Does Water Get Old or Go Bad Over Time?

Water molecules themselves do not expire, degrade, or go stale. A water molecule is the same two hydrogen atoms bonded to one oxygen atom whether it formed yesterday or a billion years ago. But the water sitting in your glass, your bottle, or your emergency supply kit absolutely changes over time in ways that affect its safety and taste. Microbes multiply, disinfectant residuals fade, containers leach trace chemicals, and dissolved gases shift. The water itself is fine; it is everything happening inside and around it that makes stored water “go bad.”

Microbes Start Growing Almost Immediately

The moment treated tap water leaves the pressurized distribution system and enters a container, a holding tank, or even a stretch of unused plumbing, bacteria begin to multiply. One study measuring what happens to drinking water left overnight in household taps found that cell concentrations roughly doubled or tripled, while culturable bacteria increased by as much as several hundred fold depending on the measurement method.¹ That is not a month of sitting around. That is a single night.

Extended stagnation makes things considerably worse. When university buildings sat unused during COVID-19 lockdowns, researchers found that bacteria counts in the tap water exceeded Chinese national standards by more than a hundredfold, and it took anywhere from four to 54 days of flushing before water quality returned to normal levels.² The problem was not just a few extra harmless bacteria. Residual chlorine had vanished, metals like iron and zinc had leached from the pipes, and turbidity had increased.

Stagnant water also creates favorable conditions for more dangerous organisms. A review of 24 studies found that 22 showed a positive link between stagnation zones and increased colonization by Legionella, the bacterium behind Legionnaires’ disease. Dead-end pipes, storage tanks, and buildings with intermittent use were all risk factors.³ This is why building managers are advised to flush plumbing after any prolonged shutdown.

Your Container Is Part of the Problem

Bacteria do not just float in the water. They attach to container walls and form biofilms, thin sticky layers of microbial communities that are much harder to remove than free-floating cells. Research on household water storage containers found visible biofilm formation on inner surfaces within 24 hours when untreated water was used.⁴ Even with treated water, the container material matters. A South African study comparing polyethylene and galvanized steel containers found that polyethylene supported substantially more bacterial regrowth, especially when the source water was already questionable.⁵

Beyond microbial concerns, containers also leach chemicals. Plastic bottles made from PET, which includes most single-use water bottles, contain antimony as a byproduct of manufacturing. At room temperature and over normal shelf-life periods, the amount of antimony that leaches into the water is tiny and well below safety limits.⁶ But temperature changes the equation dramatically. At higher temperatures, antimony release accelerates in a predictable way: at around 65°C, the kind of heat that builds up inside a car parked in the sun in a hot climate, levels can exceed the drinking water standard within a few weeks.⁷ One study modeling extreme conditions found that storing PET-bottled water at 80°C for a week pushed antimony to nearly four times the maximum allowable concentration for drinking water.⁸

UV exposure also plays a role. Health risks from bottled water consumption increase after UV exposure, and the combination of sunlight and heat is worse than either alone.⁹ This is the reason behind the common advice not to drink from a water bottle that has been baking on your dashboard. Reuse also matters: repeatedly refilling and disinfecting PET bottles increased antimony leaching linearly, though even after 27 reuses the concentrations remained below daily acceptable intake limits.⁶

For what it is worth, the bacteria do not much care whether the bottle is glass or plastic. A study tracking bottled mineral water over three months found that total and viable bacterial counts rose by one to two orders of magnitude within the first two weeks after bottling and then stabilized, regardless of whether the bottle was PET or glass.¹⁰ Glass avoids the chemical leaching issue but does not stop microbial colonization.

The Chlorine Clock

Most municipal tap water contains a small amount of residual chlorine meant to keep microbes in check as water travels through pipes to your faucet. Once water leaves the tap and sits in an open container, that chlorine starts disappearing. One study tracking stored drinking water found that free residual chlorine dropped by roughly 45% after 24 hours, 68% after 48 hours, and 85% after 72 hours.¹¹ Container type and initial chlorine concentration affected the exact numbers, but the direction was always the same: downhill fast.

This is the central mechanism behind why tap water “goes bad.” It is not that the water deteriorates. It is that the chemical guard keeping microbes at bay evaporates and reacts away, and nothing replaces it. Within a couple of days at room temperature, stored tap water is essentially unchlorinated, and the bacterial population starts doing what bacteria do in warm, nutrient-containing water.

There is a secondary wrinkle involving disinfection byproducts. Chlorine reacts with organic matter in water to form compounds called trihalomethanes. These byproducts can accumulate over time in the distribution network, and their concentrations vary with temperature, organic content, pH, and contact time.¹² In boiled tap water stored in a closed container, trihalomethanes actually kept increasing for the first few hours before eventually declining as they broke down through hydrolysis.¹³ At the concentrations typically found in treated water, these byproducts are not an acute health concern, but they are one more way that stored water changes chemically over time.

Why Water Starts to Taste Stale

Even when stored water is still perfectly safe to drink, most people notice it tastes different after sitting out for a day or two. Several things drive this.

Dissolved gases, especially carbon dioxide, escape from water that is sitting at room temperature in an open container. This subtly shifts the pH and changes the way the water tastes. It is the same principle that makes flat soda taste different from fresh, just less dramatic because tap water has far less dissolved CO₂ to begin with.

