Introduction to aquifers and related ideas. Porosity & permeability Aquifers and aquicludes London’s impact

Porosity and permeability - what’s the difference? Porous and permeable Porous - is to do with having holes (pores) in which water can collect. Permeability is to do with transmitting water - can water flow through it effectively? Some sediments - like clay - have huge volumes of pore space. Up to 60% of a clay soil can consist of tiny pores between microscopic clay flakes. The holes are so small though that clay is impermeable. Conversely some rocks can have little porosity - like crystalline limestone - but may contain cracks and fissures which make them very permeable. And some rocks, like chalk, are both porous AND permeable, making them very important aquifers. Eg sandstone, sands, gravels Porous but impermeable Eg silts, mudstones, clays Non-porous but permeable Eg limestone home

Typical aquifer in the English Chalklands - eg South Downs. Springline Aquifer Aquiclude Water table Springline Rain landing on the permeable (and porous) chalk seeps down into the aquifer, sandwiched between the clay strata. This water has entered long term storage. By contrast the rain landing on the clay cannot infiltrate so flows off the surface into streams. This can be stored in surface reservoirs instead. Reservoirs on the surface can be effective but they take up large areas of space and can be prone to pollution. On the other hand they can be important amenities too for wildlife, fishing or boating. The upper surface of the saturated rock is called the water table. Springlines form where this meets the surface. home

London’s impact on its aquifers: North Downs South Downs “Hydrostatic head” In the 1800s London’s water supply was “artesian” - ie it flowed up out the ground whenever a well was dug. This was due to the hydrostatic pressure of the water high up in the chalk downs. Urban growth in the 20th century created a huge increase in water demand. The water table declined as water was removed faster than it could be replaced by rainfall. As the hydrostatic pressure declined, the artesian wells no longer flowed and London needed to pump up water from underground. In the future the state of London’s water supply will depend on natural factors - like the rainfall and evaporation balance - and human factors. These will include changes in demand, water conservation and the ability to recycle water effectively. home

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