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Infinitely useful, water is often taken for granted until it’s either not there or there is too much of it. Droughts, floods or contaminated drinking water make headlines. But for most of us, filling a cup of water from the faucet, flushing a toilet, or taking a shower are daily activities where little thought is given to where that water originates. 

Where does our freshwater come from, and how endangered is it?

Freshwater Sources

Roughly 71% of the Earth’s surface is water, with nearly 97.5% of that puddled in saltwater lakes and oceans. Fresh water—water containing less than 1000 milligrams per liter of dissolved solids—comprises only 2.5% of the surface water. More than 68% of that 2.5% is found in glaciers and ice caps, and the rest in lakes and groundwater.

While we might consider water a renewable resource, fresh water is dependent on the water cycle. Water evaporates from the oceans, and then returns to earth as precipitation—rain, freezing rain, sleet, hail, or snow. As the oceans warm, more water is vaporized. This water vapor is then deposited as fresh water in the form of glaciers or as rain and snow that seep into the ground to fill our wells.

Too much of a good thing, however—especially all at once—isn’t always beneficial. If the oceans lose too much water, they could become so salty that ocean life is threatened. Plus, water precipitating too quickly can cause floods or dangerous snowfall events. 

Glacial meltwater replenishes freshwater sources and feeds the water back to the oceans. Glaciers are, in a sense, gatekeepers of freshwater sustainability.

Water-Created Cities

The largest cities around the globe are surrounded by water. In New York City, the Atlantic Ocean rests to the east, while the Hudson River, which originates in the Adirondacks, winds its way past Manhattan to the ocean. London, Cairo, Dublin, Beijing, Sydney—it is no accident that these large cities are located near water. For centuries, access to oceans has allowed for trade, and freshwater rivers and lakes are vital for raising crops and animals.

A nighttime map of Earth illustrates the density of human settlements around water sources. From the water-rich eastern United States to the line of bright lights that highlight settlements along the Nile River in northeastern Africa: People gather where water is plentiful.

Water Access

Threats to drinking water supplies might seem like just another disaster movie trope, but global water security is a real concern.

If fresh water is the key to life, and that key’s supply is limited, then it’s no surprise that water rights often spark conflict. To an agrarian culture, water is as precious as gold. 

In sub-Saharan Africa, many people don’t have access to an improved water source—a water source that is protected andand generally safe for drinking. Instead, they often walk more than 5 kilometers to collect water.

Consider how much water you consume daily: 60 liters to flush the toilet, 40 liters to shower, and another 40 liters to fill your water bottle, cook, or wash your hands. When you convert that volume to mass of water (assuming a density of water of 1.0 kilogram per liter), the weight of that much water is close to 140 kilograms, or just about twice the average person’s body mass. Imagine having to carry twice your weight for 5 kilometers!

Water is an invisible resource to many around the world, and we reach for the tap without worrying that water won’t flow out of it. But as the world warms—causing glaciers and ice sheets to melt, salt water to flow up rivers, the Great Lakes to turn brackish—slightly salty—or the oceans to flood our coasts—can chemistry help us maintain a secure, steady supply of freshwater? It’s open for discussion!