Happy Drains Blog

What is a water table?

The water table is simply the level at which the ground is saturated with water. As rain falls, the tiny spaces in the soil become fuller until a whole layer of soil is saturated. Above the water table the ground is drier and excess water will drain through, below the water table the ground is permanently saturated.  Beneath the water table the layers of rock holding water are known as aquifers.

The water table is not a fixed level though, it can rise and fall dependent on rainfall, flooding, and the combined effects of evaporation and transpiration (water being drawn up through the roots of trees and plants and evaporating through the leaves) that result in water leaving the ground and into the atmosphere. In this country the water table is usually a lot lower than in winter, sometimes by as much as a metre.

Knowing where the water table lies can be important when undertaking building work and putting in drainage and foundations. A high water table could make that area unsuitable for construction. For example, putting a septic tank below the water table is not a good idea as the tank may not be stable. If it shifts too much then the tank could rupture or spill, with environmentally damaging consequences! A high water table area is also unsuitable for soakaways, as the soakaway will simply fill with groundwater rather than the surface water that you’re trying to get rid of.

A water table test can be carried out fairly easily by drilling test holes. However this is best done in the Spring after frosts have passed and when the water tables are usually at their peak. Something to think about if you are planning on having any building work done, putting in drainage, or having a septic tank installed.

The water table is also the cause of a certain type of flooding known as groundwater flooding. This is where the water table rises so far that the surface of the ground becomes saturated. It’s often the cause of basements or cellars flooding. Unlike flooding caused by tidal action, drains being overwhelmed, or rivers bursting their banks, groundwater flooding can appear where you don’t expect it, such as on hillsides rather than the valley bottoms. It’s more common in areas where the underlying rock is chalk, sand or gravel. And often there’s a delay between periods of heavy rainfall and groundwater flooding appearing.

Managing the risk of groundwater flooding is the responsibility of local authorities, with the Environment Agency having a strategic overview. But for help with any drainage issues caused by flooding, give us a call first at Happy Drains on 0800 849 8099.