Residence Time of Groundwater
The length of time water spends in the groundwater portion of the hydrologic cycle may be as little as days, or as much as 10,000 years or more. This is called “residence time.” For example: A raindrop may fall to the earth’s surface and seep down through the soil to a saturated zone or aquifer only to pumped back to the surface and sprayed from a garden hose and back down again.
Estimated depth and residence time of the world’s water supply
| Water | Equivalent depth (meters) | Residence Time |
|---|---|---|
| Oceans/Seas | 2500 | ~4000 years |
| Lakes/Reservoirs | .25 | ~10 years |
| Swamps | .007 | ~1-10 years |
| Rivers | .003 | ~2 weeks |
| Soil Moisture | .13 | ~2 weeks-1 year |
| Groundwater | 120 | ~2 weeks-10,000 years |
| Ice caps/Glaciers | 60 | 10-1000 years |
| Atmospheric water | .025 | ~10 days |
| Biospheric water | .001 | ~1 week |
How Much Water Exists?
The Earth is 70% Water. Less than 1% is groundwater. In fact, over 99% of all water is not available for our use. So where is all the water? The Earth’s water is all around you. Water is in streams, lakes, oceans, and rivers. This water is called “surface water.” In addition to surface water, water is located beneath our feet, in the ground as soil moisture and in aquifers.