A more noticeable flavor change comes from microbial activity. Certain bacteria and actinomycetes produce compounds called geosmin and 2-methylisoborneol, which are responsible for the characteristic earthy or musty smell sometimes found in drinking water.¹⁴ These compounds are terpenoids, and they are produced as metabolic byproducts of organisms that thrive in stagnant conditions.¹⁵ Humans can detect geosmin at astonishingly low concentrations, in the low parts-per-trillion range, so even modest microbial activity can make water smell “off.”

In more extreme stagnation scenarios, sulfate-reducing bacteria can take over and produce hydrogen sulfide, the compound behind the classic rotten-egg smell. Research on plumbing systems with extended stagnation found that sulfate-reducing bacteria became dominant in water heater tanks and pipes left unused for long periods.¹⁶ This is the explanation for why a vacation home sometimes smells sulfurous when you first turn on the faucet after weeks away.

Practical Rules for Storing Water

For everyday life, the story is simple: a glass of tap water left on the counter overnight is not going to make you sick. The bacterial growth that occurs over eight to twelve hours at room temperature is almost entirely from organisms already present in the water and your plumbing, and in an otherwise healthy person these are harmless. The taste change you notice is real but not dangerous.

For longer-term storage, like emergency preparedness kits, the rules tighten. Organizations like the U.S. Federal Emergency Management Agency generally recommend replacing stored tap water every six months, which lines up with the chemistry: chlorine fades, container leaching accumulates, and bacterial communities shift. Using food-grade containers, keeping them in a cool, dark place, and sealing them tightly all extend the window. Research on treated rainwater stored for emergency use on isolated islands found that biosafety could be maintained for about seven days with proper treatment, versus much shorter periods for untreated water.¹⁷

Commercially packaged water can deteriorate too. An evaluation of packaged water brands found that some showed high coliform counts, increasing turbidity, and noticeable color and taste changes during their shelf period, in some cases before the printed expiration date.¹⁸ The expiration dates on commercial water bottles are mostly about container degradation and taste, not about the water itself “expiring,” but the research suggests those dates are not entirely meaningless either.

One low-tech option for disinfecting water in the field is solar disinfection, or SODIS: filling clear PET bottles and leaving them in direct sunlight. The UV radiation inactivates most bacteria within an hour under strong sun, though dormant forms like spores and cysts take longer and some require up to six hours.¹⁹ Field tests in Ethiopia demonstrated that even large 25-liter containers could produce safe drinking water in less than three hours when turbidity was low.²⁰ The tradeoff is that PET bottles used for SODIS may leach slightly more antimony due to the UV and heat exposure, though the immediate safety benefit of killing pathogens usually outweighs the trace chemical risk in settings where people lack clean water.²¹

Water That Has Been Sitting for a Million Years

If stored water “goes bad” in days, you might wonder what happens to water that sits for geological timescales. The answer is revealing. Deep beneath the Earth’s surface, vast quantities of groundwater have been trapped in rock formations for extraordinary periods. Researchers analyzing carbon isotope data from more than 6,000 wells around the world found that fossil groundwaters, those recharged thousands to millions of years ago, make up roughly 42 to 85 percent of total groundwater stored in the upper kilometer of the crust.²² Deep groundwater can have residence times ranging from tens of thousands to over a billion years.²³

This ancient water is not “bad” in the way your bedside glass gets stale. Sealed in rock with no light, limited nutrients, and intense pressure, microbial growth is negligible. Noble gas studies in places like the Paris Basin have used radiogenic helium accumulation to estimate groundwater ages, finding residence times far longer than surface hydrology models predict.²⁴ The chemistry of this water is strange by drinking-water standards, often extremely mineral-rich or salty, but the water molecules are perfectly intact. Water does not decompose. It is one of the most chemically stable substances on the planet.

This gets at the heart of the question. Water molecules do not age. What changes is the environment around them. Give bacteria warmth, nutrients, and time, and they will colonize. Remove chlorine and seal water in plastic, and chemical exchanges slowly accumulate. Strip away all of those factors, as deep rock formations do, and water sits essentially unchanged for eons.

Recycled Water on the Space Station

Perhaps the most extreme test of whether water “goes bad” plays out 400 kilometers above Earth. Astronauts on the International Space Station drink water that has been recycled from humidity, sweat, and urine through the Environmental Control and Life Support System, which achieves up to 93% water recovery.²⁵ The same water molecules cycle through human bodies and back into the drinking supply repeatedly.

Maintaining the quality of this endlessly recycled water requires constant monitoring. Chemical quality is tracked in real time using an onboard organic carbon analyzer, while microbial content is assessed through capture devices and periodic samples sent back to Earth for ground-based analysis.²⁶ The system works because it uses iodine or silver-based biocides in place of chlorine, filtered continuously rather than dosed once and left to decay. It is a useful illustration of the general principle: water does not go bad on its own. It goes bad when the conditions that keep it clean stop being maintained. Astronauts drinking water that has been through the system dozens of times are not drinking “old” water in any meaningful sense, because the purification resets the clock every cycle. The same logic applies on the ground. Your stored water is only as good as whatever was keeping it clean at the start, and for how long that protection lasts.