticides, or volatile organic compounds (VOCs). These contaminants are often introduced through industrial discharge, agricultural runoff, or mining activities.
Exposure to chemicals, including heavy metals like mercury and lead, poses a long-term health risk. The first step in managing chemical risk is to avoid high-risk sources entirely. Avoid lakes near active or abandoned industrial sites, large agricultural fields, or areas prone to toxic algae blooms. If the water source is near any of these areas, consider it chemically contaminated.
Advanced purification methods, such as reverse osmosis or distillation, are the only ways to reliably remove a broad spectrum of dissolved chemical contaminants. Since these methods are often impractical in emergency settings, the most prudent action is to avoid water where contamination is suspected. If the water has an unusual color, smell, or taste not removed by clarification, it indicates potential chemical contamination and should not be used for drinking.