We routinely throw away a glass of water left overnight yet think nothing of drinking groundwater that may have been underground for thousands of years. Science tells us that water itself does not expire; its safety depends on how it is stored and handled, not its age. As Pakistan faces growing water scarcity, challenging our perceptions about “fresh” and “stale” water may be just as important as developing new water resources.
When did water start expiring?
Most of us have been conditioned to believe that water left overnight becomes “stale” or unsafe to drink. We empty yesterday’s jug into the sink without a second thought, while happily drinking groundwater that may have been stored beneath the Earth’s surface since long before recorded human history.
Science tells a very different story. Pure water does not deteriorate simply because time has passed. Unlike milk, juice or cooked food, water does not spoil on its own. What changes is not the water itself, but the conditions in which it is stored. The idea that water has an “expiry date” is less a scientific reality than a human perception.
Bottled water: is the date about the water or the bottle?
Many consumers assume that the date printed on bottled water indicates when the water becomes unsafe to drink. In reality, it relates far more to the packaging than to the water itself.
The U.S. Food and Drug Administration (FDA) states that commercially bottled water, when properly sealed and stored, has an essentially indefinite shelf life. Manufacturers are not required under federal regulations to print an expiry date on bottled water. The practice became widespread after a 1987 law in the U.S. state of New Jersey required expiration dates on all packaged food products, including bottled water. The requirement was later repealed because there was no scientific basis for assigning an expiry date to water.
So why do many bottles still carry one? The answer lies in the plastic. Over time, particularly when exposed to sunlight or high temperatures, polyethene terephthalate (PET) bottles can gradually release trace chemical compounds into the water. While these changes may affect taste or odour, they are caused by the container, not the water itself. The question, therefore, is not How old is the water? It is: How has the water been stored?
 Is overnight water really “stale”?
Water left uncovered overnight often tastes different the next morning. That difference is real, but it is not evidence of spoilage. As water remains exposed to air, it absorbs small amounts of carbon dioxide, forming a weak carbonic acid that slightly alters its pH. At the same time, chlorine added during municipal treatment gradually dissipates. These natural chemical processes make the water taste flatter or less “fresh.”
From a health perspective, however, clean drinking water stored in a hygienic container generally remains safe to consume for at least 24 hours. The real risks arise when the container is dirty, when people repeatedly drink directly from it, or when bacteria are introduced through poor handling. Once again, hygiene, not time, is what determines water quality.
The oldest water we drink
Perhaps the greatest irony lies beneath our feet. Much of the groundwater extracted from wells is far older than most people realise. Scientists use isotopes such as Carbon-14 and other environmental tracers to estimate groundwater age. Research from aquifers around the world, including the Ogallala Aquifer in North America and the Nubian Sandstone Aquifer beneath North Africa, has shown that some groundwater has remained underground for tens of thousands, and in certain cases even hundreds of thousands, of years. This ancient groundwater is commonly known as fossil water.
Remarkably, it is often cleaner than younger groundwater because it has been naturally protected from surface pollution, agricultural runoff, industrial contaminants and many modern pollutants. As groundwater slowly moves through layers of rock, it dissolves natural minerals that influence its taste and composition, but age itself does not reduce water quality. In many cases, the oldest water is among the safest.
Reservoirs store water for years
The same principle applies to water stored behind dams and reservoirs. Water captured during one monsoon season may remain in storage for months or even years before it is released for irrigation, industry or domestic use. During prolonged dry periods, these reservoirs become essential buffers against drought and water shortages.
Their value is determined not by the water’s age but by effective management, monitoring, and treatment. Freshness is not measured by a calendar.
Why this matters in Pakistan
This conversation carries particular significance in Pakistan. Around 92 percent of the country’s land area falls within arid and semi-arid climatic zones, while the overwhelming majority of its freshwater resources originate from the Indus River system. Groundwater has become indispensable for agriculture, domestic supply and industry, yet extraction continues to increase at unsustainable rates.
Pakistan is consistently ranked among the world’s most water-stressed countries. More than 80 percent of its population experiences severe water scarcity for at least one month every year, while agriculture consumes the vast majority of available freshwater resources. In a country facing such immense pressure on its water resources, our everyday habits deserve greater attention.
Throwing away a jug of perfectly drinkable water because it has been left overnight, or discarding half-finished water bottles after weddings, conferences and public events, may appear insignificant at the individual level. Collectively, however, these habits represent a culture that treats water as disposable rather than precious. The challenge Pakistan faces is not only one of water availability; it is equally a challenge of water stewardship.
Rethinking our relationship with water
Water itself does not expire. What expires is our perception of it. Where water is abundant, yesterday’s glass is easily dismissed as “stale.” Where the next drink is uncertain, that same glass becomes invaluable.
Science reminds us that the value of water has very little to do with its age and everything to do with its quality, storage and availability. As climate change intensifies, groundwater reserves decline and populations continue to grow, societies will increasingly need to move from a culture of convenience to one of conservation.
Every litre of water unnecessarily discarded represents far more than just wasted drinking water. It embodies the energy required to pump it from rivers and aquifers, the infrastructure needed to treat and distribute it, and the growing pressure on increasingly fragile water resources.
Perhaps it is time to stop asking whether water has expired and start asking whether our habits have. Because in the end, the expiry date was never written on the water. It was written in our minds.
