Scientists generally agree that the amount of water on Earth is finite, and that the water cycle constantly recycles water through the Earth’s systems. But how long does this take? How long does water stay in any one part of the cycle? These are questions that many scientists investigate, and the answers might just surprise you!
The water cycle is dynamic and always active, but that doesn’t mean every molecule of water is constantly moving through the system. In fact, water is stored in various parts of the cycle, often referred to as reservoirs. These might be as large as water in the oceans, or, on a smaller scale, water can be ‘trapped’ in an iceberg or a lake. Much more water is in storage than actively moving through the water cycle.
How long water spends in each reservoir is called ‘residence time’. These are some estimated average residence times, but it’s important to remember that some water will spend much longer or shorter time than this.
| Reservoir | Estimated residence time |
|---|---|
| Atmosphere | 7–9 days |
| Soil moisture | 14–28 days |
| Snow | 30–150 days |
| Rivers | weeks–months |
| Groundwater | months–10,000 years |
| Glaciers | 40–400 years |
| Lake | 100 years |
| Ocean | 3,000–3,500 years |
Why is residence time important?
These different residence times are important to remember when we are trying to minimise human impacts on the environment, for example, knowing that water might spend months to 10,000 years in the groundwater before seeping out of the ground.
Imagine an old garbage dump has been found to be leaking toxins into the groundwater but testing of a nearby lake has shown no traces of these toxins. If efforts are made to fix the pollution so that no more toxins can enter the groundwater system, how long should the lake be monitored?
It might be that there is never a problem, or contamination may occur 1,000 years later. This has major impacts when we are trying to manage our past contamination problems and trying to remedy things for the future.
Activity ideas
In Constructing an aquifer model students build an aquifer model and examine how water gets into an aquifer system.
In Groundwater contamination students build an aquifer model to look at point source and non-point source pollution.
