Last edited 24 Sep 2020


An aquifer is a geological formation that is sufficiently porous and permeable to store and yield a significant quantity of water to a borehole, well or spring.

A confined aquifer is an aquifer that is confined between two layers of soil or rock that are much less pervious than the aquifer.

Principal aquifers are layers of rock or drift deposits that have high intergranular and/or fracture permeability, meaning they usually provide a high level of water storage and transmission. They may support water supply and/or river base flow on a strategic scale. In most cases, principal aquifers are aquifers previously designated as major aquifers.

There are two types of secondary aquifer designation:

  • Secondary A: permeable layers capable of supporting water supplies at a local rather than strategic scale, and in some cases forming an important source of base flow to rivers. These are generally aquifers formerly classified as minor aquifers.
  • Secondary B: predominantly lower permeability layers which may store and yield limited amounts of groundwater due to localised features such as fissures, thin permeable horizons and weathering. These are generally the water-bearing parts of the former non-aquifers.

Ref HS2 London-West Midlands Environmental Statement,November 2013, Volume 1 Glossary of terms and list of abbreviations.

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