Groundwater comes from rainfall that has filtered down through the ground and is stored in permeable rocks, known as aquifers. The amount of water aquifers receive (known as recharge) varies throughout the year. Recharge rates and water levels are lowest in the summer and early autumn, when there is less rain and higher temperatures. In these conditions more water is evaporated at the surface and plants take more water from the ground, so less is available for recharge. Recharge rates are highest in the winter and groundwater levels reach a peak in early spring. Changes in groundwater levels lag behind rainfall, and this allows springs and rivers to continue to be fed by groundwater over the summer, which in turn causes the groundwater level to fall. Failure of winter rainfall over one or more years can lead to shortages in groundwater.
The main aquifers in England and Wales are the Chalk in the south and east of England, sandstone in the west of England and Wales, and limestone. Most water circulates slowly in the upper 100 or 200 metres of an aquifer. In some places it can penetrate several kilometres below the surface, although at this depth the water becomes too salty to drink. Although at least 40 billion m3 of water is contained in the top 20 metres of the two main aquifers alone, over 16 times more than the total capacity of all surface reservoirs in the UK, only some of this is available to use.
Groundwater flows through aquifers and out into rivers, lakes and the sea. The water table usually follows the shape of the ground surface above it, and gravity causes water to flow from high to low areas.
Groundwater is vitally important to both our environment and our water resources. It forms the base flow of the majority of the UK’s rivers and streams. This keeps rivers flowing in periods of low rainfall and helps maintain wetlands.
Groundwater is also abstracted for public and private water supplies, as well as a range of industrial and agricultural uses. Across England and Wales approximately 35% of all public water supplies are from groundwater. In parts of the country this rises to over 80% of the water supplied. There are also numerous small (<20m3/d) unlicensed private abstractions from wells, boreholes and springs. 78% of abstracted groundwater is supplied to households and businesses by water companies. Industry (12%), aquaculture (5%) and agriculture (4%) also directly abstract large volumes.
Often groundwater only needs simple disinfection before being used in the drinking supply. Natural processes within rocks and soils maintain groundwater at high quality in many of our aquifers. However, groundwater pollution will occur if contaminants enter the ground faster than the natural capacity to attenuate them. Unlike surface water, once groundwater is polluted it’s very difficult (often impossible) to return it to its natural unpolluted state, even if millions of pounds are spent on investigations and remediation. Pollutants can remain in the ground in pore spaces, fractures within the rock and in the groundwater for many tens or hundreds of years. This is why it’s so important to protect groundwater and prevent pollution from occurring in the first place.
Groundwater is protected by direct regulation through the Groundwater Directive (80/68/EEC). In England and Wales this Directive is transposed into UK law by the Groundwater Regulations 1998. The Directive specifies contaminants, split into two Lists, which can have an adverse impact on groundwater. List I substances are the most toxic (for example chlorinated solvents, pesticides and sheep dip), and must be prevented from entering groundwater. List II substances are less toxic and their entry into groundwater must be controlled to prevent pollution. List II substances include compounds such as ammonia, fluorides and nitrites.
The Environment Agency has produced a report entitled 'The State of Groundwater in England and Wales'. The report highlights diffuse pollution, growing demand for water, land use, climate change and pesticides as the main issues affecting groundwaters. The report can be found here.
The UK Groundwater Forum provides a source of information and a platform for information exchange on groundwater topics in the UK. For more details, click here.
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