Old Water: Residence Time and the Cypress Bench Aquifer

Water is always moving. It rises into the sky, falls as rain or snow, flows across the land, and sinks into the ground. This constant movement is the water cycle. Scientists often ask a simple question about it. How long does water stay in one place before it moves on? The answer is called the residence time. Residence time is the average time water spends in one part of the cycle.

Some residence times are very short. A drop of water in the atmosphere stays there for about nine days on average. In just over a week, it falls back to the surface as rain or snow. Rivers also move water quickly. A single drop may travel from a mountain stream to the sea in only weeks or months.

Other residence times are much longer. Water frozen in a glacier can stay locked in the ice for thousands of years. Water deep underground can stay there even longer. In some aquifers, the water has rested below the surface for centuries. In a few places, it has been there for many thousands of years. Scientists sometimes call this old water, because it soaked into the ground long before anyone alive today was born.

To see why this matters, picture how little fresh water the planet keeps ready to use. About 97 percent of Earth's water sits in the oceans, which are too salty to drink. Roughly 2 percent is frozen in ice caps and glaciers. That leaves less than 1 percent as fresh water that people can reach in rivers, lakes, and shallow ground. Groundwater makes up a large share of that small slice.

An aquifer is a layer of rock and sand that holds water in the spaces between its grains. Rain and melting snow slowly soak down and refill it. This refilling is called recharge. In many aquifers, recharge is slow. Water may seep down only a few inches each year. When recharge is that slow, the aquifer acts almost like a savings account that earns very little interest.

Now imagine a town that depends on such an aquifer. Ellsworth is a small, fictional farming town on a dry plain. It draws nearly all of its water from the Cypress Bench aquifer below it. For years, the wells ran without trouble. But the aquifer recharges slowly, while the town and its farms pump from it every day.

When a community pumps water faster than the aquifer recharges, the water table falls. The water table is the top of the saturated ground. As it drops, wells must be drilled deeper, and pumping uses more energy. If the level keeps falling, shallow wells can go dry. Because the water is old and recharge is slow, the aquifer cannot quickly refill what people remove. The water taken out this year may have taken centuries to arrive.

For this reason, residence time shapes real choices about water. A town that draws from a fast river can plan around the seasons, because the river refills within weeks. A town like Ellsworth must think in longer terms. Its water source refills over many human lifetimes rather than a few seasons. If the town wants its wells to last, it has to use water at a pace the aquifer can sustain.

People have several ways to respond. They can measure the water table each year and set limits on pumping. They can water crops with methods that lose less water. They can capture rain and let it soak back into the ground to aid recharge. None of these steps is easy, and each has a cost. But they all rest on one lesson from the water cycle. Some water moves in days, and some moves in centuries, and a community must match its use to the speed of its own supply.

1. About how long does water typically stay in the atmosphere, according to the passage?
2. What share of Earth's water is in the oceans, and what share is fresh water that people can reach?
3. Why does the passage describe deep groundwater as old water? Use a detail from the text.
4. Explain why pumping the Cypress Bench aquifer faster than it recharges causes problems for Ellsworth.
5. Using details from the passage, explain how the aquifer's slow recharge shapes the choices Ellsworth must make about its water.
6. The passage uses the term residence time. In your own words, what does it mean, and give one example of a short and one of a long residence time from the text.