Following onfrom our last issue,

one of our consultant

hydrogeologists, James Dodds.

discusses more of the technical

issues relating to underground water.

60

Sponges,

Drawdown & Cones

Rocks and soils are

made up of three parts;

the solid rock, soil and

clay particles, water

and air. The spaces

between the rock

particles are where the

water and air sit. The

total volume of the

spaces is called the

Porosity. In terms of

the water supply to a

well or borehole, what

is important is how

much water we can

take from the porosity.

In almost all rocks and

soils we cannot take all the water held in the

porosity, this is because some of the water

sticks to the soil particles as a result of surface

tension and other forces. The amount of water

held by the rock or soil is called the Specific

Retention. In soils and rocks containing a

significant amount of clay, this retained water

can amount to a large proportion of the porosity.

In a very sandy or clay poor material. then the

retained water will be a small proportion. The

amount of water that will freely drain from a

rock or soil is called the Storativity (Storage) or

the Specific Yield.

UK GROUNDWATER FORUM

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clay

In the last article.

we talked about what

groundwater is, how it

moves through the ground

and the term aquifer.

However, water does not

only move through the

ground. it is also stored in

the ground, much like a

sponge holds on to water.

This so called STORAGE is

a very important aspect of

a groundwater system and

is one of the more difficult

concepts to understand.

100

gravel

Why is this important? Well, the interaction

between the storage and the permeability of the

ground defines how a borehole will respond to

pumping. That is to say, the fall in the water

level in a borehole when it is pumped is

controlled by these two physical properties of

the aquifer. That means it is these two

properties that control the rate at which water

can be pumped.

When a borehole pump is switched on water is

immediately drawn from the water stored in the

borehole. This causes the water level in the

borehole to fall. The water level in the borehole

is now lower than the water level just outside

the borehole and water flows into the borehole

from the aquifer. The aquifer we now know is

like sponge and stores water. Initially water

flows from this storage, which causes the water

table to fall, because we are emptying the

sponge, and the water table marks the top of the

saturated ground. As the water table falls

groundwater is forced to flow from areas of

higher water table to areas of lower water table

and ultimately to the centre of pumping, the

borehole. Because this flow of water is coming

from all directions, the flow is in the form of a

circle around the borehole and the water table

develops an upside down cone shape, with the

deepest point at the borehole and getting

shallower all around. This cone is referred to as

the cone of depression or the area of influence.

The cone of depression will expand outwards

until the water flowing into the cone equals the

water being pumped out. At this stage no water

will be released from storage and all the water

being pumped will be balanced by water

flowing through the aquifer. The sources of the

water flowing through the aquifer will be

discussed in the next issue.

So, what happens when the pump is switched

off? Exactly the opposite. When the pump is

switched off, the water level in the borehole is

still low. Water continues to flow toward the

borehole to fill it up. But it isn't only the

borehole that needs to fill. the Storage in the

aquifer also needs to be replenished. So water

flows from the edge of the cone of depression

toward the centre, filling the space created by

pumping as it goes.

If you have a question about any groundwater

issue then let us know and we will ask James to

answer some of the questions received in the

next issue. Alternatively, you can visit James'

web site at www.jdih.co.uk.