How Groundwater occurs
Groundwater constitutes about 98 percent of the fresh water on our planet, discounting that in the polar icecaps. This makes groundwater fundamental to human life and economic development, so a brief
introduction to how it occurs is appropriate.
When rain falls, some infiltrates the soil, the remainder evaporating or running off to rivers. While the roots
of plants will take up a proportion of this moisture, some will infiltrate more deeply, eventually accumulating
above an impermeable bed, saturating available pore space and forming an underground reservoir.
Underground strata that can both store and transmit accumulated groundwater are termed aquifers. The
water table marks the level to which the ground is fully saturated (the saturated zone). Above the water table
the ground is known as the unsaturated zone.
An aquifer’s productivity depends on the fundamental characteristics of being able to both store and transmit
water, and these qualities may vary (see below). Unconsolidated granular sediments (i), such as sands or
gravels contain pore space between the grains and thus the water content can exceed 30% of their volume,
but this reduces progressively both with the proportion of finer materials such as silt or clay and with
cementation of the grains (ii). In highly consolidated rocks (iii) groundwater is found only in fractures and
rarely exceeds 1% of the volume of the rock mass. However, in the case of limestones (iv), these fractures
may become enlarged, by solution and preferential flow, to form fissures and caverns. Even then the total
storage is relatively small compared to unconsolidated aquifers, and one result is that there is less water
available to dilute contaminant pulses.
SECONDARY POROSITY
PRIMARY POROSITY
(i) unconsolidated
well-sorted sand
with high porosity
(ii) unconsolidated sand but
porosity reduced by admixture
of fines or cementation
fines admixture
A.
(iii) consolidated rock
rendered porous by
fracturing
(iv) consolidated fractured
rock with porosity
increased by solution
cementation
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Rock texture and porosity of typical aquifer materials (modified from Meinzer, 1923)
An additional noteworthy feature is that a number of major British aquifers are a combination of types (ii) and
(iii) or (ii) and (iv) in that the rock matrix provides a certain proportion of the total storage capacity of the
system, while locally or regionally the fractures provide the dominant flow-path. The most widespread of
these combinations are called dual permeability aquifers, where some regional flow can occur through the
matrix, and either selected horizons/structural features may facilitate flow or individual boreholes/wellfields
may become extra-productive through preferential near-well development of local fracture systems.
Examples include some Jurassic limestones, the Permo-Triassic sandstones and the Magnesian Limestone.
The Chalk is an important special case called a dual porosity aquifer in that the microporous nature of the
limestone provides very large but relatively immobile storage and all flow is through fractures. This
arrangement greatly modifies pollutant movement, the water in the matrix being relatively immobile
compared with that in the fissures.
Double (dual) porosity
Fracture
Matrix
Flow
Geochemical
exchange
by diffusion
Immobile water
in matrix
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B. Schematic representation of double porosity aquifer (modified from Barker 1993)
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All of these groundwater settings are found in the aquifers of Britain, but consolidated rocks comprise the
most important group for both public and private water supply. Although groundwater in Britain is drawn from
a large number of different rock formations, there are seven major aquifer systems that are regarded as the
most productive (see map C):
Chalk and Greensand
Jurassic limestones
Permo Trias sandstones
Magnesian Limestone
Carboniferous Limestone
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C.
Major aquifer systems used for water supply in England and Wales
- the Chalk of southern and eastern England
- the Permo-Triassic sandstones of central, northwest England and southern Scotland
- the Lower Greensand of southern England
- the Magnesian Limestone of central and northern England
- the Jurassic limestones of southern, central and northern England
- the Carboniferous Limestone of southwestern and central England and south Wales
While these six rock formations are the most important for public water supply, there are more than 160 other
locally important aquifers distributed throughout England and Wales which are also tapped for drinking water
supplies (Jones et al 2000). For the very smallest supplies e.g. for an individual dwelling, retail outlet or
small camp-site, it is possible to tap numerous low-yield formations that are not productive enough to be
3
considered as aquifers but are able in favourable circumstances to provide one or two m /d of water. Many
supplies tap quite shallow depths and may draw at least some of their water from patches of much more
recent glacial or alluvial deposits which overlie the much older formations and are present throughout the
UK.
For more detailed information see the map displays on this site.
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