In what will be a regular series, one of our consultant hydrogeologists, James

Dodds, discusses some of the technical issues relating to underground water

What is groundwater?.

"

> ,

-- ,

-- '

-- ,

-- ,

-- '

.

-- :

R<>

What is groundwater?

,,<,",,.,,,

Some would consider

Groundwater to be any

water that is found in the

ground, This however, is

not true. Groundwater is

part of the hydrological

cycle, which is shown in

the figure below. Rainfall

hits the ground and some

is returned to the

atmosphere by

evaporation and plants,

and some flows over the

ground as streams and

rivers. The remainder

percolates into the

ground and into the

Groundwater. As

Groundwater, the water

flows through the rocks

and emerges in rivers,

as springs or at the sea unless it is pumped out

of the ground from wells and boreholes.

Spring

SAnJRATED ROCK.

UNSATURA1FD ROCK

Groundwater is defined by Fetter (Applied

Hydrogeology) as "water stored in the zone of

saturation". The fact that the zone is saturated

is important, as water will only drain freely

from saturated material and we need the water

to drain freely in order for it to enter a well or

borehole. When water flows into a well the

level to which the water rises is called the

"water table". Groundwater is the water below

the water table.

The water table is close to the surface in valleys

and actually intersects the ground surface where

rivers, lakes and marshes occur, but is much deeper

under hills. It is often said that the water table

reflects the land surface but in a subdued form.

Rocks from which significant quantities of

water can be obtained are known as aquifers.

Aquifers are grouped into two main types -

Primary and Secondary. A primary aquifer can

be thought of like a bucket of marbles. The

water is located in the gaps between each of the

marbles. A typical primary aquifer would be a

sand or sandstone. A secondary aquifer can be

imagined as a bucket of concrete. If the

concrete is solid then there will be no water

within it. However, if the concrete has cracks in

it, then there may be small amounts of water. If

the concrete has many cracks then a moderate

amount of water may be present and if it is

pulverised then it would look very similar to the

bucket of marbles and could have a lot of water

in it. A typical secondary aquifer would be a

limestone.

In order for water to be able to flow through an

aquifer there must be a pathway of

interconnected spaces (pores). In a primary

aquifer the pathway will be made by the

connections made between the pores. In a clean

sand these connections will give free passage to

the water, but if clay and silt particles are

present then these will partially fill the pores

and cause blockages. In this case it would be far

more difficult for water to flow through the

material and the permeability would be low.

In a secondary aquifer the fractures must be

connected and large enough to allow water to

flow through them. The fractures act as little

pipes a fraction of a millirnetre in diameter. In

very fractured rocks then water can be

transmitted quickly. An extreme example of this

would be a limestone cave or cavern, which in

reality is a very large fracture.

In the next article, we will discuss aquifer

storage and what happens to the

,

groundwater when a borehole is pumped.

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.