"We need to learn how to value water"
Kofi Annan, UN Secretary General
See below for June 2003 UNEP report details.
When rain falls to the ground, the water doesn't stop there. Some of it moves below the surface to become groundwater.
Go to the site of THE GROUNDWATER FOUNDATION. (Yes they do exist, and find the answers to the following questions...)
1. Define the term 'groundwater'
2. What is meant by the 'water table'. What factors affect the position of the water table beneath the ground.
3. What happens when the water table rises to the surface ?
4. What percentage of water used for agriculture comes from groundwater ?
5. What percentage of drinking water for i) urban, and ii) rural population comes from groundwater ?
6. How can groundwater become contaminated ?
Groundwater is commonly used to supply London, as the Thames Basin contains rocks known as AQUIFERS which have the potential to store water. This water is often under such pressure that if a well is sunk, the water rises to the surface naturally without having to be pumped.
(Groundwater management in different locations features in the Synoptic Unit section of the AS Fluvial Environments topic..)
Groundwater is water that is found underground, in the interstices (pore spaces) in soil and certain rocks.
Some of it is used by plants, some evaporates and some moves due to gravity into the nearest river.
The area beneath the ground where all the available spaces are filled is called the SATURATED ZONE (Phreatic) The top of this zone is called the WATER TABLE. The Water Table takes on the shape of the overlying topography. The water table may be at, or very close to the surface or hundreds of metres down.
Level of water table varies depending on the antecedent conditions. After heavy rain the water table may rise to the surface. Dry weather, or large scale abstraction may lower the level of the water table.
Rocks which are able to store water are called AQUIFERS. The speed at which water flows into these rocks is determined by the size of the pore spaces and how well the spaces are connected. Aquifers consist of gravel, sand, sandstone and fractured rock. The total volume of pore spaces in a rock is called the POROSITY. If rocks are too permeable they allow water to flow through them (you wouldn't try to carry water in a colander..) The speed of flow of water through rocks is controlled by Darcy's Law, named after a French civil engineer.
Typical speeds of flow for groundwater are between 1 metre/year and 1 metre/day, although in fractured karst (limestone areas as you well know...) the speed of flow can be comparable to that of rivers...
Groundwater flow follows a curved path through an aquifer from areas of high water levels to those where water levels are low. The slope on the water table is called the HYDRAULIC GRADIENT. When there is a borehole, the surface of the water table is depressed by drawdown.
Groundwater is likely to be old. An average (who works these things out) is about 236 years. Some of the rainfall in the London basin is around 20 000 years old, and fell during the Ice Age.
When a hole is dug in permeable rocks at a depth below the water table, water will begin to flow into the hole. Well, well... The surface of the water that accumulates is the water table, the water below that level is groundwater.
Groundwater supplies are replenished or RECHARGED by rainfall and snowmelt. Ideally, rainfall arrives at a rate below the infiltration capacity so that the water can soak in. Heavy rainfall will tend to runoff into rivers and not replenish groundwater.
Anglian Water uses a lot of groundwater to supply their customers. This comes from the Chalk which underlies large parts of East Anglia.
In many cases the water doesn't have to be pumped up. It is already under pressure so when the borehole is sunk, the water comes up it naturally due to artesian pressure.
GROUNDWATER SUPPLIES IN LONDON
London is underlain by an artesian basin. See diagram - usually a cross section showing the geology: permeable area with impermeable layer beneath to trap the water.
The Key to managing Groundwater supplies is that "You can't take out more than is put in" Works like any other storage system, but unlike a bank, you can't go overdrawn.
When industrial abstraction was reduced, groundwater levels started to rise. This adds to the problems of leakage from ancient Victorian pipe and sewer systems. Liverpool and Birmingham have similar problems. The water table has risen by as much as 20m due to a 46% reduction in abstraction. Such a rise has implications:
increase in spring and river flows
re-emergence of springs
surface water flooding
pollution of surface waters which then spreads into the groundwater (contamination)
flooding of basements and underground spaces e.g Tube, and cellars of industrial premises
reduction in stability of these structures, and of foundations
swelling of clays as they absorb water affecting structures
chemical attack on building foundations
The UK has a surplus of water in the NW and a deficit in the SE. The Thames region serves 20% of the nation's population but is located in one of the driest parts of the country. This is likely to only get worse if the various forecasts on the impact of climate change.
THAMES BASIN
| Rainfall | Evaporation | |
| Jan | 70 | 6 |
| Feb | 48 | 13 |
| Mar | 48 | 33 |
| Apr | 49 | 55 |
| May | 59 | 84 |
| Jun | 55 | 100 |
| Jul | 57 | 98 |
| Aug | 72 | 80 |
| Sep | 65 | 49 |
| Oct | 67 | 22 |
| Nov | 79 | 7 |
| Dec | 72 | 3 |
Tasks:
1. Add up the totals for Rainfall and Evaporation (both are measured in mm)
2. In which months will groundwater be used to make up for a shortfall in precipitation (soil moisture utilisation)
3. Why is evaporation so low in the winter months ?
4. In which months are the highest values for a) rainfall, and b) evaporation
5. In which months will groundwater be recharged ?
Thames Basin is one of the most intensively studied basins in the world due to the large population.
Climate is changing, becoming drier.
Aquifer is one of the most important in the country.
We also use the example of Bangladesh, where wells have been contaminated by arsenic. Read about the story HERE.
Another example of Groundwater issues in an LEDC that could be used as a case study is a recent series of stories on the poisoning of people in India by FLUORIDE. There has long been a suspicion that adding fluoride to water could lead to problems. Excess fluoride has apparently been affecting villages in Andhra Pradesh in Southern India. The BBC NEWS story gives some useful background information, and for more on FLUOROSIS you can go to the site called FLUORIDE ALERT.
An article in 'The Guardian' in June 2003 carried the headline "Trickling away....a life and death commodity"
Groundwater resources are diminishing around the world according to a report by UNEP.
It quotes the example of Mexico City, where so much groundwater has been pumped from beneath the city that buildings have sunk 2 metres.
The water table under the high plains in the American Mid-West has fallen on average by 3 metres a decade - up to 30m in places.
Aquifers are at risk of contamination by sea water in Florida.
Some cities in the Arabian Gulf have become waterlogged because of leaking pipes running from desalination plants.
Twelve cities with populations of 10 million and over, including Bangkok, Calcutta, London and Shanghai rely on groundwater reserves.
2 billion people, and 40% of agriculture are dependent on groundwater.
In rural India, 50% of irrigation water and 80% of drinking water comes from underground
Around 96% of Saudi Arabia's water comes from underground
There are 1300 boreholes beneath Dhaka, the capital of Bangladesh - in some cases the water table has fallen by 40 metres
One person in 6 can't rely on safe drinking water
More than 2 billion people have no adequate sanitation
Waterborne diseases kill a child every 8 seconds
Water means food: it takes 1000 tonnes of water to grow a tonne of wheat, and 2000 tonnes of water to grow a tonne of rice
Kofi Annan, UN secretary general said, "We need to learn how to value water."
Desert oases in Jordan are drying up"
Go HERE for the UNEP report in full. It makes fascinating reading.