Infiltration and groundwater

There is more fresh water under the ground than in all the world's freshwater lakes and rivers together, by more than a hundred times.

Not an underground river

Many people picture groundwater as rivers and lakes in caves. A few exist, in limestone country, but most groundwater is nothing like that. The USGS compares it to water in a sponge. It fills the small spaces between grains of sand and gravel and the cracks in rock. Dig a hole on a beach and the bottom fills with water: you have reached the level below which the sand is full of water.

From the surface to the water table

Infiltration is rain soaking into the ground. Some of it stays in the top layer as soil moisture, where roots take it up and plants send it back to the air by transpiration. The rest keeps moving down, which hydrologists call percolation, through a zone where the gaps hold both air and water. Eventually it reaches a depth where every gap is full. The top of that saturated zone is the water table, drawn as a dashed line in the model.

The water table is not flat. It roughly follows the shape of the land above, higher under hills and lower near valleys, though more gently. Where it meets the surface you get springs, wetlands and rivers that keep flowing in dry weather. Rock or sediment that holds and passes enough water to supply a well is called an aquifer. Sand and gravel make good aquifers; clay holds water but passes it so slowly that it acts as a barrier.

Slow, deep and large

Gravity and pressure push groundwater downhill and sideways through the gaps. The USGS describes it emerging back at the surface, into rivers and into the oceans to keep the cycle going. Because it moves so slowly, groundwater can stay underground for years or even thousands of years. It is the second-largest store of fresh water after ice. The USGS table puts fresh groundwater at about 10,530,000 km³, 30.1% of all fresh water and more than a hundred times the 93,100 km³ in fresh lakes and rivers.

That makes groundwater precious and easy to misuse. NOAA explains that some sources take a long time to replenish, so pumping faster than rain refills an aquifer can lower the water table, dry up wells and springs, and reduce the flow in rivers that depend on it. Pollution is the other risk: what seeps into the ground with the rain, such as fuel or fertiliser, can travel with the groundwater for years.

Not all groundwater is fresh. In the USGS table about 12,870,000 km³ of groundwater is saline, slightly more than the fresh part. A well works by reaching below the water table, so that water flows from the surrounding gaps into the hole. Pumping lowers the water level around the well in the shape of a cone, and many wells pumping close together can lower the water table across a whole region. Near the coast, fresh groundwater flowing towards the sea meets salty water underneath, and pumping too hard can draw that salt water into the wells.