From Cloud to Tap

Earth holds about 332 million cubic miles of water. Sounds like plenty. But about 97 percent of it is salty ocean water, and of the fresh water that’s left, more than two-thirds is locked up in ice caps and glaciers. Most of the rest is hidden underground.

Those two tablespoons are all we’ve ever had. The planet just keeps recycling them, and the machine that does it is the water cycle. The clever part: when water evaporates, it leaves salt and most impurities behind. The sun is running the biggest water distiller on the planet.

From the moment our drop leaves the cloud, it starts picking up hitchhikers. It falls through air carrying dust, soot, and pollution. Then it lands, and the real road trip begins.

Now, the amounts we’re talking about are tiny. Some contaminants are measured in parts per million, some in parts per billion, and PFAS in parts per trillion. That’s why a treatment plant’s job isn’t just to make water look clean. It has to remove things you will never, ever see.

Our drop ends up in a river, lake, or reservoir. That’s where most Americans on public water systems get their water; everyone else pumps it up from underground. Either way, the trip into the treatment plant starts at an intake, where big screens catch sticks, leaves, trash, and the occasional confused fish. Then pumps lift the water into the plant for a series of steps that are honestly kind of brilliant.

By the end of filtration, our drop looks crystal clear. But looking clean and being safe are two very different things. That’s what disinfection is for, and it’s where some germs fight back.

Our drop is clean, but it’s still miles from your house. Powerful pumps push it into large transmission mains, which branch into smaller distribution mains under your streets. Altogether, the U.S. has more than 2 million miles of drinking water pipe: enough to wrap around the Earth more than 80 times.

Some of that water gets pumped up into storage tanks and water towers. And water towers are genius, because they run on gravity.

But a lot of these pipes are old. Like, really old. Some early American cities piped water through hollowed-out logs, and many still use cast iron mains laid a century ago, alongside newer ductile iron and plastic pipe. Old pipes break, and leaks waste billions of gallons of treated water every day.

Breaks aren’t just wasteful. When pressure drops, dirty water from the soil around a cracked pipe can get sucked inside. That’s why a big main break often comes with a boil-water notice.

Even in good pipes, water changes on the way. Most of it reaches you within a day or two, but at the far edges of some systems it can be more than a week old. Along the way, the chlorine bodyguard slowly wears out, and slimy films of bacteria called biofilm cling to the pipe walls.

Finally, our drop turns off the main and into a small pipe called a service line, which runs under your yard and into your home. And for millions of homes, that service line is made of lead.

Lead was the go-to plumbing material for centuries: soft, easy to bend, and long-lasting. The word plumbing even comes from the Latin word for lead, plumbum. The problem: there’s no known safe level of lead exposure, especially for kids. It can harm the developing brain, and that damage can’t be undone.

Remember the protective coating the treatment plant builds inside pipes? This is where it matters. In 2014, Flint, Michigan switched its water source to save money, without adding corrosion control. The new water ate away the protective scale inside old pipes, and lead flowed straight into people’s homes. Same pipes. Different chemistry. A full-blown public health crisis.

Since then, utilities have had to map every service line, and many post the maps online. You can also check the pipe yourself, where it enters your house.

Lead can also come from inside the house: old lead solder on copper pipes, and certain brass faucets and fittings, especially in homes built before 1986.

And one more thing loves home plumbing: Legionella. This bacterium thrives in warm, stagnant water, like water heaters set too low or pipes nobody uses. Breathed in as a fine mist, say from a shower, it can cause a severe pneumonia called Legionnaires’ disease.

So after all that, what’s actually in your glass? First, the good news: tap water in the U.S. is among the safest in the world. Utilities monitor for more than 90 regulated contaminants under the Safe Drinking Water Act. For each one, the EPA sets a health goal, the level with no known risk, and an enforceable legal limit, set as close to that goal as is practical.

Depending on where you live, a few things can still make it into your glass:

• Disinfection byproducts, usually at low, regulated levels.
• Lead, if your home has a lead service line or old plumbing.
• Nitrates and arsenic, especially in farm country and certain groundwater regions.
• PFAS. A USGS study estimated at least 45% of U.S. tap water contains one or more. In 2024 the EPA set the first national limits, as low as 4 parts per trillion, but it has since proposed scaling that rule back, and the deadlines are still being fought over.
• Microplastics, which aren’t regulated yet. Scientists are still working out what they mean for our health.

And then there’s stuff that sounds scary but mostly isn’t.

So, what can you actually do? Here are five steps you can start today. Tick them off as you go.

So next time you fill a glass, think about the trip it took: a cloud, a river, coagulation, filtration, disinfection, miles of pipe, a water tower, and the last few feet into your kitchen. When it goes down the drain, the journey starts over: to a wastewater plant, back into a river, and up into the sky.

But some contaminants don’t cycle out so easily. One family of chemicals is so persistent that scientists call them forever chemicals. That’s the next lesson.