puricarexfiles: Groundwater and surface water are intimately connected such that pollution which enters groundwater can emerge in surface water. Equally, groundwater can be affected by polluted surface water. Groundwater is used for drinking water supply and by industry. Pumping too much groundwater can cause rivers to dry up and the level of lakes to fall unacceptably low.

Photographed in the winter of 2000/01
during the period when rainfall and subsequent
recharge to many aquifers in England
exceeded previously recorded quantities.
As a result, groundwater levels rose
to their highest recorded levels leading
to extensive groundwater flooding mainly
in the upper the dip slope, normally dry valleys
on of the Chalk escarpment.

Groundwater depletion,

deterioration of water quality and saltwater intrusion

Groundwater is the name used to describe water which is stored underground in permeable rocks.

The period of storage may be a few weeks or several years depending on the geology. Most groundwater flows and emerges at the ground surface as a spring or within a river.

Groundwater and surface water are intimately connected such that pollution which enters groundwater can emerge in surface water.

Equally, groundwater can be affected by polluted surface water. Groundwater is used for drinking water supply and by industry. Pumping too much groundwater can cause rivers to dry up and the level of lakes to fall unacceptably low.

Near the coast, over abstraction can cause salt water to move into the aquifer rocks reducing the quality of the freshwater.

What is Groundwater?

Groundwater is water that has drained through surface layers of soil and rock until it reaches a layer of rock material through which it cannot pass, or can pass only very slowly.

This results in the accumulation of water in the rock layers above this impermeable layer. The water is stored in gaps in the rock, or between the particles of which the rock is composed. Rock which retains water in this way is called an aquifer.

Rock types which can hold water include chalk, limestone and sandstone. Rock types which do not hold water but can prevent water moving out of a permeable rock type include granite, basalt and mudstone.

Groundwater supplies springs and wells and, when there has been a period with little rain, it is often groundwater that maintains flow in rivers rather than surface drainage from land.

In more remote areas, where there is no public water supply, a household may take its water supply direct from groundwater via a borehole and use it with little or no treatment if the aquifer is of good quality.

Industry and agriculture also abstract water directly from aquifers either because they cannot, or it is not convenient, to take water from the main supply or because they do not need to use water of potable quality.

Pressures and Impacts

Pollution

An aquifer may be as little as 30m from the surface, or as much as 300m. It costs more to pump water from the deeper aquifer but the water quality in the deeper one may be better than the shallower since contaminants which the water may be carrying are removed as the water moves through the rocks..

Historically, groundwater has been naturally very clean because of this filtering effect. However, increased use of chemicals in everyday life means that groundwater is now vulnerable to the same pressures as water at the surface.

Mazaki Automatic Pump Controller

Since water continuously flows from one to the other, groundwater can become polluted with nutrients or chemicals when surface water carrying these substances drains into the groundwater environment.

Diffuse sources of groundwater contamination include pesticide and fertilizer use. Point-specific sources include industrial spills, leaking oil and chemical storage tanks and landfill sites.

Normally it is easier to remedy a point-specific source since its point of origin can be relatively easily identified.

Over-abstraction

When groundwater is used at rates which are greater than the rate at which it is replaced by natural processes, its level within an aquifer drops. This will happen naturally during the summer and may be protracted in drought years.

However, prolonged abstraction for water supply purposes above the rate that an aquifer can support can impact rivers, wells and wetlands connected to the aquifer. Wildlife habitat and recreational fisheries and boating will be negatively affected where excessive groundwater use impinges on river water supply.

Where there is less water entering a river, the chemical quality of the river can suffer since the dilution of any chemicals present will be reduced and this can lead to fish death.

Over-abstraction is being more widely identified as a problem and water companies and environmental protection bodies are increasingly working together to remedy the situation.

Saltwater Intrusion

Freshwater in an aquifer near the coast will exist in balance with the salt water which is contained in the aquifer beneath the sea. Salt water is heavier than fresh water so at the margin between fresh and salt waters it will lie underneath the fresh water.

The boundary between the two moves through the year, moving further out to sea when winter rainfall increases the pressure and volume of freshwater in the landward side of the aquifer.
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Where freshwater abstraction from the coastal aquifer is not in equilibrium with the flow and ebb of these two bodies of water, salt water can intrude into the zone of freshwater abstraction and reduce the quality of water supply. This is termed saltwater intrusion and it is particularly prevalent in hot climates where fresh water is in short supply.

Where freshwater abstraction from the coastal aquifer
is not in equilibrium with the flow and ebb of these
two bodies of water, salt water can intrude into the
zone of freshwater abstraction and reduce the quality
of water supply

The southern coast of Spain suffers saline intrusion because freshwater abstraction for the tourist and horticultural industries exceeds the ability of the freshwater aquifer to supply. However, saltwater intrusion also occurs in the UK where there is excessive abstraction from coastal aquifers.

Groundwater Protection

There are several ways that groundwater may be protected from contamination. The spillage of substances which may cause pollution should be avoided through the use of good management practice.

Bladder-Type Pressure Tanks

Fertilizers should be used in a fashion which maximizes their use by the crop and minimizes leaching losses to rivers and groundwater.
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Drinking water supplies may be protected by defining source protection zones within which development is limited in order to reduce the chance of spillage of potentially polluting substances.
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The size of the source protection zone will depend on the geology of the aquifer, including how fissured (cracked) the rock is, and hence how fast the groundwater is traveling through it.

The degree of sensitivity is dependent on the proximity, with development being more restricted close to the point of abstraction. Prohibited development in these zones would include a petrol station or landfill site.

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