What is
groundwater?
When a hole is dug in permeable (or porous) soils, at a particular
depth water begins to flow in. The surface of the water that
accumulates in the hole is the water table and the water in the ground
below the water table is called groundwater.
The variations in the shape of the water table reflect the topography
(the Earth’s surface features). The water table is near the ground in
valleys actually intersecting the ground surface where rivers, lakes
and marshes occur but it is at much greater depths below hills.
Groundwater makes up about seventy percent of all the world's
freshwater.
(Source: UK Groundwater Forum)
The study of the distribution and movement of groundwater is called
hydrogeology.
Where does it come from?
Water is continually moving through the environment - we call this the
water cycle.
(Source: UK
Groundwater Forum)
Water evaporates
from the oceans, condenses into clouds and then falls on the land
surface as rain and snow (precipitation) and soaks into the soil, with
excess rainfall flowing overland to rivers and back into the seas and
oceans.
Part of the
rainfall returns to the atmosphere through evaporation and
transpiration by vegetation and once the needs of plant roots and soil
moisture have been satisfied, the remaining water will soak further
into the ground - a process called infiltration and trickle downward
into the rocks, becoming groundwater.
These underground rock layers have the capacity to let water flow
through them, either through large cracks and openings in the rock, or
through tiny spaces between individual rock grains. Such bodies of rock
are known as aquifers.
The best way of imagining an aquifer is as a giant sponge. Water aided
by gravity naturally fills the aquifer from the bottom upwards. The
bottom part of the aquifer has spaces that are completely filled by
water termed the 'saturated zone'. In the top part of the aquifer, the
rock spaces contain air as well as water and here the aquifer zone is
called 'unsaturated'. The margin where the aquifer changes from
unsaturated to saturated is marked by the 'water table'.
Water in an aquifer does not sit still - it flows through the spaces
and cracks in the rock, being pulled by gravity and pushed by the
force of the water above and behind it. The water moves from an area
where water enters the aquifer (a recharge zone) to an area where
water exits the aquifer (a discharge zone). Groundwater flows through
these aquifers to outlets in rivers, at springs and in the sea.
Groundwater makes a significant contribution to the flow of rivers and
it is responsible for maintaining river flows during extended periods
of dry weather.
What can it be used for?
Groundwater is withdrawn from the ground by drilling down into the
water-bearing rock layers or aquifers and pumping the water out at
extraction wells.
As groundwater is generally very clean, it often requires little or no
treatment before being used. The level of treatment depends on what it
is to be used for. This makes groundwater a very cheap source of 'raw
water' for public supply and it currently provides about one third of
public water supplies in England and Wales.
Approximately 2,400 million m3 of groundwater (enough to fill 640,000
Olympic-size swimming pools) is abstracted annually in the UK,
including that used by industry and agriculture which rely on
groundwater in many areas. It is still the source of water supply for
many rural communities.
The Environment Agency controls the abstraction of groundwater in
England and Wales by issuing licences which are required for all major
sources of water supply.
Why are groundwater levels monitored in south Cardiff?
During the feasibility and planning stages of the Cardiff Bay Barrage
project, concerns were raised about the possibility of groundwater
levels rising in south Cardiff resulting in properties in the area
being damaged. This concern was based on the premise that the average
Bay level following impoundment would be approximately 4.0m higher
than the average level under the tidal regime which was then in
existence.
As a result, a comprehensive groundwater protection scheme was
incorporated into the Cardiff Bay Barrage Act 1993. The Act obliged
Cardiff Bay Development Corporation, now Cardiff Harbour Authority to
monitor groundwater levels for a period before (pre-impoundment) and
after (post-impoundment) the construction of the Barrage, so that the
impact of impoundment on groundwater levels could be determined.
A monitoring system was established in 1995 to measure groundwater
levels during both pre- and post-impoundment periods.
Information regarding the Harbour Authority’s
groundwater
monitoring and
groundwater control measures are
provided in this section together with details of the
groundwater protection
scheme.
Further reading:
www.groundwateruk.org/html/depth.htm
www.groundwateruk.org/html/forum/groundwater_valuable_resource.pdf
www.environment-agency.gov.uk
For children:
www.groundwater.org/kc/whatis.html
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