The safety of “running water” depends entirely on its source and the journey it takes before reaching the glass. This common phrase refers to two very different types of water: the treated supply delivered through municipal pipes and the untreated, naturally flowing water found in streams and rivers. While one is rigorously processed for human consumption, the other carries inherent risks. The determination of safety is never absolute, but rather a factor of infrastructure, regulation, and environmental conditions.
How Municipal Treatment Makes Tap Water Safe
Source water, often taken from reservoirs, rivers, or aquifers, undergoes a multi-stage purification process before it is considered potable. The initial steps involve coagulation and flocculation, where chemicals are added to neutralize the electrical charges of suspended dirt and organic matter. These particles clump together, forming larger, heavier masses called floc.
The water then moves into sedimentation basins, allowing the floc to settle out by gravity. Following this, the water is passed through large filter beds composed of sand, gravel, and activated charcoal. This filtration step physically removes remaining fine particles, parasites, and bacteria.
The final stage is disinfection, typically achieved by adding chlorine or chloramine. These disinfectants destroy remaining disease-causing microorganisms, such as bacteria and viruses, ensuring the water remains safe as it travels through the distribution network. This entire process is overseen by federal and state regulatory bodies, which establish maximum contaminant levels for public water systems under acts like the Safe Drinking Water Act. These standards dictate precise testing frequencies and acceptable limits for hundreds of potential contaminants.
Contaminants Originating Within Home Plumbing
Even after municipal treatment, tap water safety can be compromised by infrastructure within a private building. The water’s composition, specifically its acidity, can make it corrosive, causing plumbing materials to dissolve into the flow. This leaching is a primary concern, particularly in older homes utilizing lead service lines or lead-soldered fixtures.
Lead and copper are the most common contaminants introduced this way, increasing in concentration when water stands motionless in pipes. During periods of stagnation, such as overnight, the contact time between the water and the pipe material is maximized. This extended exposure allows higher levels of these metals to accumulate directly at the tap.
A simple mitigation strategy is to flush the cold water tap before using it for drinking or cooking, especially first thing in the morning. Running the water for 30 seconds to two minutes clears the standing water and draws in fresh, treated water from the main distribution line. This practice is important for taps drawing from older brass fixtures or pipes.
Risks Associated with Untreated Natural Water Sources
Natural running water—sources like rivers, streams, and creeks—is inherently unsafe for direct consumption, regardless of how clean it appears. The primary danger lies in microscopic biological threats, including pathogenic bacteria, viruses, and protozoa. Even in remote areas, these water bodies can be contaminated by animal waste or upstream human activity.
Pathogens like Giardia lamblia and Cryptosporidium parvum are common protozoan parasites that cause severe gastrointestinal illness. Cryptosporidium oocysts are particularly hardy, resisting standard chlorine disinfection and requiring specialized filtration or ultraviolet treatment. Contamination by Escherichia coli (E. coli) bacteria, often from fecal matter, also poses a significant health risk.
Natural running water is also susceptible to chemical contamination from agricultural or industrial runoff. Pesticides, herbicides, or heavy metals can dissolve into the flow, often being invisible, odorless, and tasteless. Detecting these contaminants requires specialized testing equipment.
Any water taken from a natural source must be treated before ingestion. Boiling remains the most reliable field purification method, as sustained heat effectively kills all waterborne pathogens. If boiling is not possible, a combination of specialized water filters and chemical disinfectants, such as iodine or chlorine dioxide, must be used.
Practical Steps During Boil Water Advisories
A boil water advisory is an official warning issued when there is a known or suspected failure in the municipal distribution system. These advisories are typically triggered by events such as a water main break, loss of system pressure, or power outages. Such failures can allow outside contaminants to enter the clean water lines through backflow or intrusion.
During an advisory, all water intended for consumption, cooking, or making ice must be sterilized. The definitive method is to bring the water to a full, rolling boil for one minute, which is sufficient to inactivate all common pathogens. At altitudes above 6,560 feet (2,000 meters), the boiling time should be extended to three minutes due to the lower boiling point.
Once boiled, the water should be allowed to cool before being transferred to a clean, covered container for storage. Bottled water is a simple alternative for drinking and for tasks like brushing teeth. Dishwashing machines using high-temperature cycles are generally safe, but hand-washed items should be rinsed with boiled or disinfected water.