Groundwater is one of four causes of damp.
What is groundwater?
Why does groundwater act different to rainwater?
Groundwater is defined as; “subsurface water below the water table”.Paul l. Younger; Groundwater in the Environment
There is 100 times more groundwater than water in all the lakes and rivers of the world.
Water is pumped out for urban and agricultural use. For example the water table is 65M below London's Trafalgar Square. The water table is about 45M below sea level - below the lowest tube lines.
Groundwater is not surface water
It is important to understand why groundwater is quite different to surface water. That is water in the soil zone above the water table.
The water cycle in simplistic terms is:
- Precipitation dampens the soil, with excess water draining away to rivers.
- Some water evaporates and some is taken up by animals and plants.
- Eventually some water will be drawn by gravity through the sub-soil zone to the water table to become groundwater.
- Groundwater spills over as springs, geysers, is extracted for drinking water or irrigation, or flows to the sea, above or underground.
Surface soil can become saturated, either temporarily when land floods, or permanently such as in the fenlands.Houses are not built on soil constantly saturated by water.
Characteristics of groundwater
Water is water wherever it comes from. However, groundwater has very different characteristics to water in the soil zone, for the following reasons:
Above the water table
- Pores above the water table are unsaturated.
- The pressure above the water table is below atmospheric pressure.
The pores of a brick sitting on unsaturated soil are competing with the pores in soil, pressure and gravity, therefore the rise of water from unsaturated soil is negligible.
In the Capillary fringe
- Pores become increasingly saturated in the capillary fringe.
- The height of the capillary fringe varies for different types of earth from a few mm for silt to about 1M for mud.
The pores of a brick sitting in the middle of the capillary fringe will absorb roughly half the amount of water that it would sitting in groundwater.
Below the water table
- Pores below the water table are saturated.
- The pressure below the water table is above atmospheric pressure.
Try it yourself
Take two buckets, put 10mm of water in the base.
- Fill 1st with gravel or silt to about about 100mm (making sure water is not rising above the silt/gravel), then put brick onto gravel/silt with soil around it. This represents the brick in soil, out of contact with the capillary fringe.
- Fill 2nd with soil to about 100mm (making sure water is not above the soil), then put the brick onto the soil and pack soil around it. This represents a brick in the capillary fringe.